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

[Alexander E. Patrakov]

14.09.2014 01:31, Alexander E. Patrakov wrote:
> 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.

Well, after looking again, I see that the multi plugin became especially 
problematic. Previously, it did not forward hwsync requests to slaves 
other than master_slave. Now it does.

Please revert PATCH 8/9. It needs more discussion.

>
> 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