[alsa-devel] [RFC] ASOC: HDMI audio info frame speaker allocation

[Takashi Iwai]

On Wed, 30 Nov 2016 23:27:22 +0100,
Pierre-Louis Bossart wrote:
> 
> On 11/30/16 11:54 AM, Arnaud Pouliquen wrote:
> > Hello,
> >
> > I'm trying to understand how to implement the speaker allocation for
> > HDMI output.
> >
> > input:
> >  - HDMI sink provides the speaker presence in ELD structure
> >  - audio source can also contain channels position.
> >
> > output:
> >  Audio info frame (IF) should contain audio channel allocation (
> > hdmi_audio_infoframe structure).
> >
> > So if i well understand the concept, application should be in charge of
> > aligning channels with HDMI sink speakers...
> 
> Yes. However some HDMI IPs provide hardware means to swap the channels
> - 
> mainly because the order of channels varies between OSes and encoding
> schemes. The notion of 'aligning' might be limited in those cases to
> setting the relevant ALSA controls, not necessarily an active software
> channel swap and change of the channel map handled by the application.

The application may just leave the channel mapping as the driver
default, too.  It's a freedom for user-space whether to accept the
default channel mapping as is, or choose the own preferred one.

For some IPs that can't handle the channel maps fully, the limitation
can be implemented in the driver side, I suppose.  It just needs to
pass the limited set of channel-map array the hardware may accept,
instead of the arrays that are created by parsing the ELD.

> > Regarding HDA driver, audio IF channel allocation is computing based on
> > "Playback Channel Map"control . Application set the speaker information
> > of the audio IF through this control.
> >
> > On ASOC side, in hdmi-codec,
> >  -'ELD" control exists  to export speaker information to application
> >  - info frame channel allocation is hardware coded in
> > hdmi_codec_hw_params. The configuration is set by default depending on
> > the number of channels. Drawback is that application is not aware of the
> > configuration selected by the driver.
> >
> > So it seems that something is missing to do the link between EDID
> > information and audio source channel mapping on one side and audio IF
> > configuration on other side.
> >
> > What should be the best strategy to support feature in ASoC?
> > - No update in hdmi-codec, consider that application should know
> > the configuration available for 2,4,6 and 8 channels
> >   => seems not very flexible...
> >
> > - Update hdmi-codec to implement "Playback Channel Map" in read-only, to
> > provide channel mapping information to application.
> >  => impact is light, but application can get the correct mapping only
> > after the update of the number of channels (so after hw_params ops call)
> >
> > - Update hdmi-codec to implement "Playback Channel Map" in read and
> > write access.
> >  => HDA implementation. Seems the more flexible solution from my windows...
> >
> > - Implement control in drm drivers
> >   => seems not reasonable.
> >
> > - Other?

We can start from the read-only chmap ctls, with a plan to enhance
to the read/write later on.  I guess only few applications require the
channel map change, and the hardest part is the proper integration in
ASoC, not the write support itself.


thanks,

Takashi