Linux ALSA 音訊系統：物理鏈路篇
2020-08-13 13:20:07
本文轉載自：https://blog.csdn.net/zyuanyun/article/details/59170418
1. Overview
硬體平臺及軟體版本：
- Kernel - 3.4.5
- SoC - Samsung exynos
- CODEC - WM8994
- Machine - goni_wm8994
- Userspace - tinyalsa
Linux ALSA 音訊系統架構大致如下：
+--------+ +--------+ +--------+
|tinyplay| |tinycap | |tinymix |
+--------+ +--------+ +--------+
| ^ ^
V | V
+--------------------------------+
| ALSA Library API |
| (tinyalsa, alsa-lib) |
+--------------------------------+
user space ^
-------------------------------|---------------------
kernel space V
+--------------------------------+
| ALSA CORE |
| +-------+ +-------+ +------+ |
| | PCM | |CONTROL| | MIDI |...|
| +-------+ +-------+ +------+ |
+--------------------------------+
|
+--------------------------------+
| ASoC CORE |
+--------------------------------+
|
+--------------------------------+
| hardware driver |
| +-------+ +--------+ +-----+ |
| |Machine| |Platform| |Codec| |
| +-------+ +--------+ +-----+ |
+--------------------------------+
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- Native ALSA Application：tinyplay/tinycap/tinymix，這些使用者程式直接呼叫 alsa 使用者庫介面來實現放音、錄音、控制
- ALSA Library API：alsa 使用者庫介面，常見有 tinyalsa、alsa-lib
- ALSA CORE：alsa 核心層，向上提供邏輯裝置（PCM/CTL/MIDI/TIMER/…）系統呼叫，向下驅動硬體裝置（Machine/I2S/DMA/CODEC）
- ASoC CORE：asoc 是建立在標準 alsa core 基礎上，爲了更好支援嵌入式系統和應用於移動裝置的音訊 codec 的一套軟體體系
- Hardware Driver：音訊硬體裝置驅動，由三大部分組成，分別是 Machine、Platform、Codec
本主題不遵循自頂而下的原則，而先從硬體裝置驅動說起，畢竟這些是看得見摸得着聽得到的東西，容易對其有着直觀的理解。
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ALSA/ASoC 中硬體裝置關係：
+------------------------------------------+
| Machine |
| +--------------+ +--------------+ |
| | Platform | | Codec | |
| | | I2S | | |
| | cpu_dai|<---->|codec_dai | |
| | | | | |
| +--------------+ +--------------+ |
+------------------------------------------+
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-
Platform：指某款 SoC 平臺的音訊模組，如 exynos、omap、qcom 等等。Platform 又可細分兩部分：
- cpu dai：在嵌入式系統裏面通常指 SoC 的 I2S、PCM 匯流排控制器，負責把音訊數據從 I2S tx FIFO 搬運到 CODEC（這是回放的情形，錄製則方向相反）。cpu_dai 通過
snd_soc_register_dai()來註冊。注：DAI 是 Digital Audio Interface 的簡稱，分爲 cpu_dai 和 codec_dai，這兩者通過 I2S/PCM 匯流排連線；AIF 是 Audio Interface 的簡稱，嵌入式系統中一般是 I2S 和 PCM 介面。 - pcm dma：負責把 dma buffer 中的音訊數據搬運到 I2S tx FIFO。值得留意的是：某些情形下是不需要 dma 操作的，比如 Modem 和 CODEC 直連，因爲 Modem 本身已經把數據送到 FIFO 了，這時只需啓動 codec_dai 接收數據即可；該情形下，Machine 驅動 dai_link 中需要設定
.platform_name = "snd-soc-dummy",這是虛擬 dma 驅動，實現見sound/soc/soc-utils.c。音訊 dma 驅動通過snd_soc_register_platform()來註冊，故也常用 platform 來指代音訊 dma 驅動（這裏的 platform 需要與 SoC Platform 區分開）。
- cpu dai：在嵌入式系統裏面通常指 SoC 的 I2S、PCM 匯流排控制器，負責把音訊數據從 I2S tx FIFO 搬運到 CODEC（這是回放的情形，錄製則方向相反）。cpu_dai 通過
-
Codec：對於回放來說，userspace 送過來的音訊數據是經過採樣量化的數位信號，在 codec 經過 DAC 轉換成模擬信號然後輸出到外放或耳機，這樣我們就可以聽到聲音了。Codec 字面意思是編解碼器，但晶片裏面的功能部件很多，常見的有 AIF、DAC、ADC、Mixer、PGA、Line-in、Line-out，有些高階的 codec 晶片還有 EQ、DSP、SRC、DRC、AGC、Echo-Canceller、Noise-Suppression 等部件。
-
Machine：指某款機器，通過設定 dai_link 把 cpu_dai、codec_dai、modem_dai 各個音訊介面給鏈結成一條條音訊鏈路，然後註冊
snd_soc_card。和上面兩個不一樣，Platform 和 CODEC 驅動一般是可以重用的，而 Machine 有它特定的硬體特性，幾乎是不可重用的。所謂的硬體特性指：SoC Platform 與 Codec 的差異；DAIs 之間的鏈結方式；通過某個 GPIO 開啓 Amplifier；通過某個 GPIO 檢測耳機插拔；使用某個時鐘如 MCLK/External-OSC 作爲 I2S、CODEC 的時鐘源等等。
從上面的描述來看，對於回放的情形，PCM 數據流向大致是：
copy_from_user DMA I2S DAC
^ ^ ^ ^
+---------+ | +----------+ | +-----------+ | +-----+ | +------+
|userspace+-------->DMA Buffer+------->I2S TX FIFO+------->CODEC+------->SPK/HP|
+---------+ +----------+ +-----------+ +-----+ +------+
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幾個音訊物理鏈路的概念：
dai_link：machine 驅動中定義的音訊數據鏈路，它指定鏈路用到的 codec、codec_dai、cpu_dai、platform。比如對於 goni_wm8994 平臺的 media 鏈路：codec="wm8994-codec"、codec_dai="wm8994-aif1"、cpu_dai="samsung-i2s"、platform="samsung-audio"，這四者就構成了一條音訊數據鏈路用於多媒體聲音的回放和錄製。一個系統可能有多個音訊數據鏈路，比如 media 和 voice，因此可以定義多個 dai_link 。如 WM8994 的典型設計，有三個 dai_link，分別是 AP<>AIF1 的 「HIFI」（多媒體聲音鏈路），BP<>AIF2 的 「Voice」（通話語音鏈路），以及 BT<>AIF3（藍牙 SCO 語音鏈路）。
程式碼如下：
static struct snd_soc_dai_link goni_dai[] = {
{
.name = "WM8994",
.stream_name = "WM8994 HiFi",
.cpu_dai_name = "samsung-i2s.0",
.codec_dai_name = "wm8994-aif1",
.platform_name = "samsung-audio",
.codec_name = "wm8994-codec.0-001a",
.init = goni_wm8994_init,
.ops = &goni_hifi_ops,
}, {
.name = "WM8994 Voice",
.stream_name = "Voice",
.cpu_dai_name = "goni-voice-dai",
.codec_dai_name = "wm8994-aif2",
.codec_name = "wm8994-codec.0-001a",
.ops = &goni_voice_ops,
},
};
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hw constraints：指平臺本身的硬體限制，如所能支援的通道數/採樣率/數據格式、DMA 支援的數據週期大小（period size）、週期次數（period count）等，通過 snd_pcm_hardware 結構體描述：
static const struct snd_pcm_hardware dma_hardware = {
.info = SNDRV_PCM_INFO_INTERLEAVED |
SNDRV_PCM_INFO_BLOCK_TRANSFER |
SNDRV_PCM_INFO_MMAP |
SNDRV_PCM_INFO_MMAP_VALID |
SNDRV_PCM_INFO_PAUSE |
SNDRV_PCM_INFO_RESUME,
.formats = SNDRV_PCM_FMTBIT_S16_LE |
SNDRV_PCM_FMTBIT_U16_LE |
SNDRV_PCM_FMTBIT_U8 |
SNDRV_PCM_FMTBIT_S8,
.channels_min = 2,
.channels_max = 2,
.buffer_bytes_max = 128*1024,
.period_bytes_min = PAGE_SIZE,
.period_bytes_max = PAGE_SIZE*2,
.periods_min = 2,
.periods_max = 128,
.fifo_size = 32,
};
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hw params：使用者層設定的硬體參數，如 channels、sample rate、pcm format、period size、period count；這些參數受 hw constraints 約束。
sw params：使用者層設定的軟體參數，如 start threshold、stop threshold、silence threshold。
2. ASoC
ASoC：ALSA System on Chip，是建立在標準 ALSA 驅動之上，爲了更好支援嵌入式系統和應用於移動裝置的音訊 codec 的一套軟體體系，它依賴於標準 ALSA 驅動框架。內核文件 Documentation/alsa/soc/overview.txt 中詳細介紹了 ASoC 的設計初衷，這裏不一一參照，簡單陳述如下：
- 獨立的 codec 驅動，標準的 ALSA 驅動框架裏面 codec 驅動往往與 SoC/CPU 耦合過於緊密，不利於在多樣化的平臺/機器上移植複用
- 方便 codec 與 SoC 通過 PCM/I2S 匯流排建立鏈接
- 動態音訊電源管理 DAPM，使得 codec 任何時候都工作在最低功耗狀態，同時負責音訊路由的建立
- POPs 和 click 音抑制弱化處理，在 ASoC 中通過正確的音訊部件上下電次序來實現
- Machine 驅動的特定控制，比如耳機、麥克風的插拔檢測，外放功放的開關
在概述中已經介紹了 ASoC 硬體裝置驅動的三大構成：Codec、Platform 和 Machine，下面 下麪列舉各驅動的功能構成：
ASoC Codec Driver：
- Codec DAI 和 PCM 的設定資訊
- Codec 的控制介面，如 I2C/SPI
- Mixer 和其他音訊控制元件
- Codec 的音訊介面函數，見
snd_soc_dai_ops結構體定義 - DAPM 描述資訊
- DAPM 事件處理控制代碼
- DAC 數位靜音控制
ASoC Platform Driver： 包括 dma 和 cpu_dai 兩部分：
- dma 驅動實現音訊 dma 操作，具體見
snd_pcm_ops結構體定義 - cpu_dai 驅動實現音訊數位介面控制器的描述和設定
ASoC Machine Driver：
- 作爲鏈結 Platform 和 Codec 的載體，它必須設定 dai_link 爲音訊數據鏈路指定 Platform 和 Codec
- 處理機器特有的音訊控制元件和音訊事件，例如回放時開啓外放功放
硬體裝置驅動相關結構體：
- snd_soc_codec_driver：音訊編解碼晶片描述及操作函數，如控制元件/微件/音訊路由的描述資訊、時鐘設定、IO 控制等
- snd_soc_dai_driver：音訊數據介面描述及操作函數，根據 codec 端和 soc 端，分爲 codec_dai 和 cpu_dai
- snd_soc_platform_driver：音訊 dma 裝置描述及操作函數
- snd_soc_dai_link：音訊鏈路描述及板級操作函數
下面 下麪是 goni_wm8994 類圖，從這個類圖中，我們可以大致瞭解 goni_wm8994 整個音訊驅動組成：
3. Codec
上一章提到 codec_drv 的幾個組成部分，下面 下麪逐一介紹，基本是以內核文件 Documentation/sound/alsa/soc/codec.txt 中的內容爲脈絡來分析的。Codec 的作用，之前已有描述，本章主要羅列下 Codec driver 中重要的數據結構及註冊流程。
我們先看看 Codec 的硬體框圖，以 WM8994 爲例：
其中有着各種功能部件，包括但不限於 ：
| Widget | Description |
|---|---|
| ADC | 把麥克風拾取的模擬信號轉換成數位信號 |
| DAC | 把音訊介面過來的數位信號轉換成模擬信號 |
| AIF | 音訊數位介面，用於 Codec 與其他器件（如AP、BB等）之間的數據傳輸 |
| MIXER | 混音器，把多路輸入信號混合成單路輸出 |
| DRC | 動態範圍調節 |
| LHPF | 高低通濾波 |
3.1. Codec DAI and PCM configuration
codec_dai 和 pcm 設定資訊通過結構體 snd_soc_dai_driver 描述，包括 dai 的能力描述和操作介面，snd_soc_dai_driver 最終會被註冊到 soc-core 中。
/* * Digital Audio Interface Driver. * * Describes the Digital Audio Interface in terms of its ALSA, DAI and AC97 * operations and capabilities. Codec and platform drivers will register this * structure for every DAI they have. * This structure covers the clocking, formating and ALSA operations for each * interface. */ struct snd_soc_dai_driver { /* DAI description */ const char *name; unsigned int id; int ac97_control;<span class="token comment">/* DAI driver callbacks */</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>probe<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span>dai<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>remove<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span>dai<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>suspend<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span>dai<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>resume<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span>dai<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">/* ops */</span> <span class="token keyword">const</span> <span class="token keyword">struct</span> snd_soc_dai_ops <span class="token operator">*</span>ops<span class="token punctuation">;</span> <span class="token comment">/* DAI capabilities */</span> <span class="token keyword">struct</span> snd_soc_pcm_stream capture<span class="token punctuation">;</span> <span class="token keyword">struct</span> snd_soc_pcm_stream playback<span class="token punctuation">;</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> symmetric_rates<span class="token punctuation">:</span><span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">/* probe ordering - for components with runtime dependencies */</span> <span class="token keyword">int</span> probe_order<span class="token punctuation">;</span> <span class="token keyword">int</span> remove_order<span class="token punctuation">;</span>
};
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- name：codec_dai 的名稱標識，dai_link 通過設定 codec_dai_name 來找到對應的 codec_dai；
- probe：codec_dai 的初始化函數，註冊音效卡時回撥；
- playback：回放能力描述，如回放裝置所支援的聲道數、採樣率、音訊格式；
- capture：錄製能力描述，如錄製裝置所支援聲道數、採樣率、音訊格式；
- ops：codec_dai 的操作函數集，這些函數集非常重要，用於 dai 的時鐘設定、格式設定、硬體參數設定。
例子，wm8994 有三個 dai，這裏只列其一：
static const struct snd_soc_dai_ops wm8994_aif1_dai_ops = {
.set_sysclk = wm8994_set_dai_sysclk,
.set_fmt = wm8994_set_dai_fmt,
.hw_params = wm8994_hw_params,
.shutdown = wm8994_aif_shutdown,
.digital_mute = wm8994_aif_mute,
.set_pll = wm8994_set_fll,
.set_tristate = wm8994_set_tristate,
};
static struct snd_soc_dai_driver wm8994_dai[] = {
{
.name = 「wm8994-aif1」,
.id = 1,
.playback = {
.stream_name = 「AIF1 Playback」,
.channels_min = 1,
.channels_max = 2,
.rates = WM8994_RATES,
.formats = WM8994_FORMATS,
.sig_bits = 24,
},
.capture = {
.stream_name = 「AIF1 Capture」,
.channels_min = 1,
.channels_max = 2,
.rates = WM8994_RATES,
.formats = WM8994_FORMATS,
.sig_bits = 24,
},
.ops = &wm8994_aif1_dai_ops,
},
// …
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3.2. Codec control IO
移動裝置的音訊 Codec，其控制介面一般是 I2C 或 SPI，控制介面用於讀寫 codec 的暫存器。在 snd_soc_codec_driver 結構體中，有如下欄位描述 Codec 的控制介面：
/* codec IO */
unsigned int (*read)(struct snd_soc_codec *, unsigned int);
int (*write)(struct snd_soc_codec *, unsigned int, unsigned int);
int (*display_register)(struct snd_soc_codec *, char *,
size_t, unsigned int);
int (*volatile_register)(struct snd_soc_codec *, unsigned int);
