[alsa-devel] rate plugin issue

Stas Sergeev stsp at aknet.ru
Wed Nov 14 21:10:02 CET 2007


Takashi Iwai wrote:
> The problem is that the condition of poll() is indeed broken without
> the hack.  That is, it's no problem of snd_pcm_write_areas().
> snd_pcm_write_areas() just calls snd_pcm_wait(), which calls nothing
> but poll().  If your program calls poll() by itself, you would see the
> similar problem.
Yes. But I am not sure how should
it behave. For example, if the prog
sets avail_min=1, then it should
expect the snd_pcm_wait() to no longer
work. But I think the write() should
not be affected. Yes, snd_pcm_write_areas()
just calls snd_pcm_wait(), but should it?
Should write() depend on avail_min, or
only snd_pcm_wait() should?
If write() should depend on avail_min
(which doesn't look logical to me), then
this does imply mpg123 is doing the wrong
thing by setting avail_min=1? And so,
what would be the fix?

> True, it should be fixed in a better way.
Unfortunately, I have no idea how's the
better fix should look like.
My patch implements exactly the logic
I had in mind: leave snd_pcm_wait() to
depend on avail_min, but make writei()
to not depend on it (use period_size
instead). Since you don't agree with that
logic, please let me know what logic you
Furthermore, as you said earlier, avail_min
doesn't really work as expected. It gets
"rounded" to period_size anyway, because
that's where the interrupt arrives. Taking
that into account, I can propose the following:
deprecate avail_min and always set it to
period_size, no matter what the program
thinks. That will pretty much return the
old behaveour (the hack), although this
time - without the underruns.
But I don't suppose you are going to like
also this approach. :)

> BTW, could you provide a bit more information how to reproduce the
> bug?
Simply play some mp3 with mpg123, run "top".

> (Or, a test-case code would be best, as you know  ;) 
Well, stripping down the mpg123 code is
not a rocket science, I can do that. But
I am a bit busy at the moment, this will
have to wait a few days. But I think there
are really no problems reproducing it. :)

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