linux下alsa音频数据流分析

1. 前言
本文，我们将以回放（Playback，播放音频）为例，讲解PCM Data是如何从用户空间到内核空间，{BANNED}{BANNED}最佳佳后传递到Codec。
在Linux 音频驱动(一) ASoC音频框架简介中，我们给出了回放（Playback）PCM数据流示意图：






对于Linux来说，由于分为 user space 和kernel space，而且两者之间不能随便互相访问。因此用户如果播放音频，则需要调用copy_from_user()将用户数据从user space拷贝到kernel space (DMA Buffer)。
DMA 负责将DMA Buffer中的音频数据搬运到I2S TX FIFO。
通过I2S总线，将音频数据传送到Codec。
Codec内部经过DAC转换，将模拟信号传到扬声器SPK（头戴式耳机HP、耳塞式耳机Earp）。
下面基于源码讲解PCM Data Flow。


2. PCM Data Flow
内核版本：Kernel 版本：3.10
内核源码文件：
         ./kernel-3.10/sound/core/device.c
         ./kernel-3.10/sound/core/init.c
         ./kernel-3.10/sound/core/pcm.c
         ./kernel-3.10/sound/core/pcm_lib.c
         ./kernel-3.10/sound/core/pcm_native.c
         ./kernel-3.10/sound/soc/soc-pcm.c
Tinyalsa源码文件：
         ./external/tinyalsa/pcm.c
         ./external/kernel-headers/original/uapi/sound/asound.h


User Space


在我的源码包里，用户空间应用程序使用的是 tinyalsa提供的接口write PCM Data，即播放音频文件。
Write PCM逻辑设备是通过 ioctl() 函数完成的，即应用程序将需要播放的音频数据通过pcm_write() --> ioctl() 传递到内核。


// ./external/kernel-headers/original/uapi/sound/asound.h, line 448
struct snd_xferi {
snd_pcm_sframes_t result;
void __user *buf;
snd_pcm_uframes_t frames;
};
// ./external/tinyalsa/pcm.c, line 483
int pcm_write(struct pcm *pcm, const void *data, unsigned int count)
{
    struct snd_xferi x;
    ......
    x.buf = (void*)data;
    x.frames = count / (pcm->config.channels *
                        pcm_format_to_bits(pcm->config.format) / 8);
    ......
    ioctl(pcm->fd, SNDRV_PCM_IOCTL_WRITEI_FRAMES, &x);
    ......
}

音频数据中的几个重要概念：
Format：样本长度（采样精度 or 采样深度），音频数据{BANNED}{BANNED}最佳佳基本的单位，常见的有 8 位和 16 位；
Channel：声道数，分为单声道 mono 和立体声stereo；
Frame：帧，构成一个完整的声音单元，Frame = Format * Channel；
Rate：又称 sample rate：采样率，即每秒的采样次数，针对帧而言；
Period size：周期，每次硬件中断处理音频数据的帧数，对于音频设备的数据读写，以此为单位；
Buffer size：数据缓冲区大小，这里指runtime 的 buffer size，而不是结构体 snd_pcm_hardware 中定义的buffer_bytes_max；一般来说 buffer_size = period_size * period_count，period_count 相当于处理完一个 buffer 数据所需的硬件中断次数。


为了通过系统调用ioctl()传递音频数据，定义了struct snd_xferi x，x.buf指向本次要播放的音频数据，x.frames表示本次音频数据总共有多少帧(frame)。


Kernel Space


通过系统调用ioctl()传递数据到内核，在内核空间是PCM逻辑设备对应的snd_pcm_f_ops[0].unlocked_ioctl()。


// ./kernel-3.10/sound/core/pcm_native.c, line 3481
const struct file_operations snd_pcm_f_ops[2] = {
{
.owner = THIS_MODULE,
.write = snd_pcm_write,
        ......
.unlocked_ioctl = snd_pcm_playback_ioctl,
        ......
},
{
.owner = THIS_MODULE,
.read = snd_pcm_read,
        ......
.unlocked_ioctl = snd_pcm_capture_ioctl,
        ......
}
};