int (*readable_register)(struct snd_soc_codec *, unsigned int);
int (*writable_register)(struct snd_soc_codec *, unsigned int);
unsigned int reg_cache_size;
short reg_cache_step;
short reg_word_size;
const void *reg_cache_default;
short reg_access_size;
const struct snd_soc_reg_access *reg_access_default;
enum snd_soc_compress_type compress_type;
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- read：讀暫存器；
- write：寫暫存器；
- volatile_register：判斷指定的暫存器是否是 volatile 屬性；假如是，則讀取暫存器時不是讀 cache，而直接存取硬體；
- readable_register：判斷指定的暫存器是否可讀；
- reg_cache_default：暫存器的預設值；
- reg_cache_size：預設的暫存器值陣列大小；
- reg_word_size：暫存器寬度。
在 Linux-3.4.5 中，很多 codec 的控制介面都改用 regmap 了。soc-core 中判斷是否用的是 regmap，如果是，則呼叫 regmap 介面，見如下函數：
int snd_soc_update_bits(struct snd_soc_codec *codec, unsigned short reg, unsigned int mask, unsigned int value) { bool change; unsigned int old, new; int ret;<span class="token keyword">if</span> <span class="token punctuation">(</span>codec<span class="token operator">-></span>using_regmap<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 當前使用 regmap，呼叫 regmap 介面，其中 codec->control_data 是 regmap 私有數據</span> ret <span class="token operator">=</span> <span class="token function">regmap_update_bits_check</span><span class="token punctuation">(</span>codec<span class="token operator">-></span>control_data<span class="token punctuation">,</span> reg<span class="token punctuation">,</span> mask<span class="token punctuation">,</span> value<span class="token punctuation">,</span> <span class="token operator">&</span>change<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span> <span class="token comment">// 非 regmap，呼叫 snd_soc_codec_driver 實現的 read/write 回撥</span> ret <span class="token operator">=</span> <span class="token function">snd_soc_read</span><span class="token punctuation">(</span>codec<span class="token punctuation">,</span> reg<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>ret <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span> old <span class="token operator">=</span> ret<span class="token punctuation">;</span> new <span class="token operator">=</span> <span class="token punctuation">(</span>old <span class="token operator">&</span> <span class="token operator">~</span>mask<span class="token punctuation">)</span> <span class="token operator">|</span> <span class="token punctuation">(</span>value <span class="token operator">&</span> mask<span class="token punctuation">)</span><span class="token punctuation">;</span> change <span class="token operator">=</span> old <span class="token operator">!=</span> new<span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>change<span class="token punctuation">)</span> ret <span class="token operator">=</span> <span class="token function">snd_soc_write</span><span class="token punctuation">(</span>codec<span class="token punctuation">,</span> reg<span class="token punctuation">,</span> new<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>ret <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span> <span class="token keyword">return</span> change<span class="token punctuation">;</span>
}
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使用 regmap，使得控制介面抽象化，codec_drv 不用關心當前控制方式是什麼；regmap 線上偵錯目錄是 /sys/kernel/debug/regmap。關於 wm8994 的 regmap 描述，請自行查閱 driver/mfd/wm8994-regmap.c。
3.3. Mixers and audio controls
音訊控制元件多用於部件開關和音量的設定，音訊控制元件可通過 soc.h 中的宏來定義，例如單一型控制元件：
#define SOC_SINGLE(xname, reg, shift, max, invert) \
{ .iface = SNDRV_CTL_ELEM_IFACE_MIXER, .name = xname, \
.info = snd_soc_info_volsw, .get = snd_soc_get_volsw,\
.put = snd_soc_put_volsw, \
.private_value = SOC_SINGLE_VALUE(reg, shift, max, invert) }
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這種控制元件只有一個設定量，一般用於部件開關。宏定義的參數說明：
- xname：控制元件的名稱標識；
- reg：控制元件對應的暫存器地址；
- shift：控制元件控制位在暫存器中的偏移；
- max：控制元件設定值範圍；
- invert：設定值是否取反。
其他型別控制元件類似，不一一介紹了。
上述只是宏定義，音訊控制元件真正的結構是 snd_kcontrol_new：
struct snd_kcontrol_new {
snd_ctl_elem_iface_t iface; /* interface identifier */
unsigned int device; /* device/client number */
unsigned int subdevice; /* subdevice (substream) number */
const unsigned char *name; /* ASCII name of item */
unsigned int index; /* index of item */
unsigned int access; /* access rights */
unsigned int count; /* count of same elements */
snd_kcontrol_info_t *info;
snd_kcontrol_get_t *get;
snd_kcontrol_put_t *put;
union {
snd_kcontrol_tlv_rw_t *c;
const unsigned int *p;
} tlv;
unsigned long private_value;
};
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Codec 初始化時，通過 snd_soc_add_codec_controls() 把所有定義好的音訊控制元件註冊到 alsa-core ，上層可以通過 tinymix、alsa_amixer 等工具檢視修改這些控制元件的設定。
3.4. Codec audio operations
Codec 音訊操作介面通過結構體 snd_soc_dai_ops 描述：
struct snd_soc_dai_ops { /* * DAI clocking configuration, all optional. * Called by soc_card drivers, normally in their hw_params. */ int (*set_sysclk)(struct snd_soc_dai *dai, int clk_id, unsigned int freq, int dir); int (*set_pll)(struct snd_soc_dai *dai, int pll_id, int source, unsigned int freq_in, unsigned int freq_out); int (*set_clkdiv)(struct snd_soc_dai *dai, int div_id, int div);<span class="token comment">/* * DAI format configuration * Called by soc_card drivers, normally in their hw_params. */</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>set_fmt<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span>dai<span class="token punctuation">,</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> fmt<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>set_tdm_slot<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span>dai<span class="token punctuation">,</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> tx_mask<span class="token punctuation">,</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> rx_mask<span class="token punctuation">,</span> <span class="token keyword">int</span> slots<span class="token punctuation">,</span> <span class="token keyword">int</span> slot_width<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>set_channel_map<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span>dai<span class="token punctuation">,</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> tx_num<span class="token punctuation">,</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> <span class="token operator">*</span>tx_slot<span class="token punctuation">,</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> rx_num<span class="token punctuation">,</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> <span class="token operator">*</span>rx_slot<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>set_tristate<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span>dai<span class="token punctuation">,</span> <span class="token keyword">int</span> tristate<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">/* * DAI digital mute - optional. * Called by soc-core to minimise any pops. */</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>digital_mute<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span>dai<span class="token punctuation">,</span> <span class="token keyword">int</span> mute<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">/* * ALSA PCM audio operations - all optional. * Called by soc-core during audio PCM operations. */</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>startup<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_pcm_substream <span class="token operator">*</span><span class="token punctuation">,</span> <span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">void</span> <span class="token punctuation">(</span><span class="token operator">*</span>shutdown<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_pcm_substream <span class="token operator">*</span><span class="token punctuation">,</span> <span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>hw_params<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_pcm_substream <span class="token operator">*</span><span class="token punctuation">,</span> <span class="token keyword">struct</span> snd_pcm_hw_params <span class="token operator">*</span><span class="token punctuation">,</span> <span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>hw_free<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_pcm_substream <span class="token operator">*</span><span class="token punctuation">,</span> <span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>prepare<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_pcm_substream <span class="token operator">*</span><span class="token punctuation">,</span> <span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>trigger<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_pcm_substream <span class="token operator">*</span><span class="token punctuation">,</span> <span class="token keyword">int</span><span class="token punctuation">,</span> <span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">/* * For hardware based FIFO caused delay reporting. * Optional. */</span> snd_pcm_sframes_t <span class="token punctuation">(</span><span class="token operator">*</span>delay<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_pcm_substream <span class="token operator">*</span><span class="token punctuation">,</span> <span class="token keyword">struct</span> snd_soc_dai <span class="token operator">*</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
};
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註釋比較詳細的了，Codec 音訊操作介面分爲 5 大部分：時鐘設定、格式設定、數位靜音、PCM 音訊介面、FIFO 延遲。着重說下時鐘設定及格式設定介面：
- set_sysclk：codec_dai 系統時鐘設定，當上層開啓 pcm 裝置時，需要回撥該介面設定 Codec 的系統時鐘，Codec 才能 纔能正常工作；
- set_pll：Codec FLL 設定，Codec 一般接了一個 MCLK 輸入時鐘，回撥該介面基於 MCLK 來產生 Codec FLL 時鐘，接着 codec_dai 的 sysclk、bclk、lrclk 均可從 FLL 分頻出來（假設 Codec 作爲 master）；
- set_fmt：codec_dai 格式設定，具體見
soc-dai.h；SND_SOC_DAIFMT_I2S：音訊數據是 I2S 格式，常用於多媒體音訊；SND_SOC_DAIFMT_DSP_A：音訊數據是 PCM 格式，常用於通話語音；SND_SOC_DAIFMT_CBM_CFM：Codec 作爲 master，BCLK 和 LRCLK 由 Codec 提供；SND_SOC_DAIFMT_CBS_CFS：Codec 作爲 slave，BCLK 和 LRCLK 由 SoC/CPU 提供；
- hw_params：codec_dai 硬體參數設定，根據上層設定的聲道數、採樣率、數據格式，來設定 codec_dai 相關暫存器。
以上介面一般在 Machine 驅動中回撥，我們看看 Machine 驅動 goni_wm8994.c 的 goni_hifi_hw_params() 函數：
