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

[Shuah Khan]

On 10/21/2014 10:05 AM, Takashi Iwai wrote:
> At Tue, 21 Oct 2014 17:42:51 +0200,
> Hans Verkuil wrote:

>>
>> Quite often media apps open the alsa device at the start and then switch
>> between TV, radio or DVB mode. If the alsa device would claim the tuner
>> just by being opened (as opposed to actually using the tuner, which happens
>> when you start streaming),
> 
> What about parameter changes?  The sound devices have to be configured
> before using.  Don't they influence on others at all, i.e. you can
> change the PCM sample rate etc during TV, radio or DVB is running?

Yes. kaffeine uses  snd_usb_capture_ops ioctl -> snd_pcm_lib_ioctl

Other v4l and vlc (dvb) uses open/close as well as trigger start and
stop. trigger start/stop is done by a special audio thread in some
cases. open/close happens from the main thread.

> 
>> then that would make it impossible for the
>> application to switch tuner mode. In general you want to avoid that open()
>> will start configuring hardware since that can quite often be slow. Tuner
>> configuration in particular can be slow since several common tuners need
>> to load firmware over i2c. You only want to do that when it is really needed,
>> and not when some application (udev!) opens the device just to examine what
>> sort of device it is.
> 
> But most apps close the device soon after that, no?
> Which programs keep the PCM device (not the control) opened without
> actually using?
> 
>> So claiming the tuner in the trigger seems to be the right place. If
>> returning EBUSY is a poor error code for alsa, then we can use something else
>> for that. EACCES perhaps?
> 
> Sorry, I'm not convinced by that.  If the device has to be controlled
> exclusively, the right position is the open/close.  Otherwise, the
> program cannot know when it becomes inaccessible out of sudden during
> its operation.
> 
> 

Let me share my test matrix for this patch series. Hans pointed out
one test case I didn't know about as a result missed testing. Please
see if any of the tests miss use-cases or break them: you can scroll
down to the proposal at the end, if this is too much detail :)

Digital active and analog starting testing:
kaffeine running
- v4l2-ctl --all - works
- Changing channels works with the same token hold, even when
  frequency changes. Tested changing channels that force freq
  change.
- vlc resource is busy with no disruption to kaffeine
- xawtv - tuner busy when it tries to do ioctls that change
  tuner settings - snd_usb_pcm_open detects device is busy
  ( pcm open called from the same thread, trigger gets called
    from another thread )
- tvtime - tuner busy when it tries to do ioctls that change
  tuner settings with no disruption to kaffeine
  ( pcm open called from the same thread, trigger gets called
    from another thread )
- vlc - audio capture on WinTV HVR-950 - device is busy
  start vlc with no channels for this test
- arecord to capture on WinTV HVR-950 - device busy

vlc running
vlc -v channels.xspf
- v4l2-ctl --all - works
- Changing channels works with the same token hold, even when
  frequency changes. Tested changing channels that force freq
  change.
- kaffeine resource is busy with no disruption to vlc
- xawtv - tuner busy when it tries to do ioctls that change
  tuner settings - snd_usb_pcm_open detects device is busy
  ( pcm open called from the same thread, trigger gets called
    from another thread )
- tvtime - tuner busy when it tries to do ioctls that change
  tuner settings with no disruption to kaffeine
  ( pcm open called from the same thread, trigger gets called
    from another thread )
- vlc - audio capture on WinTV HVR-950 - device is busy
- arecord to capture on WinTV HVR-950 - device busy

Analog active and start digital testing:
xawtv -noalsa -c /dev/video1
- v4l2-ctl --all - works
- start kaffeine - fails with device busy and no disruption
- start vlc - fails with device busy and no disruption
- tvtime - tuner busy when it tries to do ioctls that change
  tuner settings with no disruption to kaffeine
- vlc - audio capture on WinTV HVR-950 - device is busy
- arecord to capture on WinTV HVR-950 - device busy

tvtime
- v4l2-ctl --all - works
- start kaffeine - fails with device busy and no disruption
- start vlc - fails with device busy and no disruption
- xawtv - tuner busy when it tries to do ioctls that change
  tuner settings with no disruption to kaffeine
- vlc - audio capture on WinTV HVR-950 - device is busy
- arecord to capture on WinTV HVR-950 - device busy

The following audio/video start/stop combination tests:
( used arecord as well to test these cases, arecord )

- tvtime start/vlc start/vlc stop/tvtime stop
  no disruption to tvtime
- tvtime start/vlc start/tvtie stop/vlc stop
  One tvtime stops, could trigger capture manually
- vlc start/tvtime start/tvtime stop/vlc stop
  vlc audio capture continues, tvtime detect tuner busy
- vlc start/tvtime start/vlc stop/tvtime start
  when vlc stops, tvtime could open the tuner device

Repeated the above with kaffeine and vlc and arecord audio
capture combinations.

Hans pointed out I am missing:

v4l2-ctl -f 180 --sleep=10
While it is sleeping you must still be able to set the frequency from
another console.

And it doesn't matter which of the two v4l2-ctl processes ends first,
as long as one has a reference to the tuner you should be blocked
from switching the tuner to a different mode (radio or dvb)

I think this above fails because tuner token ownership doesn't
span processes. Token should act as a lock between dvb/audio/v4l,
and v4l itself has mechanisms to handle the multiple ownership token
shouldn't interfere with.

Provided the only thing that is breaking is the v4l case, I think
I can get it to work with the bit field approach.

Here is what I propose for patch v3:
- make it a module under media
- collapse tuner and audio tokens into media token
- change names (get rid of abbreviated tkn stuff)
- Make other changes Takashi/Lars pointed out in pcm
- hold token in pcm open/close
- add a bitfield to struct v4l2_fh to handle
  the v4l specific multiple ownership cases.
- v4l-core and au0828-videp patches will see changes.
- dvb patch should still be good (crossing fingers)

thanks,
-- Shuah

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