[alsa-devel] ALSA PulseAudio plugin: snd_pcm_rewindable() returns >0, but snd_pcm_rewind() is broken

Nikolay Nikolov nickysn at gmail.com
Mon Oct 24 18:55:34 CEST 2011

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).
> 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 

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