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Date: Fri, 24 Oct 2014 08:44:46 -0600 From: Shuah Khan <shuahkh@....samsung.com> To: Devin Heitmueller <dheitmueller@...nellabs.com>, Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>, Takashi Iwai <tiwai@...e.de>, Lars-Peter Clausen <lars@...afoo.de>, Hans Verkuil <hverkuil@...all.nl>, Mauro Carvalho Chehab <m.chehab@...sung.com> CC: alsa-devel@...a-project.org, Linux Media Mailing List <linux-media@...r.kernel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 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> Subject: Re: [alsa-devel] [PATCH v2 5/6] sound/usb: pcm changes to use media token api 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@....samsung.com | (970) 217-8978 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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