[alsa-devel] [RFC/PATCH v6 03/12] media: Entities, pads and links

Mark Brown broonie at opensource.wolfsonmicro.com
Thu Nov 25 16:28:28 CET 2010

On Thu, Nov 25, 2010 at 04:21:38PM +0100, Laurent Pinchart wrote:
> On Thursday 25 November 2010 10:38:05 Clemens Ladisch wrote:

> > ALSA has PCM and MIDI devices, and several types of mixer controls.
> > (It also has hardware dependent and timer devices, but I don't think
> > these would need topology information.)  So we need at least these:

> I agree about PCM and MIDI, but I'm not sure about controls. If controls are 
> part of an entity, the entity will be reported through the media controller 
> API. If information about that entity can be queried through existing APIs 
> (ALSA, V4L, ...) those APIs should be used. For instance, on the V4L side, 
> V4L2 sub-devices are mapped to entities and also have a device node which can 
> be used to enumerate the controls supported by the subdev. The media 
> controller only reports the entity -> major:minor mapping to let applications 
> find the device and query it directly.

For audio we don't currently have a sensible API for associating
controls with any sort of map of how the device is laid out, userspace
has to play guessing games.

> I think we will need a new ioctl in the media controller API to report 
> advanced information about an entity. This could be used to report controls 
> implemented by an ALSA element if ALSA doesn't provide a way to do that 
> directly. Another use case, which make me think that such an ioctl would be 
> needed, is to report UVC extension units type (16-byte GUID) to userspace.

That seems reasonable.

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