[alsa-devel] [PATCH RFC v2 10/13] sound/core: add DRM ELD helper

[Russell King - ARM Linux]

On Sun, Apr 05, 2015 at 06:46:13PM +0200, Takashi Iwai wrote:
> At Sun, 5 Apr 2015 17:20:34 +0100,
> Russell King - ARM Linux wrote:
> > Since (afaik) ALSA has a lack of support for dynamic reconfiguration
> > according to the attached device changing, the best we can do without
> > a huge amount of re-work of HDMI support across all adapters is to
> > process the capabilities of the attached device at prepare time
> > according to the current capabilities.
> 
> Yeah, reconfiguration is tricky.  BTW, how is the HDMI unplug handled
> during playback?

We don't handle it right now - and we don't have any notification to
the audio drivers that that has happened.  Even if we did have such a
notification, I'm not sure what the audio driver could do with it at
the moment.

> > Implementing dynamic reconfiguration in ALSA is not something I want to
> > get involved with, and as we /already/ have HDMI support merged in the
> > kernel, this is not a blocker for at least trying to get some semblence
> > of sanity, rather than having every driver re-implementing stuff like
> > this.
> 
> Well, I didn't mean about the dynamic reconfiguration.  I thought of
> rather min/max pairs, but it was just a wrong assumption.  Scratch
> it.
> 
> One another question: don't we need to deal with the sample bits in
> sad[2]?

It should, but I'm very wary about doing that without seeing more
examples of real SADs.  Right now, all my examples only support
one SAD with either 2 channel or 6 channel audio at the standard
(basic) 32, 44.1 and 48kHz rates.

The HDMI / CEA specs are very loose in their wording about the
short audio descriptors.  I've no idea whether a sink can provide
(for example) descriptors such as:

	LPCM, 6 channel 32, 44.1, 48kHz
	LPCM, 2 channel, 32, 44.1, 48, 96, 192kHz

or whether have to describe that as a single descriptor.  I only
have two TVs to test with here.

What I'm concerned about is that when the ALSA parameter refining
starts, we start with (eg) 2-8 channels, 32-192kHz.  Given that,
if we invoke the channel restriction before the rate restriction,
we would end up limiting to 2 channel at 32-192kHz.  If we apply
the restrictions in the opposite order, we'd restrict to 6
channel, 32-48kHz.  Neither are obviously correct in this
circumstance, and I don't really see a way to solve it given my
understanding of the way ALSA's parameter refinement works.

I suspect this is why most HDMI drivers are implemented such that
they take the maximum capabilities over all SADs, which would end
up restricting audio in the above case to: up to 6 channels, at
32, 44.1, 48, 96 and 192kHz, even though 6 channel @ 192kHz isn't
hasn't been indicated as supported.

Most of this is speculation though, based off what is in the
documentation.  As I say, I don't have enough real-world examples
to get a feel for what manufacturers _actually_ do to give a hint
as to how the documentation should be interpreted.

So, maybe I should just copy what everyone else does and take the
maximum of all descriptors...

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