static int goni_hifi_hw_params(struct snd_pcm_substream *substream, struct snd_pcm_hw_params *params) { struct snd_soc_pcm_runtime *rtd = substream->private_data; struct snd_soc_dai *codec_dai = rtd->codec_dai; struct snd_soc_dai *cpu_dai = rtd->cpu_dai; unsigned int pll_out = 24000000; // 這是 MCLK 的時鐘頻率，Codec 的源時鐘 int ret = 0;<span class="token comment">/* set the cpu DAI configuration */</span> ret <span class="token operator">=</span> <span class="token function">snd_soc_dai_set_fmt</span><span class="token punctuation">(</span>cpu_dai<span class="token punctuation">,</span> SND_SOC_DAIFMT_I2S <span class="token operator">|</span> SND_SOC_DAIFMT_NB_NF <span class="token operator">|</span> SND_SOC_DAIFMT_CBM_CFM<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>ret <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span> <span class="token comment">/* set codec DAI configuration */</span> ret <span class="token operator">=</span> <span class="token function">snd_soc_dai_set_fmt</span><span class="token punctuation">(</span>codec_dai<span class="token punctuation">,</span> SND_SOC_DAIFMT_I2S <span class="token operator">|</span> SND_SOC_DAIFMT_NB_NF <span class="token operator">|</span> SND_SOC_DAIFMT_CBM_CFM<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>ret <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span> <span class="token comment">/* set the codec FLL */</span> ret <span class="token operator">=</span> <span class="token function">snd_soc_dai_set_pll</span><span class="token punctuation">(</span>codec_dai<span class="token punctuation">,</span> WM8994_FLL1<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> pll_out<span class="token punctuation">,</span> <span class="token function">params_rate</span><span class="token punctuation">(</span>params<span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token number">256</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>ret <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span> <span class="token comment">/* set the codec system clock */</span> ret <span class="token operator">=</span> <span class="token function">snd_soc_dai_set_sysclk</span><span class="token punctuation">(</span>codec_dai<span class="token punctuation">,</span> WM8994_SYSCLK_FLL1<span class="token punctuation">,</span> <span class="token function">params_rate</span><span class="token punctuation">(</span>params<span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token number">256</span><span class="token punctuation">,</span> SND_SOC_CLOCK_IN<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>ret <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
}
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其中 snd_soc_dai_set_fmt() 實際上會呼叫 cpu_dai 或 codec_dai 的 set_fmt() 回撥， snd_soc_dai_set_pll()、snd_soc_dai_set_sysclk() 也類似。
- MCLK 作爲 Codec 的源時鐘，頻率爲 24Mhz；
- 設定 cpu_dai 和 codec_dai 格式：數據格式是 I2S；Codec 作爲 master，BCLK 和 LRCLK 由 Codec 提供；
- 設定 codec_dai 的 FLL1：時鐘源是 MCLK，時鐘源頻率是 24Mhz，目的時鐘頻率是 256fs（fs 是採樣頻率）；
- 設定 codec_dai 的系統時鐘：時鐘源是 FLL1，系統時鐘頻率是 256fs。
對於 dai（codec_dai 和 cpu_dai），都要非常留意時鐘設定，它很關鍵又複雜，設定錯誤將會導致很多問題，典型如下：
- 系統無聲：檢查 Codec 系統時鐘、codec_dai 位時鐘和幀時鐘是否使能；
- 聲音失真：檢查音訊數據的採樣率是否和 codec_dai 幀時鐘一致；
- 斷續破音：檢查 Codec 系統時鐘和位時鐘、幀時鐘是否同步，出現這種情況，可能是因爲 sysclk 和 BCLK/LRCLK 不是由同一個時鐘源分頻出來的。
如下是一個典型的音訊系統時鐘設定（Codec works as master mode）：
+---------------------------------------------------------------------- -+
| CODEC | +-----------+
| +---------+ | | |
| SLIMCLK+--> | | | |
| | | | | |
| AIFnBCLK+-> | +---------+ | | |
| | | +-----+ | | | | |
| AIFnLRCLK+> FLL_SRC +---> FLL +---+ | +---->AIFnBCLK+--> |
| | | +-----+ | | | | | |
| MCLK1+---> | | +---------+ | +---->AIFnLRCLK+-> |
| ^ | | +-------> | | | | | Processor |
| | MCLK2+> | | | | AIFn | | | |
| | ^ +---------+ SLIMCLK+--> | | | | | |
| | | | +----> <----+AIFnRX<----+ |
| | | AIFnBCLK+-> SYSCLK | | | | | |
| | | | | | +----+AIFnTX+----> |
| | | MCLK1+----> | | | | | |
| | | | | +---------+ | | |
| | | MCLK2+----> | | | |
| | | +---------+ | | |
+------+-----------------------------------------------------------------+ +-----------+
|
+-+----------+
| Oscillator |
+------------+
AIF Master Mode, Using MCLK and FLL as Reference
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- MCLK1 由外部晶振提供時鐘
- Codec FLL 選取 MCLK1 作爲時鐘源，分出想要的時鐘頻率
- SYSCLK 選取 FLL 作爲時鐘源，產生系統時鐘
- BCLK、LRCLK 由 Codec 產生提供，即 Codec 作爲 Master
3.5. DAPM description
概念：Dynamic Audio Power Management，動態音訊電源管理，爲移動 Linux 裝置設計，使得音訊系統任何時候都工作在最低功耗狀態。
目的：使能最少的必要的部件，令音訊系統正常工作。
原理：當音訊路徑發生改變（比如上層使用 tinymix 工具設定音訊通路）時，或發生數據流事件（比如啓動或停止播放）時，都會觸發 dapm 去遍歷所有鄰近的音訊部件，檢查是否存在完整的音訊路徑（complete path：滿足條件的音訊路徑，該路徑上任意一個部件往前遍歷能到達輸入端點如 DAC/Mic/Linein，往後遍歷能到達輸出端點如 ADC/HP/SPK），如果存在完整的音訊路徑，則該路徑上面的所有部件都是需要上電的，其他部件則下電。
部件上下電都是 dapm 根據策略自主控制的，外部無法幹預，可以說 dapm 是一個專門爲音訊系統設計的自成體系的電源管理模組，獨立於 Linux 電源管理之外。即使 SoC 休眠了，Codec 仍可以在正常工作，試想下這個情景：語音通話，modem_dai 連線到 codec_dai，語音數據不經過 SoC，因此這種情形下 SoC 可以進入睡眠以降低功耗，只保持 Codec 正常工作就行了。
dapm 原理及實現非常精妙，我認爲是 ALSA/ASoC 中最值得鑽研的一個點了。
如下是多媒體外放回放通路：
在這個例子中，codec 中的音訊通路是：AIF1>DAC1>OUTMIXER>SPKOUT；AIF1 是輸入端點，SPKOUT 是輸出端點，因此這條通路是一個 complete path，這通路上的所有部件都是需要上電的，與此同時，其他部件需要下電。
而音訊部件由於上下電瞬間的瞬態衝擊會產生爆破音，我們稱之爲 POPs。POPs 是電氣特性，我們無法徹底消除，只能硬體軟體上優化削弱到人耳辨識不出的程度。DAPM 中，部件的上下電有嚴格的順序以抑制爆破音，總的來說：上電次序是從輸入端點到輸出端點，下電次序是從輸出端點到輸入端點。
驅動中如何建立 dapm widget 和 dapm route？以 最典型的 mixer widget 爲例：Mixes several analog signals into a single analog signal. 它可以把幾路模擬信號混合到一路輸出，如 WM8994 的 SPKMIXL：
如圖，SPKMIXL 有 5 路輸入，分別是：MIXINL、IN1LP、DAC1L、DAC2L、MIXEROUTL，因此這裏可以構成 5 條通路。
- 如下 5 個控制元件控制 SPKMIXL 輸入：
static const struct snd_kcontrol_new left_speaker_mixer[] = {
SOC_DAPM_SINGLE("DAC2 Switch", WM8994_SPEAKER_MIXER, 9, 1, 0),
SOC_DAPM_SINGLE("Input Switch", WM8994_SPEAKER_MIXER, 7, 1, 0),
SOC_DAPM_SINGLE("IN1LP Switch", WM8994_SPEAKER_MIXER, 5, 1, 0),
SOC_DAPM_SINGLE("Output Switch", WM8994_SPEAKER_MIXER, 3, 1, 0),
SOC_DAPM_SINGLE("DAC1 Switch", WM8994_SPEAKER_MIXER, 1, 1, 0),
};
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- 定義 SPKMIXL 的 dapm widget：
SND_SOC_DAPM_MIXER_E("SPKL", WM8994_POWER_MANAGEMENT_3, 8, 0,
left_speaker_mixer, ARRAY_SIZE(left_speaker_mixer),
late_enable_ev, SND_SOC_DAPM_PRE_PMU),
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留意 WM8994_POWER_MANAGEMENT_3 暫存器的 bit8 正是控制 SPKMIXL 上下電的。
- 定義 SPKMIXL 相關路由：
static const struct snd_soc_dapm_route intercon[] = {
// ...
{ "SPKL", "DAC1 Switch", "DAC1L" },
{ "SPKL", "DAC2 Switch", "DAC2L" },
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最終上層會看到兩個控制元件：「SPKL DAC1 Switch」，「SPKL DAC2 Switch」；前者用於 「SPKL」 選中 「DAC1L」 作爲輸入，後者用於 「SPKL」 選中 「DAC2L」 作爲輸入。
但控制元件 「SPKLDAC1 Switch」 或 「SPKL DAC2 Switch」 的開啓，不代表能使得 「SPKL」 上電。只有當 「SPKL」 位於完整的音訊路徑中時，「SPKL」 纔會上電。
3.6. Codec register
當 platform_driver：
static struct platform_driver wm8994_codec_driver = {
.driver = {
.name = "wm8994-codec",
.owner = THIS_MODULE,
.pm = &wm8994_pm_ops,
},
.probe = wm8994_probe,
.remove = __devexit_p(wm8994_remove),
};
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與 .name = "wm8994-codec" 的 platform_device（該 platform_device 在 driver/mfd/wm8994-core.c 中註冊）匹配後，立即回撥 wm8994_probe() 註冊 Codec：
static int __devinit wm8994_probe(struct platform_device *pdev) { struct wm8994_priv *wm8994;wm8994 <span class="token operator">=</span> <span class="token function">devm_kzalloc</span><span class="token punctuation">(</span><span class="token operator">&</span>pdev<span class="token operator">-></span>dev<span class="token punctuation">,</span> <span class="token keyword">sizeof</span><span class="token punctuation">(</span><span class="token keyword">struct</span> wm8994_priv<span class="token punctuation">)</span><span class="token punctuation">,</span> GFP_KERNEL<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>wm8994 <span class="token operator">==</span> <span class="token constant">NULL</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span> <span class="token function">platform_set_drvdata</span><span class="token punctuation">(</span>pdev<span class="token punctuation">,</span> wm8994<span class="token punctuation">)</span><span class="token punctuation">;</span> wm8994<span class="token operator">-></span>wm8994 <span class="token operator">=</span> <span class="token function">dev_get_drvdata</span><span class="token punctuation">(</span>pdev<span class="token operator">-></span>dev<span class="token punctuation">.</span>parent<span class="token punctuation">)</span><span class="token punctuation">;</span> wm8994<span class="token operator">-></span>pdata <span class="token operator">=</span> <span class="token function">dev_get_platdata</span><span class="token punctuation">(</span>pdev<span class="token operator">-></span>dev<span class="token punctuation">.</span>parent<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token function">snd_soc_register_codec</span><span class="token punctuation">(</span><span class="token operator">&</span>pdev<span class="token operator">-></span>dev<span class="token punctuation">,</span> <span class="token operator">&</span>soc_codec_dev_wm8994<span class="token punctuation">,</span> wm8994_dai<span class="token punctuation">,</span> <span class="token function">ARRAY_SIZE</span><span class="token punctuation">(</span>wm8994_dai<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
}
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snd_soc_register_codec：將 codec_driver 和 codec_dai_driver 註冊到 soc-core。
/**
* snd_soc_register_codec - Register a codec with the ASoC core
*
* @codec: codec to register
*/
int snd_soc_register_codec(struct device *dev,
const struct snd_soc_codec_driver *codec_drv,
struct snd_soc_dai_driver *dai_drv,
int num_dai)
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- 建立一個
snd_soc_codec範例，包含 codec_drv（snd_soc_dai_driver）相關資訊，封裝給 soc-core 使用，相關程式碼段如下：
