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

On Thursday 25 November 2010 14:41:35 Mark Brown wrote:
> On Thu, Nov 25, 2010 at 10:38:05AM +0100, Clemens Ladisch wrote:
> > In USB and HD audio devices, all links are immutable, and the routing
> > is controlled by 'selector' entities that activate exactly one of their
> > input pads. In userspace, this entity shows up as a mixer control.
> > I guess it would be possible to map the ACTIVE flag onto these controls.
>
> Ditto for ASoC, mostly.
>
> > Alternatively, entities can have 'mute' mixer controls associated with
> > their pads. In this case, multiple unmuted inputs would be mixed
> > together.
> >
> > 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:
> > MEDIA_ENTITY_TYPE_NODE_ALSA_PCM
> > MEDIA_ENTITY_TYPE_NODE_ALSA_MIDI
> > MEDIA_ENTITY_TYPE_SUBDEV_ALSA_CONTROL
> >
> > Furthermore, topology information is also needed for entities not
> > associated with a mixer control, such as microphones, speakers, jacks/
> > connectors, and effect units. These entities are defined in the USB and
> > HD audio specifications, but are not yet handled by ALSA.
>
> All this and more in the embedded case - digital audio link nodes and DSP
> I/O nodes (could possibly do those as digital audio ones) spring to
> mind. Also bear in mind that embedded devices can get *very* large - a
> mobile phone audio system can have of the order of 100 nodes in the
> graph.

It depends on how you define nodes. I can certainly imagine a graph with 100
controls, but maybe several controls can be part of the same node ? On the
video side we've decided to split entities depending on the possible data
paths configurations. As I'm not a fluent ascii-art speaker, please have a
look at pages 4 and 5 of http://www.ideasonboard.org/media/20101103-lpc-
media.pdf

Page 4 shows the internal topology of the OMAP3 ISP. The major blocks in that
diagram are reported as entities. Page 5 shows the internal topology of one of
the blocks, the OMAP3 ISP preview engine. As you can see the pipeline is made
of sub-blocks that implement a single image processing function. As the
pipeline is linear (don't worry about the non-linear part in the beginning,
it's just there to take into account link configurability at the higher level)
we don't export all the sub-blocks as entities, but we expose the controls on
the preview engine entity instead.

> > ALSA devices are not addressed by their device node but with card/device/
> >
> > subdevice numbers; mixer controls have numeric IDs, unique per card:
> > struct {
> >
> > int card;
> > int device;
> > int subdevice;
> >
> > } alsa_device;
> > struct {
> >
> > int card;
> > int numid;
> >
> > } alsa_control;
>
> For the embedded stuff we also have a bunch of stuff in the graph which
> may not be visible to userspace at all at present and would just have a
> string based identifier.

That could be easily added (provided the string is not too long).

--
Regards,

Laurent Pinchart


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