[alsa-devel] [RFC PATCH 1/11] expose sound device topology information

[Takashi Iwai]

Hi Clemens,

At Tue, 28 Aug 2012 00:28:30 +0200,
Clemens Ladisch wrote:
> 
> Hi,
> 
> these are some tentative patches to allow sound drivers to provide
> routing information to userspace.  (So far, snd-usb-audio has a partial
> implementation; I haven't yet had time for snd-hda-intel.  I haven't
> even looked at ASoC.)

Thanks for patches.  This brought up interesting discussions at
Plumbers.  We (kind of) agree that the media controller API can be
pretty suitable, and even Mark didn't grumble so much against it :)
It's a good sign.

> The media controller API <linux/media.h> allows prividing routing
> information, but its implementation does not quite fit the sound
> drivers:
> * it allows reconfiguration of links, but not in a way that would be
>   useful for ALSA selector controls;
> * it allows drivers to access the entity graph and pipelines, which is
>   not needed;
> * it is rather heavyweight.
> 
> Therefore, these patches create their own implementation of the media
> controller API.  In this implementation, all entity and link information
> is completely static, and more space-efficient.

I think this is fine for USB-audio and HD-audio.  We don't expect that
the changes are done via media controller API but it'll provide just
some mapping.  But, possible embedded guys might want to make it
configurable.  (Well, in that case, they can reimplement the full
stack based on the original media controller helper code...)
Maybe Mark can give more comments on it.

> For simplicity, the ioctls hook into the ALSA control device instead
> of creating a new /dev/media* device.

This is OK, I think.  But what about multiple codecs on a single
control device?  Can they be represented well?


> The separate implementation does not allow sharing of one media
> controller device in the case of combined audio/video devices.  However,
> separate drivers would already be a problem for HDMI outputs (GPU and
> sound); it appears we might need some mechanism to connect the
> topologies of multiple media devices.

The connection between different devices could be done in a different
way.  For example, we may provide some Linux device tree connection.
(This reminds me of a TODO -- the implementation of standard Linux
 device model to ALSA components, BTW.)

> TLVs (for jack entities, and for associating controls with entities)
> are not yet defined.

OK, this is a big missing piece.  But it should be relatively easy.


Takashi

> Example output for my simple UA-1A device (not all information is
> decoded correctly):
> 
> $ media-ctl -p -d /dev/snd/controlC5
> Opening media device /dev/snd/controlC5
> Enumerating entities
> Found 4 entities
> Enumerating pads and links
> Media controller API version 0.0.0
> 
> Media device information
> ------------------------
> driver          snd-usb-audio
> model           EDIROL UA-1A
> serial
> bus info        usb-0000:00:12.2-4.3
> hw revision     0x101
> driver version  0.0.0
> 
> Device topology
> - entity 1: USB Audio (1 pad, 1 link)
>             type Node subtype ALSA
>             device node name /dev/tty             (should be hw:5,0,0)
>         pad0: Source
>                 -> "OT3":0 [ENABLED,IMMUTABLE]
> 
> - entity 3: OT3 (1 pad, 1 link)
>             type Unknown subtype Unknown          (should be Jack)
>         pad0: Sink
>                 <- "USB Audio":0 [ENABLED,IMMUTABLE]
> 
> - entity 4: IT4 (1 pad, 1 link)
>             type Unknown subtype Unknown
>         pad0: Source
>                 -> "USB Audio":0 [ENABLED,IMMUTABLE]
> 
> - entity 7: USB Audio (1 pad, 1 link)
>             type Node subtype ALSA
>             device node name /dev/tty
>         pad0: Sink
>                 <- "IT4":0 [ENABLED,IMMUTABLE]
> 
> 
>  include/linux/media.h     |    9
>  include/sound/core.h      |   13 +
>  include/sound/media.h     |  116 ++++++++++
>  include/sound/pcm.h       |    6
>  sound/core/Kconfig        |    7
>  sound/core/Makefile       |    1
>  sound/core/init.c         |    4
>  sound/core/media.c        |  425 ++++++++++++++++++++++++++++++++++++++
>  sound/core/pcm.c          |   46 ++++
>  sound/core/sound.c        |    3
>  sound/pci/hda/hda_intel.c |    2
>  sound/usb/card.c          |    2
>  sound/usb/mixer.c         |  278 ++++++++++++++++++++++++
>  sound/usb/quirks.c        |    4
>  sound/usb/stream.c        |   16 -
>  sound/usb/stream.h        |    1
>  16 files changed, 925 insertions(+), 8 deletions(-)
> 
> 
> Regards,
> Clemens
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