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

[shuah <shuah@kernel.org>]

On 1/19/19 3:30 AM, Hans Verkuil wrote:
> On 01/19/2019 02:03 AM, shuah wrote:
>> On 1/18/19 2:54 PM, shuah wrote:
>>> On 1/18/19 1:36 AM, Hans Verkuil wrote:
>>>> On 12/18/18 6:59 PM, shuah@kernel.org wrote:
>>>>> From: Shuah Khan <shuah@kernel.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@kernel.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.
>>>
>>
>> One thing to mention here is some ALSA entities get created dynamically
>> during PCM open when stream is initialized. This happens after media
>> device is registered. These get deleted when pcm close happens. There is
>> no way to avoid creating entities after media device register.
> 
> That's OK. We're missing some infrastructure (media events) to inform the
> application about topology changes.
> 
> But when registering the media device for the first time it is good practice
> to do that after creating all the entities that you can.
> 

Great. We have the same understanding. I think at least one mixer
controller entries can be created prior to registering the media
device. I am working on v10 to do that. Sorry I misread init sequence
in this patch the flow the first time around for v9.

thanks,
-- Shuah