Hello,

I'm debugging an issue where both snd_pcm_avail_update() and snd_pcm_avail() are always returning buffer_size, even after successfully calling snd_pcm_writei() repeatedly.

This is causing problems when I first start playback. My goal was to fill up the playback buffer until it's almost full when first starting, and then enter my regular playback loop where I write period_size at a time.

However, I'm not able to tell when the playback buffer is full from snd_pcm_avail() or snd_pcm_avail_update() as expected. In case it's useful, the full snd_pcm_dump() output of the playback device is available here:

http://pastebin.com/ai3e0cer

To summarize the configuration, I'm using a chain of PCM devices that looks like this:

Soft volume -> Rate conversion -> Route conversion -> HW PCM (USB audio)

(notes: buffer_size for soft volume PCM is 640 [80ms], period size is 160 [20ms]. buffer_size for HW PCM is 3846 and period_size is 960. I'm opening the playback device in blocking mode, although the same thing appears to happen in non-blocking mode)

Here is the sequence of events I'm seeing:

1) Call snd_pcm_writei() with 160 frames successfully. Function returns immediately in less than 1ms because the buffer is empty.

2) snd_pcm_avail() returns 640 (expected 640-160 = 480) snd_pcm_dump() shows the following for the HW PCM device: appl_ptr : 960 hw_ptr : 0

3) Call snd_pcm_writei() again with 160 frames successfully. Function returns immediately again.

4) snd_pcm_avail() returns 640 (expected 480-160 = 320) snd_pcm_dump() shows the following for the HW PCM device: appl_ptr : 1920 hw_ptr : 0

5) Cycle repeats two more times, with these values for apply_ptr/hw_ptr reported after each subsequent snd_pcm_writei() call: appl_ptr : 2880 hw_ptr : 288

appl_ptr : 3840 hw_ptr : 288

All along, snd_pcm_avail() returns 640.

6) Call snd_pcm_writei() with 160 frames again. This time, the buffer is actually full, so it blocks for 20 ms as expected.

7) snd_pcm_avail() returns 640 snd_pcm_dump() shows the following for the HW PCM device: appl_ptr : 4800 hw_ptr : 960

8) Continue this loop, with each subsequent snd_pcm_writei() call blocking for 20 ms, while snd_pcm_avail() always returns 640.

Any ideas on why this could be happening? Or am I misunderstanding something here?

Thanks,

Nate Case ncase@xes-inc.com