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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Sun, Apr 05, 2015 at 05:57:09PM +0200, Takashi Iwai wrote:
> > diff --git a/include/sound/pcm_drm_eld.h b/include/sound/pcm_drm_eld.h
> > new file mode 100644
> > index 000000000000..93357b25d2e2
> > --- /dev/null
> > +++ b/include/sound/pcm_drm_eld.h
> > @@ -0,0 +1,6 @@
> > +#ifndef __SOUND_PCM_DRM_ELD_H
> > +#define __SOUND_PCM_DRM_ELD_H
> > +
> > +int snd_pcm_hw_constraint_eld(struct snd_pcm_runtime *runtime, void *eld);
> > +
> > +#endif
> 
> IMO, a single line of function declaration can be merged to
> sound/pcm.h.

Ok.

> > diff --git a/sound/core/Kconfig b/sound/core/Kconfig
> > index 313f22e9d929..b534c8a6046b 100644
> > --- a/sound/core/Kconfig
> > +++ b/sound/core/Kconfig
> > @@ -6,6 +6,9 @@ config SND_PCM
> >  	tristate
> >  	select SND_TIMER
> >  
> > +config SND_PCM_ELD
> > +	bool
> 
> I don't mind much adding this one, but a new Kconfig is always a
> question.  I'd like to hear other's opinion, too.

That's more a question whether you want it always included in the build
or not, especially as it is dependent on DRM header files.

> > +printk("%s: r %d-%d c %d-%d m %x\n", __func__, r->min, r->max, c->min, c->max, rate_mask);
> 
> I guess this will be removed in the final version? ;)

Of course.

> > +static int eld_limit_channels(struct snd_pcm_hw_params *params,
> > +			      struct snd_pcm_hw_rule *rule)
> > +{
> > +	struct snd_interval *var = hw_param_interval(params, rule->var);
> > +	struct snd_interval t = { .min = 1, .max = 2, .integer = 1, };
> > +	unsigned int i;
> > +	const u8 *sad, *eld = rule->private;
> > +
> > +	sad = drm_eld_sad(eld);
> > +	if (sad) {
> > +		for (i = drm_eld_sad_count(eld); i > 0; i--, sad += 3) {
> > +			switch (sad[0] & 0x78) {
> > +			case 0x08:
> > +				t.max = max(t.max, (sad[0] & 7) + 1u);
> > +				break;
> 
> What about the minimal channel?

There isn't a minimum.  The SAD describes only the _maximum_ number of
channels.  For example, if a TV supports 5.1 audio, at 32, 44.1 and 48
kHz, it can do that with just one SAD entry which indicates support for
six channels of audio at those sample rates.  However, it will happily
accept 2 channel audio at those sample rates too.

> Ideally, we should either give a list of channel numbers or process
> the hw_constraints dynamically to narrow the min/max matching with the
> eld.

The ELD can change as a result of the HDMI sink deciding to change its
EDID (it does happen) or if the device is unplugged and re-plugged.  If
I wanted to restrict the maximum channel/rates by building some sort of
table, I'd do this at open time and avoid the complexity of having rule
callbacks.

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.

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.

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