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

Mark Brown broonie at opensource.wolfsonmicro.com
Thu Nov 25 14:41:35 CET 2010

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:

> 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

> 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.

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