2011/10/25 Nikolay Nikolov nickysn@gmail.com:

On 10/24/2011 06:34 PM, Pierre-Louis Bossart wrote:

...

You want to avoid rewinding completely. Your audio hardware might have prefetched data with the DMA subsystem. Rewinding completely might result in an inconsistent configuration and possibly underflows. If you look at the PulseAudio code, we've introduced some thresholds beyond which we don't rewind (128 bytes or 1ms off the top of my head).

This seem not enough for snd-ymfpci which update hw_ptr on 5.333 ms intervals using its own timer interrupt (i.e. mininum 256 bytes period size) at 48000Hz

http://thread.gmane.org/gmane.linux.alsa.devel/89157/focus=89167

This seem to be a design fault of alsa-pulse plugin which provide a latency (minimum period size of 128 bytes) which is less than some of the alsa sound drivers (e.g. snd-ymfpci, ...) and even PA server probe those sound cards during initialisation but there is no negotiation between PA server and client using alsa-pulse plugin

...

You might argue that snd_pcm_rewindable is broken, but it's somewhat difficult to fix as the amount of prefetched data isn't modeled in the driver and it's very much hardware-specific. Using a less aggressive approach works fine on most hardware. -Pierre

Yes, I figured this out while experimenting with real hw alsa devices and that's why I actually rewind snd_pcm_rewindable() minus some threshold, that can be configured. However with the alsa pulseaudio plugin, rewinding seems to be broken completely, even with a very large threshold.

This is quite complicated when some of alsa driver (e.g.snd-ymfpci) does not provide an accurate hw_ptr

I guess this may be the reason why aplay using 4 periods per buffer

For pulse plugin ,it will need to add two callback into snd_pcm_ioplug_callback in alsa-lib/include/pcm_ioplug.h

snd_pcm_sframes_t (*rewindable)(snd_pcm_ioplug_t *io); snd_pcm_sframes_t (*forwardwable)(snd_pcm_ioplug_t *io);

and alsa-lib/src/pcm_ioplug.c

static snd_pcm_sframes_t snd_pcm_ioplug_rewindable(snd_pcm_t *pcm) { ioplug_priv_t *io = pcm->private_data; + if (io->data->callback->rewindable) { + return io->data->callback->rewindable(io->data); return snd_pcm_mmap_hw_avail(pcm); }

static snd_pcm_sframes_t snd_pcm_ioplug_forwardable(snd_pcm_t *pcm) { ioplug_priv_t *io = pcm->private_data; + if (io->data->callback->forwardable) { + return io->data->callback->forwardable(io->data); return snd_pcm_mmap_avail(pcm); }

and add pulse_rewindable() and pulse_forwardable() to alsa-plugin/pulse/pcm_pulse.c