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

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

On 10/16/2014 08:48 AM, Takashi Iwai wrote:
> 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.

ah. ok your recommendation is to go with open and close.
Mauro has some reservations with holding at open when I discussed
my observations with pulseaudio when I was holding token in open
instead of trigger start. Maybe he can chime with his concerns.
I think his concern was breaking applications if token is held in
open().

Based on what you are seeing trigger could be worse.

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

It would be up to the application I would think. I see that arecord
quits right away when it finds the device busy. pluseaudio on the other
hand appears to retry. I downloaded pulseaudio sources to understand
what it is doing, however I didn't get too far. The way it does audio
handling is complex for me to follow without spending a lot of time.

thanks,
-- Shuah

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