struct snd_soc_codec *codec;<span class="token function">dev_dbg</span><span class="token punctuation">(</span>dev<span class="token punctuation">,</span> <span class="token string">"codec register %s\n"</span><span class="token punctuation">,</span> <span class="token function">dev_name</span><span class="token punctuation">(</span>dev<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> codec <span class="token operator">=</span> <span class="token function">kzalloc</span><span class="token punctuation">(</span><span class="token keyword">sizeof</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_codec<span class="token punctuation">)</span><span class="token punctuation">,</span> GFP_KERNEL<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>codec <span class="token operator">==</span> <span class="token constant">NULL</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span> <span class="token comment">/* create CODEC component name */</span> codec<span class="token operator">-></span>name <span class="token operator">=</span> <span class="token function">fmt_single_name</span><span class="token punctuation">(</span>dev<span class="token punctuation">,</span> <span class="token operator">&</span>codec<span class="token operator">-></span>id<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>codec<span class="token operator">-></span>name <span class="token operator">==</span> <span class="token constant">NULL</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">kfree</span><span class="token punctuation">(</span>codec<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token comment">// 初始化 Codec 的暫存器快取設定及讀寫介面</span> codec<span class="token operator">-></span>write <span class="token operator">=</span> codec_drv<span class="token operator">-></span>write<span class="token punctuation">;</span> codec<span class="token operator">-></span>read <span class="token operator">=</span> codec_drv<span class="token operator">-></span>read<span class="token punctuation">;</span> codec<span class="token operator">-></span>volatile_register <span class="token operator">=</span> codec_drv<span class="token operator">-></span>volatile_register<span class="token punctuation">;</span> codec<span class="token operator">-></span>readable_register <span class="token operator">=</span> codec_drv<span class="token operator">-></span>readable_register<span class="token punctuation">;</span> codec<span class="token operator">-></span>writable_register <span class="token operator">=</span> codec_drv<span class="token operator">-></span>writable_register<span class="token punctuation">;</span> codec<span class="token operator">-></span>ignore_pmdown_time <span class="token operator">=</span> codec_drv<span class="token operator">-></span>ignore_pmdown_time<span class="token punctuation">;</span> codec<span class="token operator">-></span>dapm<span class="token punctuation">.</span>bias_level <span class="token operator">=</span> SND_SOC_BIAS_OFF<span class="token punctuation">;</span> codec<span class="token operator">-></span>dapm<span class="token punctuation">.</span>dev <span class="token operator">=</span> dev<span class="token punctuation">;</span> codec<span class="token operator">-></span>dapm<span class="token punctuation">.</span>codec <span class="token operator">=</span> codec<span class="token punctuation">;</span> codec<span class="token operator">-></span>dapm<span class="token punctuation">.</span>seq_notifier <span class="token operator">=</span> codec_drv<span class="token operator">-></span>seq_notifier<span class="token punctuation">;</span> codec<span class="token operator">-></span>dapm<span class="token punctuation">.</span>stream_event <span class="token operator">=</span> codec_drv<span class="token operator">-></span>stream_event<span class="token punctuation">;</span> codec<span class="token operator">-></span>dev <span class="token operator">=</span> dev<span class="token punctuation">;</span> codec<span class="token operator">-></span>driver <span class="token operator">=</span> codec_drv<span class="token punctuation">;</span> codec<span class="token operator">-></span>num_dai <span class="token operator">=</span> num_dai<span class="token punctuation">;</span> <span class="token function">mutex_init</span><span class="token punctuation">(</span><span class="token operator">&</span>codec<span class="token operator">-></span>mutex<span class="token punctuation">)</span><span class="token punctuation">;</span>
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- 把以上 codec 範例插入到
codec_list鏈表中（音效卡註冊時會遍歷該鏈表，找到 dai_link 宣告的 codec 並系結）：
list_add(&codec->list, &codec_list);
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- 把 codec_drv 中的
snd_soc_dai_driver（wm8994 有 3 個 dai，分別是 aif1、aif2、aif3）註冊到 soc-core：
/* register any DAIs */
if (num_dai) {
ret = snd_soc_register_dais(dev, dai_drv, num_dai);
if (ret < 0)
goto fail;
}
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snd_soc_register_dais() 會把 dai 插入到 dai_list 鏈表中（音效卡註冊時會遍歷該鏈表，找到 dai_link 宣告的 codec_dai 並系結）：
list_add(&dai->list, &dai_list);
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最後順便提下 codec 和 codec_dai 的區別：codec 指音訊晶片共有的部分，包括 codec 初始化函數、控制介面、暫存器快取、控制元件、dapm 部件、音訊路由、偏置電壓設定函數等描述資訊；而 codec_dai 指 codec 上的音訊介面驅動描述，包括時鐘設定、格式設定、能力描述等等，各個介面的描述資訊不一定都是一致的，所以每個音訊介面都有着各自的驅動描述。
我開始時認爲：codec_dai 從屬於 codec，dai_link 沒有必要同時宣告 codec 和 codec_dai，應該可以實現codec_dai 就能找到它對應的父裝置 codec 的方法。後來想到系統上如果有兩個以上的 codec，而恰好不同 codec 上的 codec_dai 有重名的話，此時就必須同時宣告 codec 和 codec_dai 才能 纔能找到正確的音訊介面了。
4. Platform
概述中提到音訊 Platform 驅動主要用於音訊數據傳輸，這裏又細分爲兩步：
- 啓動 dma 裝置，把音訊數據從 dma buffer 搬運到 cpu_dai FIFO，這部分驅動用
snd_soc_platform_driver描述，後面分析用 pcm_dma 指代它。 - 啓動數位音訊介面控制器（I2S/PCM/AC97），把音訊數據從 cpu_dai FIFO 傳送到 codec_dai，這部分驅動用
snd_soc_dai_driver描述，後面分析用 cpu_dai 指代它。
那麼 dma buffer 中的音訊數據從何而來？保留這個問題，在後面章節 pcm native 分析。
我們瀏覽下 platform_drv 中的幾個重要結構體，其中淺藍色部分是 cpu_dai 相關的，淺綠色部分是 pcm_dma 相關的。snd_soc_dai 是 cpu_dai 註冊時所建立的 dai 範例，snd_soc_platform 是 pcm_dma 註冊時所建立的 platform 範例，這些範例方便 soc-core 管理。
4.1. cpu dai
一個典型的 I2S 匯流排控制器框圖：
各模組描述如下，摘自 S3C44B0 的數據手冊：
Bus interface, register bank, and state machine(BRFC) - Bus interface logic and FIFO access are controlled by the state machine.
3-bit dual prescaler(IPSR) - One prescaler is used as the master clock generator of the IIS bus interface and the other is used as the external CODEC clock generator.
16-byte FIFOs(TXFIFO, RXFIFO) - In transmit data transfer, data are written to TXFIFO, and, in the receive data transfer, data are read from RXFIFO.
Master IISCLK generaor(SCLKG) - In master mode, serial bit clock is generated from the master clock.
Channel generator and state machine(CHNC) - IISCLK and IISLRCK are generated and controlled by the channel state machine.
16-bit shift register(SFTR) - Parallel data is shifted to serial data output in the transmit mode, and serial data input is shifted to parallel data in the receive mode.
再回顧下 I2S 匯流排協定，這是音訊驅動開發最基本的內容了：
- BCLK：位時鐘，對應數位音訊的每一位數據；BCLK = 聲道數 * 採樣頻率 * 採樣位數；
- LRCLK：幀時鐘，構成一個完整的聲音單元；雙聲道的情況下，LRCLK = 0 時表示是左聲道的數據，= 1 時表示是右聲道的數據；LRCLK = 採樣頻率；
- DACDAT：下行數據；
- ADCDAT：上行數據；
- 數據的最高位總是出現在 LRCLK 跳變後的第 2 個 BCLK 脈衝處。
對於 cpu_dai 驅動，從上面的類圖我們可知，主要工作有：
- 實現 dai 操作函數，見
snd_soc_dai_ops定義，用於設定和操作音訊數位介面控制器，如時鐘設定set_sysclk()、格式設定set_fmt()、硬體參數設定hw_params()、啓動/停止數據傳輸trigger()等； - 實現
probe函數（初始化）、remove函數（解除安裝）、suspend/resume函數（電源管理）； - 初始化
snd_soc_dai_driver範例，包括回放和錄製的能力描述、dai 操作函數集、probe/remove回撥、電源管理相關的suspend/resume回撥； - 通過
snd_soc_register_dai()把初始化完成的snd_soc_dai_driver註冊到 soc-core：首先建立一個snd_soc_dai範例，然後把該snd_soc_dai範例插入到dai_list鏈表（音效卡註冊時會遍歷該鏈表，找到 dai_link 宣告的 cpu_dai 並系結）。
/** * snd_soc_register_dai - Register a DAI with the ASoC core * * @dai: DAI to register */ int snd_soc_register_dai(struct device *dev, struct snd_soc_dai_driver *dai_drv) { struct snd_soc_dai *dai;dai <span class="token operator">=</span> <span class="token function">kzalloc</span><span class="token punctuation">(</span><span class="token keyword">sizeof</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_dai<span class="token punctuation">)</span><span class="token punctuation">,</span> GFP_KERNEL<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>dai <span class="token operator">==</span> <span class="token constant">NULL</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span> <span class="token comment">/* create DAI component name */</span> dai<span class="token operator">-></span>name <span class="token operator">=</span> <span class="token function">fmt_single_name</span><span class="token punctuation">(</span>dev<span class="token punctuation">,</span> <span class="token operator">&</span>dai<span class="token operator">-></span>id<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>dai<span class="token operator">-></span>name <span class="token operator">==</span> <span class="token constant">NULL</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">kfree</span><span class="token punctuation">(</span>dai<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span> <span class="token punctuation">}</span> dai<span class="token operator">-></span>dev <span class="token operator">=</span> dev<span class="token punctuation">;</span> dai<span class="token operator">-></span>driver <span class="token operator">=</span> dai_drv<span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>dai<span class="token operator">-></span>driver<span class="token operator">-></span>ops<span class="token punctuation">)</span> dai<span class="token operator">-></span>driver<span class="token operator">-></span>ops <span class="token operator">=</span> <span class="token operator">&</span>null_dai_ops<span class="token punctuation">;</span> <span class="token function">mutex_lock</span><span class="token punctuation">(</span><span class="token operator">&</span>client_mutex<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token function">list_add</span><span class="token punctuation">(</span><span class="token operator">&</span>dai<span class="token operator">-></span>list<span class="token punctuation">,</span> <span class="token operator">&</span>dai_list<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token function">mutex_unlock</span><span class="token punctuation">(</span><span class="token operator">&</span>client_mutex<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
}
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dai 操作函數的實現是 cpu_dai 驅動的主體，需要設定好相關暫存器讓 I2S/PCM 匯流排控制器正常運轉，snd_soc_dai_ops 欄位的詳細說明見 3.4. Codec audio operations 章節。
cpu_dai 驅動應該算是這個系列中最簡單的一環，因此不多花費筆墨在這裏了。倒是某些平臺上，dma 裝置資訊（匯流排地址、通道號、傳輸單元大小）是在這裏初始化的，這點要留意，這些 dma 裝置資訊在 pcm_dma 驅動中用到。以 Exynos 平臺爲例，程式碼位置 sound/soc/samsung/i2s.c。
Samsung Exynos 平臺的音訊 dma 裝置資訊用 s3c_dma_params 結構體描述：
struct s3c_dma_params {
struct s3c2410_dma_client *client; /* stream identifier */
int channel; /* Channel ID */
dma_addr_t dma_addr;
int dma_size; /* Size of the DMA transfer */
unsigned ch;
struct samsung_dma_ops *ops;
};
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- client：流識別符號
- channel：通道號
- dma_addr：裝置的匯流排地址，這裏通常指向 I2S tx FIFO 或 I2S rx FIFO 首地址
- dma_size：dma 傳輸單元大小
- ops：平臺 dma 操作函數
sound/soc/samsung/i2s.c 中設定 dma 裝置資訊的相關程式碼片段：
struct i2s_dai {
// ...
/* Driver for this DAI */
struct snd_soc_dai_driver i2s_dai_drv;
/* DMA parameters */
struct s3c_dma_params dma_playback; // playback dma 描述資訊
struct s3c_dma_params dma_capture; // capture dma 描述資訊
struct s3c_dma_params idma_playback;// playback idma 描述資訊，idma 僅用於回放，用於三星平臺的 LPA（低功耗音訊）模式
// ...
