linux-kernel - Re: [PATCH v9 4/4] sound/usb: Use Media Controller API to share media resources

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Date:   Fri, 18 Jan 2019 14:54:18 -0700
From:   shuah <shuah@...nel.org>
To:     Hans Verkuil <hverkuil@...all.nl>, mchehab@...nel.org,
        perex@...ex.cz, tiwai@...e.com
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        alsa-devel@...a-project.org, shuah <shuah@...nel.org>
Subject: Re: [PATCH v9 4/4] sound/usb: Use Media Controller API to share media
 resources

On 1/18/19 1:36 AM, Hans Verkuil wrote:
> On 12/18/18 6:59 PM, shuah@...nel.org wrote:
>> From: Shuah Khan <shuah@...nel.org>
>>
>> Media Device Allocator API to allows multiple drivers share a media device.
>> This API solves a very common use-case for media devices where one physical
>> device (an USB stick) provides both audio and video. When such media device
>> exposes a standard USB Audio class, a proprietary Video class, two or more
>> independent drivers will share a single physical USB bridge. In such cases,
>> it is necessary to coordinate access to the shared resource.
>>
>> Using this API, drivers can allocate a media device with the shared struct
>> device as the key. Once the media device is allocated by a driver, other
>> drivers can get a reference to it. The media device is released when all
>> the references are released.
>>
>> Change the ALSA driver to use the Media Controller API to share media
>> resources with DVB, and V4L2 drivers on a AU0828 media device.
>>
>> The Media Controller specific initialization is done after sound card is
>> registered. ALSA creates Media interface and entity function graph nodes
>> for Control, Mixer, PCM Playback, and PCM Capture devices.
>>
>> snd_usb_hw_params() will call Media Controller enable source handler
>> interface to request the media resource. If resource request is granted,
>> it will release it from snd_usb_hw_free(). If resource is busy, -EBUSY is
>> returned.
>>
>> Media specific cleanup is done in usb_audio_disconnect().
>>
>> Signed-off-by: Shuah Khan <shuah@...nel.org>
>> ---
>>   sound/usb/Kconfig        |   4 +
>>   sound/usb/Makefile       |   2 +
>>   sound/usb/card.c         |  14 ++
>>   sound/usb/card.h         |   3 +
>>   sound/usb/media.c        | 321 +++++++++++++++++++++++++++++++++++++++
>>   sound/usb/media.h        |  74 +++++++++
>>   sound/usb/mixer.h        |   3 +
>>   sound/usb/pcm.c          |  29 +++-
>>   sound/usb/quirks-table.h |   1 +
>>   sound/usb/stream.c       |   2 +
>>   sound/usb/usbaudio.h     |   6 +
>>   11 files changed, 455 insertions(+), 4 deletions(-)
>>   create mode 100644 sound/usb/media.c
>>   create mode 100644 sound/usb/media.h
>>
> 
> <snip>
> 
>> +int snd_media_device_create(struct snd_usb_audio *chip,
>> +			struct usb_interface *iface)
>> +{
>> +	struct media_device *mdev;
>> +	struct usb_device *usbdev = interface_to_usbdev(iface);
>> +	int ret;
>> +
>> +	/* usb-audio driver is probed for each usb interface, and
>> +	 * there are multiple interfaces per device. Avoid calling
>> +	 * media_device_usb_allocate() each time usb_audio_probe()
>> +	 * is called. Do it only once.
>> +	 */
>> +	if (chip->media_dev)
>> +		goto snd_mixer_init;
>> +
>> +	mdev = media_device_usb_allocate(usbdev, KBUILD_MODNAME);
>> +	if (!mdev)
>> +		return -ENOMEM;
>> +
>> +	if (!media_devnode_is_registered(mdev->devnode)) {
> 
> It looks like you missed my comment for v8:
> 
> "You should first configure the media device before registering it."
> 
> In other words, first create the media entities, and only then do you
> register the media device. Otherwise it will come up without any alsa
> entities, which are then added. So an application that immediately
> opens the media device upon creation will see a topology that is still
> in flux.

Yes. You are right. I saw your comment and thought I got it addressed.
I will fix it. I have the logic correct in au0828, but not here.

Thanks,
-- Shuah