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

[Takashi Iwai]

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.

> +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?

> +static void init_channel_allocations(void)
> +{
> +	int i, j;
> +	struct cea_channel_speaker_allocation *p;
> +
> +	for (i = 0; i < ARRAY_SIZE(channel_allocations); i++) {
> +		p = channel_allocations + i;

You should zero-initialize p->channels and p->spk_mask here...

> +		for (j = 0; j < ARRAY_SIZE(p->speakers); j++)
> +			if (p->speakers[j]) {
> +				p->channels++;
> +				p->spk_mask |= p->speakers[j];
> +			}
> +	}

... because patch_intel_hdmi() may be called multiple times (either
multiple codecs or reconfiguration via hwdep) as below.

> @@ -455,6 +642,8 @@ static int patch_intel_hdmi(struct hda_c
>  
>  	snd_hda_eld_proc_new(codec, &spec->sink);
>  
> +	init_channel_allocations();
> +
>  	return 0;
>  }


thanks,

Takashi