};
static __devinit int samsung_i2s_probe(struct platform_device *pdev)
{
// …
// 從 platform_device 中取得 resource，得到 playback dma 通道號
res = platform_get_resource(pdev, IORESOURCE_DMA, 0);
if (!res) {
dev_err(&pdev->dev, 「Unable to get I2S-TX dma resource\n」);
return -ENXIO;
}
dma_pl_chan = res->start; // dma_pl_chan 中的 pl 是 playback 簡寫
<span class="token comment">// 從 platform_device 中取得 resource，得到 capture dma 通道號</span>
res <span class="token operator">=</span> <span class="token function">platform_get_resource</span><span class="token punctuation">(</span>pdev<span class="token punctuation">,</span> IORESOURCE_DMA<span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>res<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">dev_err</span><span class="token punctuation">(</span><span class="token operator">&</span>pdev<span class="token operator">-></span>dev<span class="token punctuation">,</span> <span class="token string">"Unable to get I2S-RX dma resource\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token operator">-</span>ENXIO<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
dma_cp_chan <span class="token operator">=</span> res<span class="token operator">-></span>start<span class="token punctuation">;</span> <span class="token comment">// dma_cp_chan 中的 cp 是 capture 的簡寫</span>
<span class="token comment">// 從 platform_device 中取得 resource，得到 playback idma 通道號</span>
res <span class="token operator">=</span> <span class="token function">platform_get_resource</span><span class="token punctuation">(</span>pdev<span class="token punctuation">,</span> IORESOURCE_DMA<span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>res<span class="token punctuation">)</span>
dma_pl_sec_chan <span class="token operator">=</span> res<span class="token operator">-></span>start<span class="token punctuation">;</span>
<span class="token keyword">else</span>
dma_pl_sec_chan <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token comment">// 從 platform_device 中取得 resource，得到 I2S 的基地址</span>
res <span class="token operator">=</span> <span class="token function">platform_get_resource</span><span class="token punctuation">(</span>pdev<span class="token punctuation">,</span> IORESOURCE_MEM<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>res<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">dev_err</span><span class="token punctuation">(</span><span class="token operator">&</span>pdev<span class="token operator">-></span>dev<span class="token punctuation">,</span> <span class="token string">"Unable to get I2S SFR address\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token operator">-</span>ENXIO<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token function">request_mem_region</span><span class="token punctuation">(</span>res<span class="token operator">-></span>start<span class="token punctuation">,</span> <span class="token function">resource_size</span><span class="token punctuation">(</span>res<span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token string">"samsung-i2s"</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">dev_err</span><span class="token punctuation">(</span><span class="token operator">&</span>pdev<span class="token operator">-></span>dev<span class="token punctuation">,</span> <span class="token string">"Unable to request SFR region\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token operator">-</span>EBUSY<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
regs_base <span class="token operator">=</span> res<span class="token operator">-></span>start<span class="token punctuation">;</span>
<span class="token comment">// ...</span>
pri_dai<span class="token operator">-></span>dma_playback<span class="token punctuation">.</span>dma_addr <span class="token operator">=</span> regs_base <span class="token operator">+</span> I2STXD<span class="token punctuation">;</span> <span class="token comment">// 設定 playback dma 裝置地址爲 I2S tx FIFO 地址</span>
pri_dai<span class="token operator">-></span>dma_capture<span class="token punctuation">.</span>dma_addr <span class="token operator">=</span> regs_base <span class="token operator">+</span> I2SRXD<span class="token punctuation">;</span> <span class="token comment">// 設定 capture dma 裝置地址爲 I2S rx FIFO 地址</span>
pri_dai<span class="token operator">-></span>dma_playback<span class="token punctuation">.</span>client <span class="token operator">=</span>
<span class="token punctuation">(</span><span class="token keyword">struct</span> s3c2410_dma_client <span class="token operator">*</span><span class="token punctuation">)</span><span class="token operator">&</span>pri_dai<span class="token operator">-></span>dma_playback<span class="token punctuation">;</span>
pri_dai<span class="token operator">-></span>dma_capture<span class="token punctuation">.</span>client <span class="token operator">=</span>
<span class="token punctuation">(</span><span class="token keyword">struct</span> s3c2410_dma_client <span class="token operator">*</span><span class="token punctuation">)</span><span class="token operator">&</span>pri_dai<span class="token operator">-></span>dma_capture<span class="token punctuation">;</span>
pri_dai<span class="token operator">-></span>dma_playback<span class="token punctuation">.</span>channel <span class="token operator">=</span> dma_pl_chan<span class="token punctuation">;</span> <span class="token comment">// 設定 playback dma 通道號</span>
pri_dai<span class="token operator">-></span>dma_capture<span class="token punctuation">.</span>channel <span class="token operator">=</span> dma_cp_chan<span class="token punctuation">;</span> <span class="token comment">// 設定 capture dma 通道號</span>
pri_dai<span class="token operator">-></span>src_clk <span class="token operator">=</span> i2s_cfg<span class="token operator">-></span>src_clk<span class="token punctuation">;</span>
pri_dai<span class="token operator">-></span>dma_playback<span class="token punctuation">.</span>dma_size <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span> <span class="token comment">// 設定 playback dma 傳輸單元大小爲 4 個位元組</span>
pri_dai<span class="token operator">-></span>dma_capture<span class="token punctuation">.</span>dma_size <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span> <span class="token comment">// 設定 capture dma 傳輸單元大小爲 4 個位元組</span>
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我們再看看 Board 初始化時，如何設定這些 resource，檔案 arch/arm/mach-exynos/dev-audio.c：
static struct resource exynos4_i2s0_resource[] = {
[0] = {
.start = EXYNOS4_PA_I2S0, // start 欄位儲存的是 I2S 基地址
.end = EXYNOS4_PA_I2S0 + 0x100 - 1,
.flags = IORESOURCE_MEM, // 標識爲 MEM 資源
},
[1] = {
.start = DMACH_I2S0_TX, // start 欄位儲存的是用於回放的 dma 通道號
.end = DMACH_I2S0_TX,
.flags = IORESOURCE_DMA, // 標識爲 DMA 資源
},
[2] = {
.start = DMACH_I2S0_RX, // start 欄位儲存的是用於錄製的 dma 通道號
.end = DMACH_I2S0_RX,
.flags = IORESOURCE_DMA, // 標識爲 DMA 資源
},
[3] = {
.start = DMACH_I2S0S_TX, // start 欄位儲存的是用於回放的 idma 通道號
.end = DMACH_I2S0S_TX,
.flags = IORESOURCE_DMA, // 標識爲 DMA 資源
},
};
struct platform_device exynos4_device_i2s0 = {
.name = 「samsung-i2s」, // platform_device 名稱標識爲 「samsung-i2s」，與 i2s.c 中的samsung_i2s_driver 匹配
.id = 0,
.num_resources = ARRAY_SIZE(exynos4_i2s0_resource),
.resource = exynos4_i2s0_resource,
.dev = {
.platform_data = &i2sv5_pdata,
},
};
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當 samsung_i2s_driver 初始化時，通過 platform_get_resource() 函數來獲取 platform_device 宣告的 resource。
struct resource 結構中我們通常關心 start、end 和 flags 這 3 個欄位，分別標明資源的開始值、結束值和型別。flags 可以爲 IORESOURCE_IO、IORESOURCE_MEM、IORESOURCE_IRQ、IORESOURCE_DMA 等。start、end 的含義會隨着 flags 而變更，如當 flags 爲 IORESOURCE_MEM 時，start、end 分別表示該 platform_device 佔據的記憶體的開始地址和結束地址；當 flags 爲 IORESOURCE_IRQ 時，start、end 分別表示該 platform_device 使用的中斷號的開始值和結束值，如果只使用了 1 箇中斷號，開始和結束值相同。對於同種型別的資源而言，可以有多份，譬如說某裝置佔據了 2 個記憶體區域，則可以定義 2 個 IORESOURCE_MEM 資源。
摘自：http://21cnbao.blog.51cto.com/109393/337609
4.2. pcm dma
PCM 數據管理可以說是 ALSA 系統中最核心的部分，這部分的工作有兩個（回放情形）：
copy_from_user把使用者態的音訊數據拷貝到 dma buffer 中；- 啓動 dma 裝置把音訊數據從 dma buffer 傳送到 I2S tx FIFO。
當數據送到 I2S tx FIFO 後，剩下的是啓動 I2S 控制器把數據傳送到 Codec，然後 DAC 把音訊數位信號轉換成模擬信號，再輸出到 SPK/HP。關於 I2S (cpu_dai) 和 Codec，在以上兩章已經描述清楚了。
爲什麼要使用 dma 傳輸？兩個原因：首先在數據傳輸過程中，不需要 cpu 的參與，節省 cpu 的開銷；其次傳輸速度快，提高硬體裝置的吞吐量。對於 ARM，它不能直接把數據從 A 地址搬運到 B 地址，只能把數據從 A 地址搬運到一個暫存器，然後再從這個暫存器搬運到 B 地址；而 dma 有突發（Burst）傳輸能力，這種模式下一次能傳輸幾個甚至十幾個位元組的數據，尤其適合大數據的高速傳輸。一個 dma 傳輸塊裏面，可以劃分爲若幹個週期，每傳輸完一個週期產生一箇中斷。
寫這個文件的初衷是爲了描述清楚 pcm 數據流向，這裏先剖析 pcm_dma 驅動，以便後面 pcm native 的分析。以 Exynos 平臺爲例，程式碼位置 sound/soc/samsung/dma.c。
- 淺綠色：pcm_dma 驅動共有的結構及介面定義
- 淺灰色：samsung exynos 平臺特有的實現
- 淺紫色：pcm native 關鍵結構
- 淺橙色：
snd_soc_platform是pcm_dma 註冊時所建立的 platform 範例
snd_pcm_substream 是 pcm native 關鍵結構體，上圖可以看出這個結構體包含了音訊數據傳輸所需的重要資訊：pcm ops 操作函數集和 dma buffer。
我們先看看 dma 裝置相關的結構，對於回放來說，dma 裝置把記憶體緩衝區的音訊數據傳送到 I2S tx FIFO；對於錄製來說，dma 裝置把 I2S rx FIFO 的音訊數據傳送到記憶體快取區。因此在 dma 裝置傳輸之前，必須確定 data buffer 和 I2S FIFO 的資訊。
snd_dma_buffer：數據快取區，用於儲存從使用者態拷貝過來的音訊數據；包含 dma buffer 的物理首地址，虛擬首地址、大小等資訊；其中實體地址用於設定 dma 傳輸的源地址（回放情形）或目的地址（錄製情形），虛擬地址用於與使用者態之間的音訊數據拷貝。
s3c_dma_params：dma 裝置描述，包括裝置匯流排地址（回放情形下爲 I2S tx FIFO 首地址，設定爲 dma 傳輸的目的地址）、dma 通道號、dma 傳輸單元大小，這些資訊在 i2s.c 中初始化（具體見上一小節）。
runtime_data：dma 執行期資訊
- state：記錄 dma 裝置狀態，啓動或停止；
- dma_loaded：dma 裝載計數，每當啓動一次 dma 傳輸，該計數加一；每當完成一次 dma 傳輸，該計數減一；
- dma_period：dma 週期數據大小；
- dma_start：指向 dma buffer 物理首地址；
- dma_pos：記錄 dma buffer 當前指針位置，當 dma 每傳輸一次，都會更新該指針；
- dma_end：dma buffer 結束位置；
- params：dma 裝置描述資訊，包括裝置匯流排地址、dma 通道號、傳輸單元大小。
4.2.1. pcm operations
操作函數的實現是本模組的主體，見 snd_pcm_ops 結構體描述：
struct snd_pcm_ops {
int (*open)(struct snd_pcm_substream *substream);
int (*close)(struct snd_pcm_substream *substream);
int (*ioctl)(struct snd_pcm_substream * substream,
unsigned int cmd, void *arg);
int (*hw_params)(struct snd_pcm_substream *substream,
struct snd_pcm_hw_params *params);
int (*hw_free)(struct snd_pcm_substream *substream);
int (*prepare)(struct snd_pcm_substream *substream);
int (*trigger)(struct snd_pcm_substream *substream, int cmd);
snd_pcm_uframes_t (*pointer)(struct snd_pcm_substream *substream);
int (*copy)(struct snd_pcm_substream *substream, int channel,
snd_pcm_uframes_t pos,
void __user *buf, snd_pcm_uframes_t count);
int (*silence)(struct snd_pcm_substream *substream, int channel,
snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
struct page *(*page)(struct snd_pcm_substream *substream,
unsigned long offset);
int (*mmap)(struct snd_pcm_substream *substream, struct vm_area_struct *vma);
int (*ack)(struct snd_pcm_substream *substream);
};
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下面 下麪介紹幾個重要的介面：
- open：開啓 pcm 邏輯裝置時，回撥該函數設定 dma 裝置的硬體約束；並申請一個私有結構，儲存 dma 裝置資源如通道號、傳輸單元、緩衝區資訊、IO 資訊等，儲存在
runtime->private_data。程式碼如下：
static const struct snd_pcm_hardware dma_hardware = {
.info = SNDRV_PCM_INFO_INTERLEAVED |
SNDRV_PCM_INFO_BLOCK_TRANSFER |
SNDRV_PCM_INFO_MMAP |
SNDRV_PCM_INFO_MMAP_VALID |
SNDRV_PCM_INFO_PAUSE |
SNDRV_PCM_INFO_RESUME,
.formats = SNDRV_PCM_FMTBIT_S16_LE |
SNDRV_PCM_FMTBIT_U16_LE |
SNDRV_PCM_FMTBIT_U8 |
SNDRV_PCM_FMTBIT_S8,
.channels_min = 2,
.channels_max = 2,
.buffer_bytes_max = 128*1024,
.period_bytes_min = PAGE_SIZE,
.period_bytes_max = PAGE_SIZE*2,
.periods_min = 2,
.periods_max = 128,
.fifo_size = 32,
};
static int dma_open(struct snd_pcm_substream substream)
{
struct snd_pcm_runtime runtime = substream->runtime;
struct runtime_data *prtd;
<span class="token function">pr_debug</span><span class="token punctuation">(</span><span class="token string">"Entered %s\n"</span><span class="token punctuation">,</span> <span class="token constant">__func__</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 設定 dma 裝置的硬體約束</span>
<span class="token function">snd_pcm_hw_constraint_integer</span><span class="token punctuation">(</span>runtime<span class="token punctuation">,</span> SNDRV_PCM_HW_PARAM_PERIODS<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">snd_soc_set_runtime_hwparams</span><span class="token punctuation">(</span>substream<span class="token punctuation">,</span> <span class="token operator">&</span>dma_hardware<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 爲 runtime_data 分配記憶體，用於儲存 dma 資源，包括緩衝區資訊、IO 裝置資訊、通道號、傳輸單元大小 </span>
prtd <span class="token operator">=</span> <span class="token function">kzalloc</span><span class="token punctuation">(</span><span class="token keyword">sizeof</span><span class="token punctuation">(</span><span class="token keyword">struct</span> runtime_data<span class="token punctuation">)</span><span class="token punctuation">,</span> GFP_KERNEL<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>prtd <span class="token operator">==</span> <span class="token constant">NULL</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span>
<span class="token function">spin_lock_init</span><span class="token punctuation">(</span><span class="token operator">&</span>prtd<span class="token operator">-></span>lock<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// runtime 的私有數據指向 runtime_data </span>
