[alsa-devel] [PATCH] Wrong latency in pulseaudio plugin breaks Adobe Flash Player

[Philip Spencer]

On Thu, 23 Feb 2012, Clemens Ladisch wrote:

> Philip Spencer wrote:
>>     3. What should happen?
>>
>>            (a) The call returns immediately, since there's lots of room
>>                in the buffer.
>>
>>            (b) The call returns after one I/O period's worth of data has
>>                been drained from the buffer.
>
> (a)
>
>> If the correct answer is (a) (ALSA's current behaviour), then is there
>> any mechanism for an app to achieve (b) -- get woken up after one I/O
>> period's worth of data have been drained (other than just doing some
>> infinite loop of sleeping then checking snd_pcm_avail periodically)?
>
> Adjust avail_min (but note that wakeups happen only at period
> boundaries).

That won't work if using the pulseaudio plugin as the backend -- it'll 
stop the first part of snd_pcm_wait from succeeding right away, but when 
it moves on to snd_pcm_wait_nocheck the poll call will return right away, 
because the pulseaudio plugin marks the file descriptor as ready whenever 
there are more than buffer_attr.minreq bytes available in the buffer.

Perhaps (though that won't help me, as I don't imagine I'll have any 
success getting Flash Player developers to change anything!, but it may
help any other similar app) that should be fixed in the pulseaudio plugin:

   - Implement sw_params

   - When sw_params is called, check the avail_min value and record it
      as a new field avail_min (and if it's different from the previous
      recorded value and the stream is running, call update_active)

   - In check_active, instead of "return wsize >= pcm->buffer_attr.minreq",
       "return wsize >= pcm->avail_min". (Question: should
       buffer_attr.minreq be changed too, or left alone?)

   - At the same time the start_threshold parameter could be implemented
     too.

I could whip up a patch for that if needed, although of course it won't 
actually help my situation -- I may just have to be resigned to using
non-live mode with about a 3/4 second latency for linux users.

Regards,

Philip

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