[alsa-devel] Polling issues with the PCM multi plugin

Devin Anderson surfacepatterns at gmail.com
Fri Dec 14 04:43:25 CET 2012

On Wed, Dec 12, 2012 at 11:25 PM, Takashi Iwai <tiwai at suse.de> wrote:
> At Wed, 12 Dec 2012 23:04:16 -0800,
> Devin Anderson wrote:
>> Are the apps really doing the wrong thing?  It seems like
>> `snd_pcm_poll_descriptors()` and `snd_pcm_poll_descriptor_revents()`
>> are meant to operate on a single group of file descriptors, without
>> interpreting what the availability of events on each particular file
>> descriptor might mean.  This leads to some problems.
> Yes, grouping multiple descriptors gives ambiguity.  It must be a part
> of the problems.
>> As an example, let's consider what the multi plugin should do when
>> `snd_pcm_poll_descriptors_revents()` is called.  It basically has two
>> options:
>> 1.) Return I/O events when one or more file descriptors have I/O events
>> This is obviously problematic, as there may only be partial data
>> available.  This is also how the multi plugin works today.
> Right, and this behavior hasn't been changed, AFAIK.
>> 2.) Return I/O events when all file descriptors have I/O events.
>> In this scenario, `poll()` will return when events are available on at
>> least one poll descriptor, leading to busy waiting via `poll()` until
>> I/O events are available on all file descriptors.
> Never implemented in such a way, so far, because this can very easily
> lead to XRUN.

So, how can this problem be solved?

I have an idea that I'd like to explore, but it's dependent on the
idea that `snd_pcm_poll_descriptors()` needs to be called before every
`poll()` call, and I'm not sure if that's a reasonable expectation.
Is `snd_pcm_poll_descriptors()` a function you can call once, and then
depend on working through multiple `poll()` calls, or is it required
that `snd_pcm_poll_descriptors()` be called for every call to refresh
the descriptors and the events they should be polled for?


Devin Anderson
surfacepatterns (at) gmail (dot) com

blog - http://surfacepatterns.blogspot.com/
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