Re: [PATCH RFC v2 10/13] sound/core: add DRM ELD helper - Russell King

From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Anssi Hannula <anssi.hannula@iki.fi>, Takashi Iwai <tiwai@suse.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>,
	alsa-devel@alsa-project.org, dri-devel@lists.freedesktop.org,
	Jaroslav Kysela <perex@perex.cz>, Mark Brown <broonie@kernel.org>,
	Yakir Yang <ykk@rock-chips.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH RFC v2 10/13] sound/core: add DRM ELD helper
Date: Thu, 7 May 2015 11:41:03 +0100	[thread overview]
Message-ID: <20150507104102.GA2067@n2100.arm.linux.org.uk> (raw)
In-Reply-To: <554A493C.5080106@iki.fi>

On Wed, May 06, 2015 at 08:02:52PM +0300, Anssi Hannula wrote:
> 05.04.2015, 20:26, Russell King - ARM Linux kirjoitti:
> > On Sun, Apr 05, 2015 at 06:46:13PM +0200, Takashi Iwai wrote:
> >>
> >> 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.
> 
> For the record, yes, multiple LPCM SADs are relatively common, and I
> think I've seen some that offered more rates on a stereo SAD than on a
> multichannel SAD (like in your example).

Okay, so we "think" we've seen it in the wild...

> > 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 this is probably less than speculation.

So, ALSA gurus, how do we handle this?  How do we arrange the parameter
resolution such that ALSA can permit _either_ 2 channels at 192kHz or 6
channels at 48kHz, but not 6 channels at 192kHz?  Right now, I don't
see how that's possible.

-- 
FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
according to speedtest.net.
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