snd_pcm_playback_ioctl() 直接调用了snd_pcm_playback_ioctl1()。


// ./kernel-3.10/sound/core/pcm_native.c, line 2784
static long snd_pcm_playback_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
struct snd_pcm_file *pcm_file;
pcm_file = file->private_data;
    ......
return snd_pcm_playback_ioctl1(file, pcm_file->substream, cmd, (void __user *)arg);
}

snd_pcm_playback_ioctl1()函数：
a. 定义struct snd_xferi xferi，为了获取用户空间传来的arg；
b. 调用put_user() 清除snd_xferi.result状态；
c. 调用copy_from_user()将取用户空间的arg拷贝到内核空间，即拷贝音频数据存储空间的指针buf和音频数据帧数frames；
d. 调用snd_pcm_lib_write()；
e. 调用put_user() 回填write结果_xferi->result。


// ./kernel-3.10/sound/core/pcm_native.c, line 2624
static int snd_pcm_playback_ioctl1(struct file *file, struct snd_pcm_substream *substream, unsigned int cmd, void __user *arg)
{
    ......
switch (cmd) {
case SNDRV_PCM_IOCTL_WRITEI_FRAMES:
{
struct snd_xferi xferi;
struct snd_xferi __user *_xferi = arg;
struct snd_pcm_runtime *runtime = substream->runtime;
snd_pcm_sframes_t result;
if (runtime->status->state == SNDRV_PCM_STATE_OPEN)
return -EBADFD;
if (put_user(0, &_xferi->result))
return -EFAULT;
if (copy_from_user(&xferi, _xferi, sizeof(xferi)))
return -EFAULT;
result = snd_pcm_lib_write(substream, xferi.buf, xferi.frames);
__put_user(result, &_xferi->result);
return result < 0 ? result : 0;
}
......
}


snd_pcm_lib_write()做一些参数检查后调用snd_pcm_lib_write1()。注意调用snd_pcm_lib_write1()时传入的{BANNED}{BANNED}最佳佳后一个参数snd_pcm_lib_write_transfer，该函数完成音频数据从 kernel space 到 DMA Buffer 的传输。


// ./kernel-3.10/sound/core/pcm_lib.c, line 2101
snd_pcm_sframes_t snd_pcm_lib_write(struct snd_pcm_substream *substream, const void __user *buf, snd_pcm_uframes_t size)
{
struct snd_pcm_runtime *runtime;
int nonblock;
......
nonblock = !!(substream->f_flags & O_NONBLOCK);
......
return snd_pcm_lib_write1(substream, (unsigned long)buf, size, nonblock, snd_pcm_lib_write_transfer);
}

如下snd_pcm_lib_write1()代码段中，第40行调用transfer()，即调用snd_pcm_lib_write_transfer()，将音频数据从 kernel space 拷贝到 DMA Buffer。然后，在第49行，调用snd_pcm_start()启动DMA传输，将音频数据从 DMA Buffer 拷贝到 I2S TX FIFO。


// ./kernel-3.10/sound/core/pcm_lib.c, line 1985
static snd_pcm_sframes_t snd_pcm_lib_write1(struct snd_pcm_substream *substream, unsigned long data,
    snd_pcm_uframes_t size, int nonblock, transfer_f transfer)
{
struct snd_pcm_runtime *runtime = substream->runtime;
snd_pcm_uframes_t xfer = 0;
snd_pcm_uframes_t offset = 0;
snd_pcm_uframes_t avail;
int err = 0;


