ALSADMA缓冲区分析.docx
ALSADMA缓冲区分析ALSA应用程序调用ALSA lib库中的函数snd_pcm_writei向声卡硬件写入交错(write后的i代表interleaved)数据。在ALSA lib中最后会调到snd_pcm_hw_writen(snd_pcm_t *pcm, void *bufs, snd_pcm_uframes_t size)函数，这个函数调用通用的ioctl接口: err = ioctl(fd, SNDRV_PCM_IOCTL_WRITEN_FRAMES, &xfern); 在ALSA驱动层中 snd_pcm_playback_ioctl1函数响应ioctl,在这个函数中某个位置会调用 result = snd_pcm_lib_writev(substream, bufs, xfern.frames);它会继续调用： snd_pcm_lib_write1(substream, (unsigned long)bufs, frames, nonblock, snd_pcm_lib_writev_transfer)函数,其中snd_pcm_lib_writev_transfer是个函数指针。这个函数很关键，其中包括下面的代码： 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; . while (size > 0) snd_pcm_uframes_t frames, appl_ptr, appl_ofs; snd_pcm_uframes_t avail; snd_pcm_uframes_t cont; if (runtime->status->state = SNDRV_PCM_STATE_RUNNING) snd_pcm_update_hw_ptr(substream); avail = snd_pcm_playback_avail(runtime); if (!avail) if (nonblock) err = -EAGAIN; goto _end_unlock; err = wait_for_avail_min(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; snd_assert(frames != 0, snd_pcm_stream_unlock_irq(substream); return -EINVAL); appl_ptr = runtime->control->appl_ptr; appl_ofs = appl_ptr % runtime->buffer_size; snd_pcm_stream_unlock_irq(substream); if (err = transfer(substream, appl_ofs, data, offset, frames) < 0) goto _end; snd_pcm_stream_lock_irq(substream); switch (runtime->status->state) case SNDRV_PCM_STATE_XRUN: err = -EPIPE; goto _end_unlock; case SNDRV_PCM_STATE_SUSPENDED: err = -ESTRPIPE; goto _end_unlock; default: break; appl_ptr += frames; if (appl_ptr >= runtime->boundary) appl_ptr -= runtime->boundary; runtime->control->appl_ptr = appl_ptr; if (substream->ops->ack) substream->ops->ack(substream); offset += frames; size -= frames; xfer += 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); if (err < 0) goto _end_unlock; _end_unlock: snd_pcm_stream_unlock_irq(substream); _end: return xfer > 0 ? (snd_pcm_sframes_t)xfer : err; snd_pcm_playback_avail是个内联函数，在include/sound/pcm.h中定义: /* * result is: 0 . (boundary - 1) */ static inline snd_pcm_uframes_t snd_pcm_playback_avail(struct snd_pcm_runtime *runtime) snd_pcm_sframes_t avail = runtime->status->hw_ptr + runtime->buffer_size - runtime->control->appl_ptr; if (avail < 0) avail += runtime->boundary; else if (snd_pcm_uframes_t) avail >= runtime->boundary) avail -= runtime->boundary; return avail; 上面的函数代码，可以帮助我们理解 alsa 中的数据结构成员所代表的含义。很显然，上面的函数的目的就是返回当前hardware buffer中用户程序可写的帧(frames)数。runtime->status-hw_ptr应该表示硬件指针，指向硬件已经处理(播放/录音)过的数据的位置，runtime->control->buffer_size是整个buffer的大小，而runtime->control->appl_ptr是用户程序已经处理(读/写）过的数据的位置。appl_ptr之后的空间，一直到buffer末尾，这一段是可以写入的，这一段长度为runtime->buffer_size - runtime->control->appl_ptr;另外hw_ptr之前的空间也是可写的，所以整个可用的长度为 runtime->status->hw_ptr + runtime->buffer_size - runtime->control->appl_ptr。
