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

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Date:	Wed, 22 Oct 2014 15:45:42 -0400
From:	Devin Heitmueller <dheitmueller@...nellabs.com>
To:	Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:	Takashi Iwai <tiwai@...e.de>, alsa-devel@...a-project.org,
	Lars-Peter Clausen <lars@...afoo.de>,
	Linux Media Mailing List <linux-media@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"ramakrmu@...co.com" <ramakrmu@...co.com>,
	Shuah Khan <shuahkh@....samsung.com>,
	Hans Verkuil <hverkuil@...all.nl>,
	Sander Eikelenboom <linux@...elenboom.it>,
	Prabhakar Lad <prabhakar.csengg@...il.com>,
	Antti Palosaari <crope@....fi>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	"sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tim Gardner <tim.gardner@...onical.com>,
	"olebowle@....com" <olebowle@....com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Mauro Carvalho Chehab <m.chehab@...sung.com>
Subject: Re: [alsa-devel] [PATCH v2 5/6] sound/usb: pcm changes to use media
 token api

> 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.

Devin

-- 
Devin J. Heitmueller - Kernel Labs
http://www.kernellabs.com
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