Android USB AUDIO初步分析

1.USB-AUDIO的声卡注册过程
usb_probe_interface    ----  driver.cusb_audio_probe    ----  card.csnd_usb_create_stream    ----  card.csnd_usb_parse_audio_interface    ----  stream.csnd_usb_add_audio_stream    ----  stream.csnd_pcm_new    ----  pcm.c_snd_pcm_new    ----  pcm.cstatic struct snd_device_ops ops = {.dev_free = snd_pcm_dev_free,.dev_register =  snd_pcm_dev_register,.dev_disconnect = snd_pcm_dev_disconnect,};snd_pcm_new_stream(...,SNDRV_PCM_STREAM_PLAYBACK,...)    ----  pcm.csnd_pcm_new_stream(...,SNDRV_PCM_STREAM_CAPTURE,...)    ----  pcm.csnd_device_new    ----  pcm.c

struct snd_pcm {//是分别对应的2条音频流，一条是playback，一条是capturestruct snd_pcm_str streams[2];
};

_snd_pcm_new中有个函数需要特别关注如下：
snd_pcm_dev_register...for (cidx = 0; cidx < 2; cidx++) {...switch (cidx) {case SNDRV_PCM_STREAM_PLAYBACK:sprintf(str, "pcmC%iD%ip", pcm->card->number, pcm->device);devtype = SNDRV_DEVICE_TYPE_PCM_PLAYBACK;break;case SNDRV_PCM_STREAM_CAPTURE:sprintf(str, "pcmC%iD%ic", pcm->card->number, pcm->device);devtype = SNDRV_DEVICE_TYPE_PCM_CAPTURE;break;}...snd_register_device_for_dev(devtype, pcm->card,pcm->device,&snd_pcm_f_ops[cidx],pcm, str, dev)...

如上是注册pcm设备的地方，同时我们需要关注pcm设备的ops，如下：
const struct file_operations snd_pcm_f_ops[2] = {{.owner =        THIS_MODULE,.write =       snd_pcm_write,.aio_write =     snd_pcm_aio_write,.open =          snd_pcm_playback_open,.release =       snd_pcm_release,.llseek =      no_llseek,.poll =          snd_pcm_playback_poll,.unlocked_ioctl =    snd_pcm_playback_ioctl,.compat_ioctl =     snd_pcm_ioctl_compat,.mmap =           snd_pcm_mmap,.fasync =     snd_pcm_fasync,.get_unmapped_area =    snd_pcm_get_unmapped_area,},{.owner =      THIS_MODULE,.read =            snd_pcm_read,.aio_read =       snd_pcm_aio_read,.open =           snd_pcm_capture_open,.release =        snd_pcm_release,.llseek =      no_llseek,.poll =          snd_pcm_capture_poll,.unlocked_ioctl = snd_pcm_capture_ioctl,.compat_ioctl =  snd_pcm_ioctl_compat,.mmap =           snd_pcm_mmap,.fasync =     snd_pcm_fasync,.get_unmapped_area =    snd_pcm_get_unmapped_area,}
};

之后pcm的录音和播放都会通过这个ops完成
2.使用tinycap的录音过程

external\tinyalsa\tinycap.c
int main(){//先预先留出wav的头部fseek(file, sizeof(struct wav_header), SEEK_SET);frames = capture_sample(file, card, device, header.num_channels,header.sample_rate, format,period_size, period_count);//写入头部fseek(file, 0, SEEK_SET);fwrite(&header, sizeof(struct wav_header), 1, file);
}

**一帧音频数据大小 = 通道数 * 采样深度**

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。

继续来看capture_sample

capture_sample打开声卡的音频设备"/dev/snd/pcmC1D0c"，"/dev/snd/pcmC1D0p"，其中"c"对应capture，"p"对应的是playpcm_open(card, device, PCM_IN, &config);通过样本大小(4个硬件周期，每个周期处理1024帧数据)获取总字节数size = pcm_frames_to_bytes(pcm, pcm_get_buffer_size(pcm));为这次样本的采集准备空间buffer = malloc(size);通过pcm_read从内核中获取音频数据到buffer中，随后将这个buffer的内容写入到文件file中，持续的读取声卡中的音频，直到结束录音while (capturing && !pcm_read(pcm, buffer, size)) {if (fwrite(buffer, 1, size, file) != size) {fprintf(stderr,"Error capturing sample\n");break;}bytes_read += size;}录音结束，释放buffer，关闭pcm设备，返回当前读取到的音频帧数free(buffer);pcm_close(pcm);return pcm_bytes_to_frames(pcm, bytes_read);

随后进入到alsa的音频库(libtinyalsa.so)中

external\tinyalsa\pcm.c
int pcm_read(struct pcm *pcm, void *data, unsigned int count)
{struct snd_xferi x;int len = 10;if (!(pcm->flags & PCM_IN))return -EINVAL;//x.buf指向的是内核返回的音频Buf数据x.buf = data;//x.frames 对应的是要收集满count个字节的音频数据，对应的声卡pcm设备需要采集的帧数x.frames = count / (pcm->config.channels *pcm_format_to_bits(pcm->config.format) / 8);for (;;) {if (!pcm->running) {if (pcm_start(pcm) < 0) {fprintf(stderr, "start error");return -errno;}}通过ioctl调度到到内核完成if (ioctl(pcm->fd, SNDRV_PCM_IOCTL_READI_FRAMES, &x)) {pcm->prepared = 0;pcm->running = 0;if (errno == EPIPE) {/* we failed to make our window -- try to restart */pcm->underruns++;continue;}return oops(pcm, errno, "cannot read stream data");}return 0;}
}

3.tinycap的录音过程进入内核

snd_pcm_capture_ioctlsnd_pcm_capture_ioctl1snd_pcm_lib_readsnd_pcm_lib_read1(substream, (unsigned long)buf, size, nonblock, snd_pcm_lib_read_transfer)

ok，我们看看snd_pcm_lib_read1这个函数snd_pcm_lib_read1....//这里实现的是一个等待队列，等待usb收集到足够的音频数据后会返回avail的值wait_for_avail(substream, &avail);//调用transfer的函数指针callback到userspacetransfer(substream, appl_ofs, data, offset, frames);

USB采集音频数据的大致流程如下：

最后回到transfer的函数指针实现

static int snd_pcm_lib_read_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 {//走这儿，很简单，就是将dma采集到的buf传递到用户空间，完事char *hwbuf = runtime->dma_area + frames_to_bytes(runtime, hwoff);if (copy_to_user(buf, hwbuf, frames_to_bytes(runtime, frames)))return -EFAULT;}return 0;
}

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Android USB AUDIO初步分析

**一帧音频数据大小 = 通道数 * 采样深度**

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。

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