Hey!
On ALSA 1.0.16 with the HDA driver I am experiencing some weird things with snd_pcm_rewind() which I think are the result of a kernel bug, but I am not sure.
Here's what I do to trigger this:
I play some PCM. Eventually I want to rewrite what I already have written. I call snd_pcm_hwsync() and then snd_pcm_avail_update() to determine how "empty" the playback buffer is. I subtract that value from the total buffer size. The resulting value should be something resembling the number of frames that I might be able to rewind. I then subtract enough frames to keep away from the playback pointer far enough (50ms, the playback buffer is 370ms, period is 185ms).
Now I pass that value to snd_pcm_rewind() and it returns something that is higher than what I passed in. Which I find quite surprising. I had expected that the returned value could be equal or smaller than what I passed in, but never higher!
So, my interpretation of the API was apparently wrong. It looks like on HDA the snd_pcm_rewind() parameter is apparently rounded *up* to the next period boundary (which in this case was equivalent to the *entire* buffer)?
Of course, rewinding that far immediately caused an XRUN. Which is why I think rounding up in this cases is a bad idea and the API shouldn't work this way. Rounding down is OK, rounding up is call for trouble.
To tape over this I added some code to immediately call snd_pcm_forward() with the difference if I noticed that the device forwarded more than I requested. This value also got rounded up -- to twice the period size. So with this backward+forward operation I ended up at a frame index that is farther ahead than where I originally came from!
So, what's the expected behaviour of snd_pcm_rewind()? Always round up? What's the rationale behind this?
If this is not a bug but expected behaviour, could we at least add something for passing the direction to round to?
What I am working on is the "glitch-free" playback model in PulseAudio (http://0pointer.de/blog/projects/pulse-glitch-free.html). To do this I am writing blocks smaller than the period size to the device via the mmap interface all the time. Given that on HDA _rewind() is apparently not granular to the frame size, is writing via mmap in small blocks like that OK? If it's not, how can I learn about this in advance?
Lennart