runtime<span class="token operator">-></span>private_data <span class="token operator">=</span> prtd<span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
}
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- hw_params：設定硬體參數時（
cmd=SNDRV_PCM_IOCTL_HW_PARAMS），回撥該函數初始化 dma 資源，包括通道號、傳輸單元、緩衝區資訊、IO 裝置資訊等。程式碼如下：
static int dma_hw_params(struct snd_pcm_substream *substream, struct snd_pcm_hw_params *params) { struct snd_pcm_runtime *runtime = substream->runtime; struct runtime_data *prtd = runtime->private_data; struct snd_soc_pcm_runtime *rtd = substream->private_data; unsigned long totbytes = params_buffer_bytes(params); struct s3c_dma_params *dma = snd_soc_dai_get_dma_data(rtd->cpu_dai, substream); // 從 cpu_dai 驅動 i2s.c 取得 dma 裝置資源 struct samsung_dma_info dma_info;<span class="token comment">/* return if this is a bufferless transfer e.g. * codec <--> BT codec or GSM modem -- lg FIXME */</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>dma<span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token comment">/* this may get called several times by oss emulation * with different params -HW */</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>prtd<span class="token operator">-></span>params <span class="token operator">==</span> <span class="token constant">NULL</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">/* prepare DMA */</span> prtd<span class="token operator">-></span>params <span class="token operator">=</span> dma<span class="token punctuation">;</span> <span class="token comment">// 該欄位儲存的是 dma 裝置資源，如 I2S tx FIFO 地址、dma 通道號、dma 傳輸單元等</span> prtd<span class="token operator">-></span>params<span class="token operator">-></span>ops <span class="token operator">=</span> <span class="token function">samsung_dma_get_ops</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 平臺的 dma 操作函數，這些操作函數實現見：arch/arm/plat-samsung/dma-ops.c</span> <span class="token comment">//...</span> prtd<span class="token operator">-></span>params<span class="token operator">-></span>ch <span class="token operator">=</span> prtd<span class="token operator">-></span>params<span class="token operator">-></span>ops<span class="token operator">-></span><span class="token function">request</span><span class="token punctuation">(</span> prtd<span class="token operator">-></span>params<span class="token operator">-></span>channel<span class="token punctuation">,</span> <span class="token operator">&</span>dma_info<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token function">snd_pcm_set_runtime_buffer</span><span class="token punctuation">(</span>substream<span class="token punctuation">,</span> <span class="token operator">&</span>substream<span class="token operator">-></span>dma_buffer<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 這裏把 dma buffer 相關資訊賦給 substream runtime，注意 dma_buffer 在建立 pcm 邏輯裝置時分配</span> runtime<span class="token operator">-></span>dma_bytes <span class="token operator">=</span> totbytes<span class="token punctuation">;</span> <span class="token function">spin_lock_irq</span><span class="token punctuation">(</span><span class="token operator">&</span>prtd<span class="token operator">-></span>lock<span class="token punctuation">)</span><span class="token punctuation">;</span> prtd<span class="token operator">-></span>dma_loaded <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> prtd<span class="token operator">-></span>dma_period <span class="token operator">=</span> <span class="token function">params_period_bytes</span><span class="token punctuation">(</span>params<span class="token punctuation">)</span><span class="token punctuation">;</span> prtd<span class="token operator">-></span>dma_start <span class="token operator">=</span> runtime<span class="token operator">-></span>dma_addr<span class="token punctuation">;</span> <span class="token comment">// dma buffer 物理首地址</span> prtd<span class="token operator">-></span>dma_pos <span class="token operator">=</span> prtd<span class="token operator">-></span>dma_start<span class="token punctuation">;</span> prtd<span class="token operator">-></span>dma_end <span class="token operator">=</span> prtd<span class="token operator">-></span>dma_start <span class="token operator">+</span> totbytes<span class="token punctuation">;</span> <span class="token function">spin_unlock_irq</span><span class="token punctuation">(</span><span class="token operator">&</span>prtd<span class="token operator">-></span>lock<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
}
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- prepare：當數據已準備好（
cmd=SNDRV_PCM_IOCTL_PREPARE），回撥該函數告知 dma 裝置數據已就緒。程式碼如下：
static int dma_prepare(struct snd_pcm_substream *substream) { struct runtime_data *prtd = substream->runtime->private_data; int ret = 0;<span class="token function">pr_debug</span><span class="token punctuation">(</span><span class="token string">"Entered %s\n"</span><span class="token punctuation">,</span> <span class="token constant">__func__</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">/* return if this is a bufferless transfer e.g. * codec <--> BT codec or GSM modem -- lg FIXME */</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>prtd<span class="token operator">-></span>params<span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token comment">/* flush the DMA channel */</span> prtd<span class="token operator">-></span>params<span class="token operator">-></span>ops<span class="token operator">-></span><span class="token function">flush</span><span class="token punctuation">(</span>prtd<span class="token operator">-></span>params<span class="token operator">-></span>ch<span class="token punctuation">)</span><span class="token punctuation">;</span> prtd<span class="token operator">-></span>dma_loaded <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token comment">// 初始化 dma 裝載計數</span> prtd<span class="token operator">-></span>dma_pos <span class="token operator">=</span> prtd<span class="token operator">-></span>dma_start<span class="token punctuation">;</span> <span class="token comment">// 設定 dma buffer 當前指針爲 dma buffer 首地址</span> <span class="token comment">/* enqueue dma buffers */</span> <span class="token function">dma_enqueue</span><span class="token punctuation">(</span>substream<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 插入到 dma 傳輸佇列中</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span>
}
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dma_enqueue() 函數，把當前 dma buffer 插入到 dma 傳輸佇列中。當觸發 trigger() 啓動 dma 裝置傳輸後，將會把 dma buffer 數據傳送到 FIFO（回放情形）。
注意：每次 dma 傳輸完一個週期的數據後，都要呼叫 snd_pcm_period_elapsed() 告知 pcm native 一個週期的數據已經傳送到 FIFO 上了，然後再次呼叫 dma_enqueue()，dma 傳輸…如此回圈，直到觸發 trigger() 停止 dma 傳輸。
- trigger：數據傳送 開始/停止/暫停/恢復 時，回撥該函數啓動或停止 dma 傳輸（當上層第一次呼叫
pcm_write()時，觸發trigger()啓動 dma 傳輸；當上層呼叫pcm_stop()或pcm_drop()時，觸發trigger()停止 dma 傳輸）。trigger()函數裏面的操作必須是原子的，不能呼叫可能睡眠的操作，並且應儘量簡單。程式碼如下：
static int dma_trigger(struct snd_pcm_substream *substream, int cmd) { struct runtime_data *prtd = substream->runtime->private_data; int ret = 0;<span class="token function">pr_debug</span><span class="token punctuation">(</span><span class="token string">"Entered %s\n"</span><span class="token punctuation">,</span> <span class="token constant">__func__</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token function">spin_lock</span><span class="token punctuation">(</span><span class="token operator">&</span>prtd<span class="token operator">-></span>lock<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">switch</span> <span class="token punctuation">(</span>cmd<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">case</span> SNDRV_PCM_TRIGGER_START<span class="token punctuation">:</span> <span class="token keyword">case</span> SNDRV_PCM_TRIGGER_RESUME<span class="token punctuation">:</span> <span class="token keyword">case</span> SNDRV_PCM_TRIGGER_PAUSE_RELEASE<span class="token punctuation">:</span> prtd<span class="token operator">-></span>state <span class="token operator">|</span><span class="token operator">=</span> ST_RUNNING<span class="token punctuation">;</span> prtd<span class="token operator">-></span>params<span class="token operator">-></span>ops<span class="token operator">-></span><span class="token function">trigger</span><span class="token punctuation">(</span>prtd<span class="token operator">-></span>params<span class="token operator">-></span>ch<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 啓動 dma 傳輸</span> <span class="token keyword">break</span><span class="token punctuation">;</span> <span class="token keyword">case</span> SNDRV_PCM_TRIGGER_STOP<span class="token punctuation">:</span> <span class="token keyword">case</span> SNDRV_PCM_TRIGGER_SUSPEND<span class="token punctuation">:</span> <span class="token keyword">case</span> SNDRV_PCM_TRIGGER_PAUSE_PUSH<span class="token punctuation">:</span> prtd<span class="token operator">-></span>state <span class="token operator">&</span><span class="token operator">=</span> <span class="token operator">~</span>ST_RUNNING<span class="token punctuation">;</span> prtd<span class="token operator">-></span>params<span class="token operator">-></span>ops<span class="token operator">-></span><span class="token function">stop</span><span class="token punctuation">(</span>prtd<span class="token operator">-></span>params<span class="token operator">-></span>ch<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 停止 dma 傳輸</span> <span class="token keyword">break</span><span class="token punctuation">;</span> <span class="token keyword">default</span><span class="token punctuation">:</span> ret <span class="token operator">=</span> <span class="token operator">-</span>EINVAL<span class="token punctuation">;</span> <span class="token keyword">break</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token function">spin_unlock</span><span class="token punctuation">(</span><span class="token operator">&</span>prtd<span class="token operator">-></span>lock<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span>
}
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- pointer：dma 每完成一次傳輸，都會呼叫該函數獲得傳輸數據的當前位置，這樣 pcm native 可計算 dma buffer 指針位置及可用空間。該函數也是原子的。程式碼如下：
static snd_pcm_uframes_t dma_pointer(struct snd_pcm_substream *substream) { struct snd_pcm_runtime *runtime = substream->runtime; struct runtime_data *prtd = runtime->private_data; unsigned long res;res <span class="token operator">=</span> prtd<span class="token operator">-></span>dma_pos <span class="token operator">-</span> prtd<span class="token operator">-></span>dma_start<span class="token punctuation">;</span> <span class="token comment">// 當前位置減去首地址，其實就是已傳輸數據的大小</span> <span class="token comment">/* we seem to be getting the odd error from the pcm library due * to out-of-bounds pointers. this is maybe due to the dma engine * not having loaded the new values for the channel before being * called... (todo - fix ) */</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>res <span class="token operator">>=</span> <span class="token function">snd_pcm_lib_buffer_bytes</span><span class="token punctuation">(</span>substream<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>res <span class="token operator">==</span> <span class="token function">snd_pcm_lib_buffer_bytes</span><span class="token punctuation">(</span>substream<span class="token punctuation">)</span><span class="token punctuation">)</span> res <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">return</span> <span class="token function">bytes_to_frames</span><span class="token punctuation">(</span>substream<span class="token operator">-></span>runtime<span class="token punctuation">,</span> res<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 單位轉化爲 frames</span>
}
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4.2.2. dma buffer allocation
在 4.2.1. pcm operations 小節，數次提及 dma buffer，即 dma 數據緩衝區。dma buffer 的分配，一般發生在 pcm_dma 驅動初始化階段 probe() 或 pcm 邏輯裝置建立階段 pcm_new()。程式碼如下：
