RK3288的buffer指针读写程序实现

以录音为例：
snd_pcm_lib_read1 -> pcm_lib.c
    snd_pcm_update_hw_ptr ->
        snd_pcm_update_hw_ptr0 -> pcm_lib.c
            substream->ops->pointer ->    //这个函数指针很关键！ 初始化时在soc_new_pcm()中注册
            soc_pcm_pointer -> soc-pcm.c
                    platform->driver->ops->pointer ->    //snd_dmaengine_pcm_register()中注册dmaengine_no_residue_pcm_platform
                    snd_dmaengine_pcm_pointer_no_residue
static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
  unsigned int in_interrupt)
{
struct snd_pcm_runtime *runtime = substream->runtime;
snd_pcm_uframes_t pos;
snd_pcm_uframes_t old_hw_ptr, new_hw_ptr, hw_base;
snd_pcm_sframes_t hdelta, delta;
unsigned long jdelta;
unsigned long curr_jiffies;
struct timespec curr_tstamp;
struct timespec audio_tstamp;
int crossed_boundary = 0;
old_hw_ptr = runtime->status->hw_ptr;
/*
* group pointer, time and jiffies reads to allow for more
* accurate correlations/corrections.
* The values are stored at the end of this routine after
* corrections for hw_ptr position
*/
//dma硬件读当前相对dma buffer address的偏移，
//每次拿到的pos相对上一次的偏移量其实就是peroid_size，
//也就是说每次读的size受peroid_size控制。
//而最多的pos是瘦buffer_size控制，当到达buffer_size之后，就会重置
pos = substream->ops->pointer(substream);
......
//runtime->hw_ptr_base以buffer_size的单位移动
hw_base = runtime->hw_ptr_base;
//得到当前实际的读取位置
new_hw_ptr = hw_base + pos;
if (in_interrupt) {
/* we know that one period was processed */
/* delta = "expected next hw_ptr" for in_interrupt != 0 */
delta = runtime->hw_ptr_interrupt + runtime->period_size;
......
}
/* new_hw_ptr might be lower than old_hw_ptr in case when */
/* pointer crosses the end of the ring buffer */
//当dma buffer完成一个buffer_size之后，pos又从0开始计算，这时就会成立。
if (new_hw_ptr < old_hw_ptr) {
//hw_base要往后挪
hw_base += runtime->buffer_size;
//防止移动后超过边界
if (hw_base >= runtime->boundary) {
hw_base = 0;
crossed_boundary++;
}
//重新更新正确的new_hw_ptr
new_hw_ptr = hw_base + pos;
}
      __delta:
    //delta其实就是0或者一个peroid_size
delta = new_hw_ptr - old_hw_ptr;
......
/* Do jiffies check only in xrun_debug mode */
if (!xrun_debug(substream, XRUN_DEBUG_JIFFIESCHECK))
goto no_jiffies_check;
......
no_jiffies_check:
......
no_delta_check:
//当dma没有读到数据，会直接返回，等待下一次中断到来更新pos.
//当用户调用读写接口不更新pos的时候，会是这种情况
if (runtime->status->hw_ptr == new_hw_ptr)
return 0;
......
if (in_interrupt) {
delta = new_hw_ptr - runtime->hw_ptr_interrupt;
if (delta < 0)
delta += runtime->boundary;
delta -= (snd_pcm_uframes_t)delta % runtime->period_size;
runtime->hw_ptr_interrupt += delta;
if (runtime->hw_ptr_interrupt >= runtime->boundary)
runtime->hw_ptr_interrupt -= runtime->boundary;
}
//更新base和hw_ptr值
runtime->hw_ptr_base = hw_base;
runtime->status->hw_ptr = new_hw_ptr;
......
return snd_pcm_update_state(substream, runtime);
}
snd_pcm_update_state（）：
int snd_pcm_update_state(struct snd_pcm_substream *substream,
struct snd_pcm_runtime *runtime)
{
snd_pcm_uframes_t avail;
if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
avail = snd_pcm_playback_avail(runtime);
else
//计算出当前总buffer有效数据空间
avail = snd_pcm_capture_avail(runtime);
if (avail > runtime->avail_max)
runtime->avail_max = avail;
if (runtime->status->state == SNDRV_PCM_STATE_DRAINING) {
if (avail >= runtime->buffer_size) {
snd_pcm_drain_done(substream);
return -EPIPE;
}
} else {
if (avail >= runtime->stop_threshold) {
xrun(substream);
return -EPIPE;
}
}
......
return 0;
}
static inline snd_pcm_uframes_t snd_pcm_capture_avail(struct snd_pcm_runtime *runtime)
{
//hw_ptr为当前硬件录音后没被读走的终点位置，appl_ptr为应用层读取的当前位置，也就是硬件录音没被读取的起点位置
snd_pcm_sframes_t avail = runtime->status->hw_ptr - runtime->control->appl_ptr;
if (avail < 0)
avail += runtime->boundary;
return avail;
}

原作者：KrisFei