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