static int preallocate_dma_buffer(struct snd_pcm *pcm, int stream) { struct snd_pcm_substream *substream = pcm->streams[stream].substream; struct snd_dma_buffer *buf = &substream->dma_buffer; size_t size = dma_hardware.buffer_bytes_max; // 緩衝區大小不得超過 hardware 中的buffer_bytes_maxbuf<span class="token operator">-></span>dev<span class="token punctuation">.</span>type <span class="token operator">=</span> SNDRV_DMA_TYPE_DEV<span class="token punctuation">;</span> buf<span class="token operator">-></span>dev<span class="token punctuation">.</span>dev <span class="token operator">=</span> pcm<span class="token operator">-></span>card<span class="token operator">-></span>dev<span class="token punctuation">;</span> buf<span class="token operator">-></span>private_data <span class="token operator">=</span> <span class="token constant">NULL</span><span class="token punctuation">;</span> buf<span class="token operator">-></span>area <span class="token operator">=</span> <span class="token function">dma_alloc_writecombine</span><span class="token punctuation">(</span>pcm<span class="token operator">-></span>card<span class="token operator">-></span>dev<span class="token punctuation">,</span> size<span class="token punctuation">,</span> <span class="token operator">&</span>buf<span class="token operator">-></span>addr<span class="token punctuation">,</span> GFP_KERNEL<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// area 欄位是 dma buffer 虛擬首地址，addr 欄位是 dma buffer 物理首地址</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>buf<span class="token operator">-></span>area<span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span> buf<span class="token operator">-></span>bytes <span class="token operator">=</span> size<span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
}
static int dma_new(struct snd_soc_pcm_runtime rtd)
{
struct snd_card card = rtd->card->snd_card;
struct snd_pcm *pcm = rtd->pcm;
int ret = 0;
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>card<span class="token operator">-></span>dev<span class="token operator">-></span>dma_mask<span class="token punctuation">)</span>
card<span class="token operator">-></span>dev<span class="token operator">-></span>dma_mask <span class="token operator">=</span> <span class="token operator">&</span>dma_mask<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>card<span class="token operator">-></span>dev<span class="token operator">-></span>coherent_dma_mask<span class="token punctuation">)</span>
card<span class="token operator">-></span>dev<span class="token operator">-></span>coherent_dma_mask <span class="token operator">=</span> <span class="token function">DMA_BIT_MASK</span><span class="token punctuation">(</span><span class="token number">32</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>pcm<span class="token operator">-></span>streams<span class="token punctuation">[</span>SNDRV_PCM_STREAM_PLAYBACK<span class="token punctuation">]</span><span class="token punctuation">.</span>substream<span class="token punctuation">)</span> <span class="token punctuation">{</span>
ret <span class="token operator">=</span> <span class="token function">preallocate_dma_buffer</span><span class="token punctuation">(</span>pcm<span class="token punctuation">,</span> <span class="token comment">// 爲回放子流分配的 dma buffer</span>
SNDRV_PCM_STREAM_PLAYBACK<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>ret<span class="token punctuation">)</span>
<span class="token keyword">goto</span> out<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>pcm<span class="token operator">-></span>streams<span class="token punctuation">[</span>SNDRV_PCM_STREAM_CAPTURE<span class="token punctuation">]</span><span class="token punctuation">.</span>substream<span class="token punctuation">)</span> <span class="token punctuation">{</span>
ret <span class="token operator">=</span> <span class="token function">preallocate_dma_buffer</span><span class="token punctuation">(</span>pcm<span class="token punctuation">,</span> <span class="token comment">// 爲錄製子流分配的 dma buffer</span>
SNDRV_PCM_STREAM_CAPTURE<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>ret<span class="token punctuation">)</span>
<span class="token keyword">goto</span> out<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
out:
return ret;
}
static struct snd_soc_platform_driver samsung_asoc_platform = {
.ops = &dma_ops,
.pcm_new = dma_new,
.pcm_free = dma_free_dma_buffers,
};
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當 soc-core 呼叫 soc_new_pcm() 建立 pcm 邏輯裝置時，會回撥 pcm_new() 完成 dma buffer 記憶體分配，注意回放子流和錄製子流有着各自的 dma buffer。
4.2.3. pcm dma register
上兩個小節，我們介紹了 pcm_dma 介面函數的作用及實現和 dma buffer 的分配，本小節分析 pcm_dma 註冊過程。
當 platform_driver：
static struct platform_driver asoc_dma_driver = {
.driver = {
.name = "samsung-audio",
.owner = THIS_MODULE,
},
.probe = samsung_asoc_platform_probe,
.remove = __devexit_p(samsung_asoc_platform_remove),
};
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與 .name = "samsung-audio" 的 platform_device（該 platform_device 在 arch/arm/plat-samsung/devs.c 中註冊）匹配後，系統會回撥 samsung_asoc_platform_probe() 註冊 platform：
static struct snd_soc_platform_driver samsung_asoc_platform = {
.ops = &dma_ops,
.pcm_new = dma_new,
.pcm_free = dma_free_dma_buffers,
};
static int __devinit samsung_asoc_platform_probe(struct platform_device *pdev)
{
return snd_soc_register_platform(&pdev->dev, &samsung_asoc_platform);
}
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snd_soc_register_platform：將 platform_drv 註冊到 soc-core。
- 建立一個
snd_soc_platform範例，包含 platform_drv（snd_soc_platform_driver）的相關資訊，封裝給 soc-core 使用； - 把以上建立的 platform 範例插入到
platform_list鏈表上（音效卡註冊時會遍歷該鏈表，找到 dai_link 宣告的 platform 並系結）。
程式碼實現：
/** * snd_soc_register_platform - Register a platform with the ASoC core * * @platform: platform to register */ int snd_soc_register_platform(struct device *dev, struct snd_soc_platform_driver *platform_drv) { struct snd_soc_platform *platform;platform <span class="token operator">=</span> <span class="token function">kzalloc</span><span class="token punctuation">(</span><span class="token keyword">sizeof</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_platform<span class="token punctuation">)</span><span class="token punctuation">,</span> GFP_KERNEL<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>platform <span class="token operator">==</span> <span class="token constant">NULL</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span> <span class="token comment">/* create platform component name */</span> platform<span class="token operator">-></span>name <span class="token operator">=</span> <span class="token function">fmt_single_name</span><span class="token punctuation">(</span>dev<span class="token punctuation">,</span> <span class="token operator">&</span>platform<span class="token operator">-></span>id<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>platform<span class="token operator">-></span>name <span class="token operator">==</span> <span class="token constant">NULL</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">kfree</span><span class="token punctuation">(</span>platform<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span> <span class="token punctuation">}</span> platform<span class="token operator">-></span>dev <span class="token operator">=</span> dev<span class="token punctuation">;</span> platform<span class="token operator">-></span>driver <span class="token operator">=</span> platform_drv<span class="token punctuation">;</span> platform<span class="token operator">-></span>dapm<span class="token punctuation">.</span>dev <span class="token operator">=</span> dev<span class="token punctuation">;</span> platform<span class="token operator">-></span>dapm<span class="token punctuation">.</span>platform <span class="token operator">=</span> platform<span class="token punctuation">;</span> platform<span class="token operator">-></span>dapm<span class="token punctuation">.</span>stream_event <span class="token operator">=</span> platform_drv<span class="token operator">-></span>stream_event<span class="token punctuation">;</span> <span class="token function">mutex_init</span><span class="token punctuation">(</span><span class="token operator">&</span>platform<span class="token operator">-></span>mutex<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token function">mutex_lock</span><span class="token punctuation">(</span><span class="token operator">&</span>client_mutex<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token function">list_add</span><span class="token punctuation">(</span><span class="token operator">&</span>platform<span class="token operator">-></span>list<span class="token punctuation">,</span> <span class="token operator">&</span>platform_list<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token function">mutex_unlock</span><span class="token punctuation">(</span><span class="token operator">&</span>client_mutex<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
}
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至此，完成了 Platform 驅動的實現。回放情形下，pcm_dma 裝置負責把 dma buffer 中的數據搬運到 I2S tx FIFO，I2S 匯流排控制器負責把 I2S tx FIFO 中的數據傳送到 Codec。至於 alsa 如何把音訊數據從 userspace 拷貝到 dma buffer，如何管理 dma buffer，如何啓動 I2S 和 DMA 傳輸，這裏面一環扣一環，見後續 pcm native 分析。
5. Machine
章節 3. Codec 和 4. Platform 介紹了 Codec、Platform 驅動，但僅有 Codec、Platform 驅動是不能工作的，需要一個角色把 codec、codec_dai、cpu_dai、platform 給鏈結起來才能 纔能構成一個完整的音訊鏈路，這個角色就由 machine_drv 承擔了。如下是一個典型的智慧手機音訊框圖：
+------------+ +---------------------+ +------------+
| | | | | |
| | + CODEC + | |
| AP +------>AIF1 AIF3+------> PA +->SPK
| | + +-----+ +-----+ + | |
| | | | DSP | | DAC | | | |
+------------+ | +-----+ +-----+ | +------------+
| +-----+ +-----+ |
| | DSP | | DAC | |
| +-----+ +-----+ |
+------------+ | +-----+ +-----+ | +------------+
| | | | DSP | | ADC | | | |
| | + +-----+ +-----+ + | |
| BB +------>AIF2 +-----+ +-----+ AIF4+------> BTSCO |
| | + | DSP | | ADC | + | |
| | | +-----+ +-----+ | | |
+------------+ +----------+----------+ +------------+
| | |
+MIC +HP +EARP
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組成了 4 個音訊鏈路（dai_link）：
- AP<>AIF1：AP（應用處理器）與 Codec 之間的鏈路，多媒體聲音
- BB<>AIF2：BB（基頻處理器）與 Codec 之間的鏈路，通話語音
- PA<>AIF3：PA（智慧功率放大器）與 Codec 之間的鏈路，外放輸出
- BTSCO<>AIF4：BTSCO（藍牙）與 Codec 之間的鏈路，藍牙耳機輸出
snd_soc_dai_link 結構體：
struct snd_soc_dai_link { /* config - must be set by machine driver */ const char *name; /* Codec name */ const char *stream_name; /* Stream name */ const char *codec_name; /* for multi-codec */ const struct device_node *codec_of_node; const char *platform_name; /* for multi-platform */ const struct device_node *platform_of_node; const char *cpu_dai_name; const struct device_node *cpu_dai_of_node; const char *codec_dai_name;<span class="token keyword">unsigned</span> <span class="token keyword">int</span> dai_fmt<span class="token punctuation">;</span> <span class="token comment">/* format to set on init */</span> <span class="token comment">/* Keep DAI active over suspend */</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> ignore_suspend<span class="token punctuation">:</span><span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">/* Symmetry requirements */</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> symmetric_rates<span class="token punctuation">:</span><span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">/* pmdown_time is ignored at stop */</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> ignore_pmdown_time<span class="token punctuation">:</span><span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">/* codec/machine specific init - e.g. add machine controls */</span> <span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token operator">*</span>init<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">struct</span> snd_soc_pcm_runtime <span class="token operator">*</span>rtd<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">/* machine stream operations */</span> <span class="token keyword">struct</span> snd_soc_ops <span class="token operator">*</span>ops<span class="token punctuation">;</span>
};
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註釋比較詳細，重點介紹如下幾個欄位：
- codec_name：音訊鏈路需要系結的 codec 名稱，音效卡註冊時會遍歷
codec_list，找到同名的 codec 並系結； - platform_name：音訊鏈路需要系結的 platform 名稱，音效卡註冊時會遍歷
platform_list，找到同名的 platform 並系結； - cpu_dai_name：音訊鏈路需要系結的 cpu_dai 名稱，音效卡註冊時會遍歷
dai_list，找到同名的 dai 並系結； - codec_dai_name：音訊鏈路需要系結的 codec_dai 名稱，音效卡註冊時會遍歷
dai_list，找到同名的 dai 並系結； - ops：重點留意 hw_params() 回撥，一般來說這個回撥是要實現的，用於設定 codec、codec_dai、cpu_dai 的數據格式和系統時鐘。在
3.4. Codec audio operations小節中有描述。
goni_wm8994.c 中的 dai_link 定義，兩個音訊鏈路分別用於 Media 和 Voice：
static struct snd_soc_dai_link goni_dai[] = {
{
.name = "WM8994",
.stream_name = "WM8994 HiFi",
.cpu_dai_name = "samsung-i2s.0",
.codec_dai_name = "wm8994-aif1",
.platform_name = "samsung-audio",
.codec_name = "wm8994-codec.0-001a",
.init = goni_wm8994_init,
.ops = &goni_hifi_ops,
}, {
.name = "WM8994 Voice",
.stream_name = "Voice",
.cpu_dai_name = "goni-voice-dai",
.codec_dai_name = "wm8994-aif2",
.codec_name = "wm8994-codec.0-001a",
.ops = &goni_voice_ops,
},
};
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除了 dai_link，機器中一些特定的音訊控制元件和音訊事件也可以在 machine_drv 定義，如耳機插拔檢測、外部功放開啓關閉等。
我們再分析 machine_drv 初始化過程：
static struct snd_soc_card goni = { .name = "goni", .owner = THIS_MODULE, .dai_link = goni_dai, .num_links = ARRAY_SIZE(goni_dai),<span class="token punctuation">.</span>dapm_widgets <span class="token operator">=</span> goni_dapm_widgets<span class="token punctuation">,</span> <span class="token punctuation">.</span>num_dapm_widgets <span class="token operator">=</span> <span class="token function">ARRAY_SIZE</span><span class="token punctuation">(</span>goni_dapm_widgets<span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token punctuation">.</span>dapm_routes <span class="token operator">=</span> goni_dapm_routes<span class="token punctuation">,</span> <span class="token punctuation">.</span>num_dapm_routes <span class="token operator">=</span> <span class="token function">ARRAY_SIZE</span><span class="token punctuation">(</span>goni_dapm_routes<span class="token punctuation">)</span><span class="token punctuation">,</span>