if (size == 0)
return 0;
    ......
runtime->twake = runtime->control->avail_min ? : 1;
if (runtime->status->state == SNDRV_PCM_STATE_RUNNING)
snd_pcm_update_hw_ptr(substream);
avail = snd_pcm_playback_avail(runtime);
while (size > 0) {
snd_pcm_uframes_t frames, appl_ptr, appl_ofs;
snd_pcm_uframes_t cont;
if (!avail) {
if (nonblock) {
err = -EAGAIN;
goto _end_unlock;
}
runtime->twake = min_t(snd_pcm_uframes_t, size,
runtime->control->avail_min ? : 1);
err = wait_for_avail(substream, &avail);
if (err < 0)
goto _end_unlock;
}
frames = size > avail ? avail : size;
cont = runtime->buffer_size - runtime->control->appl_ptr % runtime->buffer_size;
if (frames > cont)
frames = cont;
......
appl_ptr = runtime->control->appl_ptr;
appl_ofs = appl_ptr % runtime->buffer_size;
snd_pcm_stream_unlock_irq(substream);
err = transfer(substream, appl_ofs, data, offset, frames);  //将音频数据从 kernel space 拷贝到 DMA Buffer
snd_pcm_stream_lock_irq(substream);
......
offset += frames;
size -= frames;
xfer += frames;
avail -= frames;
if (runtime->status->state == SNDRV_PCM_STATE_PREPARED &&
    snd_pcm_playback_hw_avail(runtime) >= (snd_pcm_sframes_t)runtime->start_threshold) {
err = snd_pcm_start(substream);  //启动DMA传输，将音频数据从 DMA Buffer 拷贝到 I2S TX FIFO
if (err < 0)
goto _end_unlock;
}
}
    ......
}

snd_pcm_lib_write_transfer()中：
a. 如果有设定过substream->ops->copy回调函数，则执行substream->ops->copy()将音频数据从 kernel space 拷贝到 DMA Buffer。
b. 如果没有设定substream->ops->copy回调函数，则直接调用copy_from_user()将音频数据从 kernel space 拷贝到 DMA Buffer。


// ./kernel-3.10/sound/core/pcm_lib.c, line 1962
static int snd_pcm_lib_write_transfer(struct snd_pcm_substream *substream, unsigned int hwoff,
      unsigned long data, unsigned int off, snd_pcm_uframes_t frames)
{
struct snd_pcm_runtime *runtime = substream->runtime;
int err;
char __user *buf = (char __user *) data + frames_to_bytes(runtime, off);
if (substream->ops->copy) {
if ((err = substream->ops->copy(substream, -1, hwoff, buf, frames)) < 0)
return err;
} else {
char *hwbuf = runtime->dma_area + frames_to_bytes(runtime, hwoff);
if (copy_from_user(hwbuf, buf, frames_to_bytes(runtime, frames)))
return -EFAULT;
}
return 0;
}


注：本例中，substream->ops->copy回调函数是在soc_new_pcm()中设置的。在soc_new_pcm()中，如果有设定platform->driver->ops （即PCM DMA驱动操作函数集），则PCM逻辑设备的某些操作函数将会被platform->driver->ops中覆盖掉，如下代码段第26行。


// ./kernel-3.10/sound/soc/soc-pcm.c, line 2005
int soc_new_pcm(struct snd_soc_pcm_runtime *rtd, int num)
{
    ......
/* ASoC PCM operations */
if (rtd->dai_link->dynamic) {
rtd->ops.open = dpcm_fe_dai_open;
rtd->ops.hw_params = dpcm_fe_dai_hw_params;
rtd->ops.prepare = dpcm_fe_dai_prepare;
rtd->ops.trigger = dpcm_fe_dai_trigger;
rtd->ops.hw_free = dpcm_fe_dai_hw_free;
rtd->ops.close = dpcm_fe_dai_close;
rtd->ops.pointer = soc_pcm_pointer;
rtd->ops.ioctl = soc_pcm_ioctl;
} else {
rtd->ops.open = soc_pcm_open;
rtd->ops.hw_params = soc_pcm_hw_params;
rtd->ops.prepare = soc_pcm_prepare;
rtd->ops.trigger = soc_pcm_trigger;
rtd->ops.hw_free = soc_pcm_hw_free;
rtd->ops.close = soc_pcm_close;
rtd->ops.pointer = soc_pcm_pointer;
rtd->ops.ioctl = soc_pcm_ioctl;
}


if (platform->driver->ops) {
rtd->ops.ack = platform->driver->ops->ack;
rtd->ops.copy = platform->driver->ops->copy;
rtd->ops.silence = platform->driver->ops->silence;
rtd->ops.page = platform->driver->ops->page;
rtd->ops.mmap = platform->driver->ops->mmap;
}
    ......
}


