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

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Message-ID: <s5htx34ul3w.wl-tiwai@suse.de>
Date:	Thu, 16 Oct 2014 16:48:03 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Shuah Khan <shuahkh@....samsung.com>
Cc:	Lars-Peter Clausen <lars@...afoo.de>, m.chehab@...sung.com,
	akpm@...ux-foundation.org, gregkh@...uxfoundation.org,
	crope@....fi, olebowle@....com, dheitmueller@...nellabs.com,
	hverkuil@...all.nl, ramakrmu@...co.com,
	sakari.ailus@...ux.intel.com, laurent.pinchart@...asonboard.com,
	perex@...ex.cz, prabhakar.csengg@...il.com,
	tim.gardner@...onical.com, linux@...elenboom.it,
	linux-kernel@...r.kernel.org, alsa-devel@...a-project.org,
	linux-media@...r.kernel.org
Subject: Re: [alsa-devel] [PATCH v2 5/6] sound/usb: pcm changes to use media token api

At Thu, 16 Oct 2014 08:39:14 -0600,
Shuah Khan wrote:
> 
> On 10/16/2014 08:16 AM, Takashi Iwai wrote:
> > At Thu, 16 Oct 2014 08:10:52 -0600,
> > Shuah Khan wrote:
> >>
> >> On 10/16/2014 08:01 AM, Takashi Iwai wrote:
> >>> At Thu, 16 Oct 2014 07:10:37 -0600,
> >>> Shuah Khan wrote:
> >>>>
> >>>> On 10/16/2014 06:00 AM, Lars-Peter Clausen wrote:
> >>>>> On 10/14/2014 04:58 PM, Shuah Khan wrote:
> >>>>> [...]
> >>>>>>       switch (cmd) {
> >>>>>>       case SNDRV_PCM_TRIGGER_START:
> >>>>>> +        err = media_get_audio_tkn(&subs->dev->dev);
> >>>>>> +        if (err == -EBUSY) {
> >>>>>> +            dev_info(&subs->dev->dev, "%s device is busy\n",
> >>>>>> +                    __func__);
> >>>>>
> >>>>> In my opinion this should not dev_info() as this is out of band error
> >>>>> signaling and also as the potential to spam the log. The userspace
> >>>>> application is already properly notified by the return code.
> >>>>>
> >>>>
> >>>> Yes it has the potential to flood the dmesg especially with alsa,
> >>>> I will remove the dev_info().
> >>>
> >>> Yes.  And, I think doing this in the trigger isn't the best.
> >>> Why not in open & close?
> >>
> >> My first cut of this change was in open and close. I saw pulseaudio
> >> application go into this loop trying to open the device. To avoid
> >> such problems, I went with trigger stat and stop. That made all the
> >> pulseaudio continues attempts to open problems go away.
> > 
> > But now starting the stream gives the error, and PA would loop it
> > again, no?
> > 
> > 
> >>> Also, is this token restriction needed only for PCM?  No mixer or
> >>> other controls?
> >>
> >> snd_pcm_ops are the only ones media drivers implement and use. So
> >> I don't think mixer is needed. If it is needed, it is not to hard
> >> to add for that case.
> > 
> > Well, then I wonder what resource does actually conflict with
> > usb-audio and media drivers at all...?
> > 
> 
> audio for dvb/v4l apps gets disrupted when audio app starts. For
> example, dvb or v4l app tuned to a channel, and when an audio app
> starts. audio path needs protected to avoid conflicts between
> digital and analog applications as well.

OK, then concentrating on only PCM is fine.

But, I'm still not convinced about doing the token management in the
trigger.  The reason -EBUSY doesn't work is that it's the very same
error code when a PCM device is blocked by other processes.  And
-EAGAIN is interpreted by PCM core to -EBUSY for historical reasons.

How applications (e.g. PA) should behave if the token get fails?
Shouldn't it retry or totally give up?


Takashi
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