};
static int __init goni_init(void)
{
int ret;
goni_snd_device <span class="token operator">=</span> <span class="token function">platform_device_alloc</span><span class="token punctuation">(</span><span class="token string">"soc-audio"</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>goni_snd_device<span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token operator">-</span>ENOMEM<span class="token punctuation">;</span>
<span class="token comment">/* register voice DAI here */</span>
ret <span class="token operator">=</span> <span class="token function">snd_soc_register_dai</span><span class="token punctuation">(</span><span class="token operator">&</span>goni_snd_device<span class="token operator">-></span>dev<span class="token punctuation">,</span> <span class="token operator">&</span>voice_dai<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>ret<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">platform_device_put</span><span class="token punctuation">(</span>goni_snd_device<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> ret<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">platform_set_drvdata</span><span class="token punctuation">(</span>goni_snd_device<span class="token punctuation">,</span> <span class="token operator">&</span>goni<span class="token punctuation">)</span><span class="token punctuation">;</span>
ret <span class="token operator">=</span> <span class="token function">platform_device_add</span><span class="token punctuation">(</span>goni_snd_device<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>ret<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">snd_soc_unregister_dai</span><span class="token punctuation">(</span><span class="token operator">&</span>goni_snd_device<span class="token operator">-></span>dev<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">platform_device_put</span><span class="token punctuation">(</span>goni_snd_device<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> ret<span class="token punctuation">;</span>
}
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- 建立一個
.name="soc-audio"的 platform_device 範例； - 設定 platform_device 的私有數據
snd_soc_card； - 然後註冊 platform_device 到系統中；
- 再然後呢？好像沒有了…
但是真的沒有了嗎？別忘了，platform_device 還有個好夥伴 platform_driver 跟它配對。而 .name="soc-audio" 的 platform_driver 定義在 soc-core.c 中：
/* ASoC platform driver */
static struct platform_driver soc_driver = {
.driver = {
.name = "soc-audio",
.owner = THIS_MODULE,
.pm = &snd_soc_pm_ops,
},
.probe = soc_probe,
.remove = soc_remove,
};
static int __init snd_soc_init(void)
{
// …
snd_soc_util_init();
return platform_driver_register(&soc_driver);
}
module_init(snd_soc_init);
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兩者匹配後，soc_probe() 會被呼叫，繼而呼叫 snd_soc_register_card() 註冊音效卡。由於該過程很冗長，這裏不一一貼程式碼分析了，但整個流程是比較簡單的，流程圖如下：
- 取出 platform_device 的私有數據，該私有數據就是
snd_soc_card； snd_soc_register_card()爲每個 dai_link 分配一個snd_soc_pcm_runtime範例，別忘了之前提過snd_soc_pcm_runtime是 ASoC 的橋樑，儲存着 codec、codec_dai、cpu_dai、platform 等硬體裝置範例。- 隨後的工作都在
snd_soc_instantiate_card()進行： - 遍歷
dai_list、codec_list、platform_list鏈表，爲每個音訊鏈路找到對應的 cpu_dai、codec_dai、codec、platform；找到的 cpu_dai、codec_dai、codec、platform 儲存到snd_soc_pcm_runtime，完成音訊鏈路的裝置系結； - 呼叫
snd_card_create()建立音效卡； soc_probe_dai_link()依次回撥 cpu_dai、codec、platform、codec_dai 的probe()函數，完成各音訊裝置的初始化，隨後呼叫soc_new_pcm()建立 pcm 邏輯裝置（因爲涉及到本系 本係列的重點內容，後面具體分析這個函數）；- 最後呼叫
snd_card_register()註冊音效卡。
soc_new_pcm 原始碼分析：
/* create a new pcm */ int soc_new_pcm(struct snd_soc_pcm_runtime *rtd, int num) { struct snd_soc_codec *codec = rtd->codec; struct snd_soc_platform *platform = rtd->platform; struct snd_soc_dai *codec_dai = rtd->codec_dai; struct snd_soc_dai *cpu_dai = rtd->cpu_dai; struct snd_pcm_ops *soc_pcm_ops = &rtd->ops; struct snd_pcm *pcm; char new_name[64]; int ret = 0, playback = 0, capture = 0;<span class="token comment">// 初始化 snd_soc_pcm_runtime 的 ops 欄位，成員函數其實依次呼叫 machine、codec_dai、cpu_dai、platform 的回撥；如 soc_pcm_hw_params：</span> <span class="token comment">// |-> rtd->dai_link->ops->hw_params() </span> <span class="token comment">// |-> codec_dai->driver->ops->hw_params() </span> <span class="token comment">// |-> cpu_dai->driver->ops->hw_params()</span> <span class="token comment">// |-> platform->driver->ops->hw_params()</span> <span class="token comment">// 在這裏把底層硬體的操作介面抽象起來，pcm native 不用知道底層硬體細節</span> soc_pcm_ops<span class="token operator">-></span>open <span class="token operator">=</span> soc_pcm_open<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>close <span class="token operator">=</span> soc_pcm_close<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>hw_params <span class="token operator">=</span> soc_pcm_hw_params<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>hw_free <span class="token operator">=</span> soc_pcm_hw_free<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>prepare <span class="token operator">=</span> soc_pcm_prepare<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>trigger <span class="token operator">=</span> soc_pcm_trigger<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>pointer <span class="token operator">=</span> soc_pcm_pointer<span class="token punctuation">;</span> <span class="token comment">/* check client and interface hw capabilities */</span> <span class="token function">snprintf</span><span class="token punctuation">(</span>new_name<span class="token punctuation">,</span> <span class="token keyword">sizeof</span><span class="token punctuation">(</span>new_name<span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token string">"%s %s-%d"</span><span class="token punctuation">,</span> rtd<span class="token operator">-></span>dai_link<span class="token operator">-></span>stream_name<span class="token punctuation">,</span> codec_dai<span class="token operator">-></span>name<span class="token punctuation">,</span> num<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>codec_dai<span class="token operator">-></span>driver<span class="token operator">-></span>playback<span class="token punctuation">.</span>channels_min<span class="token punctuation">)</span> playback <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>codec_dai<span class="token operator">-></span>driver<span class="token operator">-></span>capture<span class="token punctuation">.</span>channels_min<span class="token punctuation">)</span> capture <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">// 建立 pcm 邏輯裝置</span> ret <span class="token operator">=</span> <span class="token function">snd_pcm_new</span><span class="token punctuation">(</span>rtd<span class="token operator">-></span>card<span class="token operator">-></span>snd_card<span class="token punctuation">,</span> new_name<span class="token punctuation">,</span> num<span class="token punctuation">,</span> playback<span class="token punctuation">,</span> capture<span class="token punctuation">,</span> <span class="token operator">&</span>pcm<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>ret <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">printk</span><span class="token punctuation">(</span>KERN_ERR <span class="token string">"asoc: can't create pcm for codec %s\n"</span><span class="token punctuation">,</span> codec<span class="token operator">-></span>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token comment">/* DAPM dai link stream work */</span> <span class="token function">INIT_DELAYED_WORK</span><span class="token punctuation">(</span><span class="token operator">&</span>rtd<span class="token operator">-></span>delayed_work<span class="token punctuation">,</span> close_delayed_work<span class="token punctuation">)</span><span class="token punctuation">;</span> rtd<span class="token operator">-></span>pcm <span class="token operator">=</span> pcm<span class="token punctuation">;</span> pcm<span class="token operator">-></span>private_data <span class="token operator">=</span> rtd<span class="token punctuation">;</span> <span class="token comment">// pcm 的私有數據指向 snd_soc_pcm_runtime</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>platform<span class="token operator">-></span>driver<span class="token operator">-></span>ops<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 初始化 snd_soc_pcm_runtime 的 ops 欄位，這些與 pcm_dma 操作相關，一般我們只用留意 pointer 回撥</span> soc_pcm_ops<span class="token operator">-></span>mmap <span class="token operator">=</span> platform<span class="token operator">-></span>driver<span class="token operator">-></span>ops<span class="token operator">-></span>mmap<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>pointer <span class="token operator">=</span> platform<span class="token operator">-></span>driver<span class="token operator">-></span>ops<span class="token operator">-></span>pointer<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>ioctl <span class="token operator">=</span> platform<span class="token operator">-></span>driver<span class="token operator">-></span>ops<span class="token operator">-></span>ioctl<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>copy <span class="token operator">=</span> platform<span class="token operator">-></span>driver<span class="token operator">-></span>ops<span class="token operator">-></span>copy<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>silence <span class="token operator">=</span> platform<span class="token operator">-></span>driver<span class="token operator">-></span>ops<span class="token operator">-></span>silence<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>ack <span class="token operator">=</span> platform<span class="token operator">-></span>driver<span class="token operator">-></span>ops<span class="token operator">-></span>ack<span class="token punctuation">;</span> soc_pcm_ops<span class="token operator">-></span>page <span class="token operator">=</span> platform<span class="token operator">-></span>driver<span class="token operator">-></span>ops<span class="token operator">-></span>page<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>playback<span class="token punctuation">)</span> <span class="token function">snd_pcm_set_ops</span><span class="token punctuation">(</span>pcm<span class="token punctuation">,</span> SNDRV_PCM_STREAM_PLAYBACK<span class="token punctuation">,</span> soc_pcm_ops<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 把 soc_pcm_ops 賦給 playback substream 的 ops 欄位</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>capture<span class="token punctuation">)</span> <span class="token function">snd_pcm_set_ops</span><span class="token punctuation">(</span>pcm<span class="token punctuation">,</span> SNDRV_PCM_STREAM_CAPTURE<span class="token punctuation">,</span> soc_pcm_ops<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 把 soc_pcm_ops 賦給 capture substream 的 ops 欄位</span> <span class="token comment">// 回撥 dma 驅動的 pcm_new()，進行 dma buffer 記憶體分配和 dma 裝置初始化</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>platform<span class="token operator">-></span>driver<span class="token operator">-></span>pcm_new<span class="token punctuation">)</span> <span class="token punctuation">{</span> ret <span class="token operator">=</span> platform<span class="token operator">-></span>driver<span class="token operator">-></span><span class="token function">pcm_new</span><span class="token punctuation">(</span>rtd<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>ret <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">pr_err</span><span class="token punctuation">(</span><span class="token string">"asoc: platform pcm constructor failed\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> pcm<span class="token operator">-></span>private_free <span class="token operator">=</span> platform<span class="token operator">-></span>driver<span class="token operator">-></span>pcm_free<span class="token punctuation">;</span> <span class="token function">printk</span><span class="token punctuation">(</span>KERN_INFO <span class="token string">"asoc: %s <-> %s mapping ok\n"</span><span class="token punctuation">,</span> codec_dai<span class="token operator">-></span>name<span class="token punctuation">,</span> cpu_dai<span class="token operator">-></span>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> ret<span class="token punctuation">;</span>
}
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可見 soc_new_pcm() 最主要的工作是建立 pcm 邏輯裝置，建立回放子流和錄製子流範例，並初始化回放子流和錄製子流的 pcm 操作函數（數據搬運時，需要呼叫這些函數來驅動 codec、codec_dai、cpu_dai、dma 裝置工作）。
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