Re: [alsa-devel] [PATCH v2 5/6] sound/usb: pcm changes to use media token api

[Shuah Khan <shuahkh@osg.samsung.com>]

On 10/22/2014 01:45 PM, Devin Heitmueller wrote:
>> this seems like a feature, not a bug. PulseAudio starts streaming before
>> clients push any data and likewise keeps sources active even after for some
>> time after clients stop recording. Closing VLC in your example doesn't
>> immediately close the ALSA device. look for module-suspend-on-idle in your
>> default.pa config file.
> 
> The ALSA userland emulation in PulseAudio is supposed to faithfully emulate
> the behavior of the ALSA kernel ABI... except when it doesn't, then it's not
> a bug but rather a feature.  :-)
> 
>> I also agree that the open/close of the alsa device is the only way to
>> control exclusion.
> 
> I was also a proponent that we should have fairly coarse locking done
> at open/close for the various device nodes (ALSA/V4L/DVB).  The challenge here
> is that we have a large installed based of existing applications that
> rely on kernel
> behavior that isn't formally specified in any specification.  Hence
> we're forced to try
> to come up with a solution that minimizes the risk of ABI breakage.
> 
> If we were doing this from scratch then we could lay down some hard/fast rules
> about things apps aren't supposed to do and how apps are supposed to respond
> to those exception cases.  Unfortunately we don't have that luxury here.
> 

Sounds like we don't have a clear direction on open/close or capture
start/stop. What I am hearing is open/close isn't acceptable for
media maintainers and capture trigger start/stop isn't acceptable
to sound maintainers. :) Fork in the road, which way do we go?

Implementation wise, supporting capture trigger start/stop approach
will be harder to maintain in longterm. It adds more variables to
the mix. Applications open sounds device from the main thread and
then create a new thread to handle streams. I can see that based
on the token hold requests that come in. So the token hold logic
will have to take that into account, leading into potential
unbalanced lock/unlock scenarios. It is not impossible to solve,
that's what I did in this patch series, but it does get complex.

What I am looking for is some consensus on let's go with an approach
and try. Doesn't matter which way we go, and how much testing I do,
I am bound to miss something and this work needs to soak for a bit in
the media experimental branch.

thanks,
-- Shuah



-- 
Shuah Khan
Sr. Linux Kernel Developer
Samsung Research America (Silicon Valley)
shuahkh@osg.samsung.com | (970) 217-8978