[alsa-devel] radeon multi-channel lpcm

Pierre Ossman pierre-list at ossman.eu
Wed Sep 5 19:22:39 CEST 2012

On Wed, 05 Sep 2012 18:39:01 +0200
Takashi Iwai <tiwai at suse.de> wrote:

> At Wed, 5 Sep 2012 18:19:23 +0200,
> Pierre Ossman wrote:
> > 
> > Noone with any insight? I'm ready to do some leg work, but I couldn't
> > find anything in either the HDA specification, or the AMD
> > documentation. :/
> Well, HD-audio HDMI codec driver just extracts the available channel
> setup from the given ELD, and the ELD data is sent from the graphics
> driver.  So, if anything missing in ELD, it's supposed to be in the
> graphics driver side.  In other words, if ELD doesn't give 6 channels,
> HD-audio codec won't follow that channel setup, too.

I see. I got a bit confused by this code though:

	/* Restrict capabilities by ELD if this isn't disabled */
	if (!static_hdmi_pcm && eld->eld_valid) {

Which made me think that the ELD was just used to make sure you didn't
send something the other end couldn't handle. IOW, eld_valid == false
would give you all features the hw was capable off.

Some follow up questions, to confirm I haven't confused things:

 - The ELD really needs to go into the hardware, not just end up in the
   hands of the audio driver? IOW, we cannot override this?

 - The HDA interface only defines reading the ELD, so writing it would
   have to be done by the graphics driver using some GPU interface?

If so, I need to poke the AMD guys as I couldn't find any information
in the public documentation about writing the ELD.

     -- Pierre Ossman

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