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

From: shuah
Date: Fri Jan 18 2019 - 20:03:32 EST

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@xxxxxxxxxx wrote:
From: Shuah Khan <shuah@xxxxxxxxxx>

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

Media specific cleanup is done in usb_audio_disconnect().

Signed-off-by: Shuah Khan <shuah@xxxxxxxxxx>
 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


+int snd_media_device_create(struct snd_usb_audio *chip,
+ÂÂÂÂÂÂÂÂÂÂÂ struct usb_interface *iface)
+ÂÂÂ struct media_device *mdev;
+ÂÂÂ struct usb_device *usbdev = interface_to_usbdev(iface);
+ÂÂÂ int ret;
+ÂÂÂ /* usb-audio driver is probed for each usb interface, and
+ÂÂÂÂ * there are multiple interfaces per device. Avoid calling
+ÂÂÂÂ * media_device_usb_allocate() each time usb_audio_probe()
+ÂÂÂÂ * is called. Do it only once.
+ÂÂÂÂ */
+ÂÂÂ if (chip->media_dev)
+ÂÂÂÂÂÂÂ goto snd_mixer_init;
+ÂÂÂ mdev = media_device_usb_allocate(usbdev, KBUILD_MODNAME);
+ÂÂÂ if (!mdev)
+ÂÂÂ 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.

snd_media_device_create() could get called after media_device_register()
happens, if au0828 registers it first.

I did find a problem in snd_media_device_create() and I will send a v10
with that fix.

-- Shuah