[alsa-devel] [PATCH 3/4] hda: HDMI channel allocations for audio infoframe

[Takashi Iwai]

At Tue, 18 Nov 2008 19:08:18 +0800,
Wu Fengguang wrote:
> 
> On Tue, Nov 18, 2008 at 11:57:08AM +0100, Takashi Iwai wrote:
> > At Tue, 18 Nov 2008 16:57:16 +0800,
> > Wu Fengguang wrote:
> > > 
> > > To play a 3+ channels LPCM/DSD stream via HDMI,
> > > 
> > > 	- HDMI sink must tell HDMI source about its speaker placements
> > > 	  (via ELD, speaker-allocation field)
> > > 	- HDMI source must tell the HDMI sink about channel allocation
> > > 	  (via audio infoframe, channel-allocation field)
> > > 
> > > (related docs: HDMI 1.3a spec section 7.4, CEA-861-D section 7.5.3 and 6.6)
> > > 
> > > This patch attempts to set the CA(channel-allocation) byte in the audio infoframe
> > > according to
> > > 	- the number of channels in the current stream
> > > 	- the speakers attached to the HDMI sink
> > > 
> > > A channel_allocations[] line must meet the following two criterions to be
> > > considered as a valid candidate for CA:
> > > 	1) its number of allocated channels = substream->runtime->channels
> > > 	2) its speakers are a subset of the available ones on the sink side
> > > 
> > > If there are multiple candidates, the first one is selected.  This simple
> > > policy shall cheat the sink into playing music, but may direct data to the
> > > wrong speakers.
> > > 
> > > Sorry, this last step is not obvious to me. Any domain experts, please?
> > 
> > The problem is that the speaker positioning isn't defined in the ALSA
> > API yet.  I think your implementation will work in practice.
> 
> OK. Let's forget it until (possibly) some one complains ;-)
> 
> > > +static struct cea_channel_speaker_allocation channel_allocations[] = {
> > > +/* channel number:  8      7     6     5     4      3     2     1 */
> > 
> > Do we need to set ca_index explicitly?
> > Any case ca_index != array index?
> 
> Yes, since we stop at the first matching item, the lines could be
> reordered somewhere to give priority to some popular ones.

Fair enough.  One thing I forgot is the initialization style.
Try to initialize in C99 style, something like

static struct cea_channel_speaker_allocation channel_allocations[] = {
	{ .ca_index = 0x00,
	  .speakers = { ..... } },   
	{ .ca_index = 0x01,
	  .speakers = { ..... } },   
	...
};

You can indent as you like, though.


thanks,

Takashi