[alsa-devel] [PATCH 0/9] Misc fixes related to rewinds

[Alexander E. Patrakov]

14.09.2014 01:14, Jaroslav Kysela wrote:
> Date 13.9.2014 20:30, Alexander E. Patrakov wrote:
>> The idea of the series is to fix the two issues that I found [1] for the
>
> I applied all your patches to alsa-lib's repo, but...
>
>> hw plugin. snd_pcm_rewindable() sometimes returned negative values that
>> are actually negative amounts of samples and not error codes. Also, it
>> bases its calculations on stale hardware position pointer, which is not
>> what PulseAudio wants (alternatively, we can document the need to call
>> snd_pcm_avail() before snd_pcm_rewindable(), but I don't like it).
>
> The hw sync is expensive and the application might do this sync multiple
> times when woken up. I think that it must be clear that:
>
> 1) only snd_pcm_avail(), snd_pcm_delay(), snd_pcm_avail_delay()
>     does the real hw sync
> 2) snd_pcm_avail(), snd_pcm_delay(), snd_pcm_avail_delay(),
>     snd_pcm_rewindable() and snd_pcm_forwardable() does
>     hw sync (and change all plugins to respect this)
>
> I don't like the situation "be somewhere between because it's good for
> one purpose"...

I understand the concern. I have specifically not added the call to 
hwsync directly to snd_pcm_rewindable implementation (although it would 
have resulted in a smaller patch), because that would indeed cause 
double-hwsync and the resulting inefficiency. I made sure that all 
plugins either make the hwsync thing themselves or rely on the slave to 
do that for them, but not both. If you find an error and/or spot a case 
of a double-hwsync in a plugin chain, please complain.

One known case of double-hwsync is the following pattern: an application 
calls snd_pcm_rewindable(), thinks about it, and then calls 
snd_pcm_rewind(). Which, due to PATCH 2/9, calls the rewindable callback 
again, resulting in the second hwsync. I don't know which way out is 
best: either ignore, or revert the intention of PATCH 2/9, or revert the 
whole PATCH 8/9 and replace it with a documentation change.

OTOH, I made a mistake of not adding David Henningsson to the CC list 
during the initial submission. If PulseAudio would need to synchronize 
hardware pointers even after conversion to snd_pcm_rewindable() for some 
other reason, then the need for PATCH 8/9 is not that obvious, and maybe 
it should be reverted and replaced with a documentation fix.

-- 
Alexander E. Patrakov