现在看一下snd_pcm_start()如何启动DMA传输的。
snd_pcm_start()直接调用了snd_pcm_action()。此处要注意snd_pcm_action()的{BANNED}中国{BANNED}中国第一个参数snd_pcm_action_start。snd_pcm_action()在多个地方会被调用，要注意其被调用时的{BANNED}中国{BANNED}中国第一个参数是什么。


// ./kernel-3.10/sound/core/pcm_native.c, line 891
static struct action_ops snd_pcm_action_start = {
.pre_action = snd_pcm_pre_start,
.do_action = snd_pcm_do_start,
.undo_action = snd_pcm_undo_start,
.post_action = snd_pcm_post_start
};
// ./kernel-3.10/sound/core/pcm_native.c, line 904
int snd_pcm_start(struct snd_pcm_substream *substream)
{
return snd_pcm_action(&snd_pcm_action_start, substream, SNDRV_PCM_STATE_RUNNING);
}

snd_pcm_action()中会调用snd_pcm_action_group()或snd_pcm_action_single()。为简化讲解，我们关注snd_pcm_action_single()。snd_pcm_action_single()函数非常简单，就是执行struct action_ops *ops指向的回调函数集，本例中为上文提到的snd_pcm_action_start。


// ./kernel-3.10/sound/core/pcm_native.c, line 785
static int snd_pcm_action(struct action_ops *ops, struct snd_pcm_substream *substream, int state)
{
int res;


if (snd_pcm_stream_linked(substream)) {
if (!spin_trylock(&substream->group->lock)) {
spin_unlock(&substream->self_group.lock);
spin_lock(&substream->group->lock);
spin_lock(&substream->self_group.lock);
}
res = snd_pcm_action_group(ops, substream, state, 1);
spin_unlock(&substream->group->lock);
} else {
res = snd_pcm_action_single(ops, substream, state);
}
return res;
}
// ./kernel-3.10/sound/core/pcm_native.c, line 765
static int snd_pcm_action_single(struct action_ops *ops,
struct snd_pcm_substream *substream,
int state)
{
int res;

res = ops->pre_action(substream, state);
if (res < 0)
return res;
res = ops->do_action(substream, state);
if (res == 0)
ops->post_action(substream, state);
else if (ops->undo_action)
ops->undo_action(substream, state);
return res;
}


我们重点看一下ops->do_action()，即snd_pcm_do_start()。在前面soc_new_pcm()代码段中，我们看到substream->ops->trigger()被设定为soc_pcm_trigger()，该函数依次调用codec_dai driver的trigger函数、pcm_dma的trigger函数、cpu_dai driver的trigger函数。


// ./kernel-3.10/sound/core/pcm_native.c, line 862
static int snd_pcm_do_start(struct snd_pcm_substream *substream, int state)
{
if (substream->runtime->trigger_master != substream)
return 0;
return substream->ops->trigger(substream, SNDRV_PCM_TRIGGER_START);
}

// ./kernel-3.10/sound/core/soc-pcm.c, line 609
static int soc_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
{
    struct snd_soc_pcm_runtime *rtd = substream->private_data;
struct snd_soc_platform *platform = rtd->platform;
struct snd_soc_dai *cpu_dai = rtd->cpu_dai;
struct snd_soc_dai *codec_dai = rtd->codec_dai;
int ret;


if (codec_dai->driver->ops->trigger) {
ret = codec_dai->driver->ops->trigger(substream, cmd, codec_dai);  //调用codec_dai driver的trigger函数 (可选的)
if (ret < 0)
return ret;
}


if (platform->driver->ops && platform->driver->ops->trigger) {
ret = platform->driver->ops->trigger(substream, cmd);  //调用pcm_dma的trigger函数
if (ret < 0)
return ret;
}


if (cpu_dai->driver->ops->trigger) {
ret = cpu_dai->driver->ops->trigger(substream, cmd, cpu_dai);  //调用cpu_dai driver的trigger函数 (可选的)
if (ret < 0)
return ret;
}
return 0;
}


