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.
> > +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.
Got it, thanks.
> > @@ -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;
> > }
Thank you,
Fengguang
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