问题：为什么执行trigger函数的顺序是codec_dai --> pcm_dma --> cpu_dai 呢 ？是否可以调整顺序 ？


思考：
是可以调整的。(下面的思考是我个人想法，其实我倾向于相信这部分的设计是大神有意为之的，可能我还没理解吧。)
首先，codec_dai和cpu_dai的trigger函数是可选的。查看struct snd_soc_dai_ops结构体的源码，可以看到其中PCM operation函数集注释说“ALSA PCM audio operations - all optional.”。
其次，假设codec_dai和cpu_dai的trigger都有定义，个人认为相对理想的顺序应该是codec_dai --> cpu_dai --> pcm_dma。因为，当我们启动DMA将音频数据拷贝到cpu_dai（I2S TX Buffer）时，如果该硬件还没有ready，那DMA搬过去的数据可能会丢失。同理，如果cpu_dai开始传数据了，而codec_dai（I2S RX Buffer）没有ready，那数据也有可能丢失。理论上应该是在启动DMA之前，数据流下一接收装置应该已经ready。所以，我才会说相对理想的trigger顺序应该是codec_dai --> cpu_dai --> pcm_dma。
（我目前使用的MTK平台没有定义codec_dai和cpu_dai的trigger函数，没有办法研究该顺序是否有意义。有条件的读者可以调整顺序，看是否会对音频播放造成问题。）


下面介绍trigger函数的内容不同的SoC平台会有差异，本例基于MTK平台的源码分析。
本例中，codec_dai driver的trigger函数、pcm_dma的trigger函数、cpu_dai driver的trigger函数分别如下：


codec_dai driver的trigger函数：mt6323_codec_trigger()，虽然该函数有定义，但是该函数没有做任何事情。


// ./kernel-3.10/sound/soc/mediatek/mt_soc_audio_v3/mt_soc_codec_63xx.c, line 1070
static struct snd_soc_dai_driver mtk_6331_dai_codecs[] =
{
    {
        .name = MT_SOC_CODEC_TXDAI_NAME,
        .ops = &mt6323_aif1_dai_ops,
        ......
    },
    ......
}
// ./kernel-3.10/sound/soc/mediatek/mt_soc_audio_v3/mt_soc_codec_63xx.c, line 1063
static const struct snd_soc_dai_ops mt6323_aif1_dai_ops =
{
    .startup    = mt63xx_codec_startup,
    .prepare   = mt63xx_codec_prepare,
    .trigger     = mt6323_codec_trigger,
};
// ./kernel-3.10/sound/soc/mediatek/mt_soc_audio_v3/mt_soc_codec_63xx.c, line 1048
static int mt6323_codec_trigger(struct snd_pcm_substream *substream , int command , struct snd_soc_dai *Daiport)
{
    switch (command)
    {
        case SNDRV_PCM_TRIGGER_START:
        case SNDRV_PCM_TRIGGER_RESUME:
        case SNDRV_PCM_TRIGGER_STOP:
        case SNDRV_PCM_TRIGGER_SUSPEND:
            break;
    }


    return 0;
}


pcm_dma的trigger函数：mtk_pcm_I2S0dl1_trigger()


// ./kernel-3.10/sound/soc/mediatek/mt_soc_audio_v3/mt_soc_pcm_dl1_i2s0Dl1.c, line 730
static struct snd_soc_platform_driver mtk_I2S0dl1_soc_platform =
{
    .ops        = &mtk_I2S0dl1_ops,
    .pcm_new    = mtk_asoc_pcm_I2S0dl1_new,
    .probe      = mtk_afe_I2S0dl1_probe,
};
// ./kernel-3.10/sound/soc/mediatek/mt_soc_audio_v3/mt_soc_pcm_dl1_i2s0Dl1.c, line 715
static struct snd_pcm_ops mtk_I2S0dl1_ops =
{
    ......
    .trigger =  mtk_pcm_I2S0dl1_trigger,
    .pointer =  mtk_pcm_I2S0dl1_pointer,
    .copy =     mtk_pcm_I2S0dl1_copy,
    ......
};
// ./kernel-3.10/sound/soc/mediatek/mt_soc_audio_v3/mt_soc_pcm_dl1_i2s0Dl1.c, line 529
static int mtk_pcm_I2S0dl1_trigger(struct snd_pcm_substream *substream, int cmd)
{
    //printk("mtk_pcm_I2S0dl1_trigger cmd = %d\n", cmd);


    switch (cmd)
    {
        case SNDRV_PCM_TRIGGER_START:
        case SNDRV_PCM_TRIGGER_RESUME:
            return mtk_pcm_I2S0dl1_start(substream);  // 启动 dma 传输
        case SNDRV_PCM_TRIGGER_STOP:
        case SNDRV_PCM_TRIGGER_SUSPEND:
            return mtk_pcm_I2S0dl1_stop(substream);  // 停止 dma 传输
    }
    return -EINVAL;
}
// ./kernel-3.10/sound/soc/mediatek/mt_soc_audio_v3/mt_soc_pcm_dl1_i2s0Dl1.c, line 493
static int mtk_pcm_I2S0dl1_start(struct snd_pcm_substream *substream)
{
    struct snd_pcm_runtime *runtime = substream->runtime;
    printk("%s\n", __func__);
    // here start digital part


    SetConnection(Soc_Aud_InterCon_Connection, Soc_Aud_InterConnectionInput_I05, Soc_Aud_InterConnectionOutput_O00);
    SetConnection(Soc_Aud_InterCon_Connection, Soc_Aud_InterConnectionInput_I06, Soc_Aud_InterConnectionOutput_O01);
    SetConnection(Soc_Aud_InterCon_Connection, Soc_Aud_InterConnectionInput_I05, Soc_Aud_InterConnectionOutput_O03);
    SetConnection(Soc_Aud_InterCon_Connection, Soc_Aud_InterConnectionInput_I06, Soc_Aud_InterConnectionOutput_O04);


    SetIrqEnable(Soc_Aud_IRQ_MCU_MODE_IRQ1_MCU_MODE, true);


    SetSampleRate(Soc_Aud_Digital_Block_MEM_DL1, runtime->rate);
    SetChannels(Soc_Aud_Digital_Block_MEM_DL1, runtime->channels);
    SetMemoryPathEnable(Soc_Aud_Digital_Block_MEM_DL1, true);


    EnableAfe(true);
    ......
    return 0;
}


cpu_dai driver的trigger函数：本例中没有实现该函数


// ./kernel-3.10/sound/soc/mediatek/mt_soc_audio_v3/mt_soc_dai_stub.c, line 98
static struct snd_soc_dai_driver mtk_dai_stub_dai[] =
{
    {
        ......
        .name = MT_SOC_DL1DAI_NAME,
        .ops = &mtk_dai_stub_ops,
    },
    ......
}
// ./kernel-3.10/sound/soc/mediatek/mt_soc_audio_v3/mt_soc_dai_stub.c, line 93
static struct snd_soc_dai_ops mtk_dai_stub_ops =
{
    .startup    = multimedia_startup,
};

3. 总结
回放（Playback）PCM数据流示意图：




{BANNED}{BANNED}最佳佳后简单总结一下PCM write时的数据传递：
  i.  应用程序调用tinyalsa提供的接口pcm_write()-->ioctl()将需要回放的音频数据指针和帧数传递给内核。
 ii.  内核在snd_pcm_lib_write_transfer()函数中使用copy_from_user()将音频数据从user space拷贝到kernel space，即从应用程序的buffer拷贝到DMA buffer。
iii.  内核在snd_pcm_start()中启动DMA传输，将音频数据从DMA buffer拷贝到I2S TX FIFO。（实质上是通过pcm_dma的trigger函数来做的。）
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原文链接：https://blog.csdn.net/yangjizhen1533/article/details/114576914