On 05/07/2015 12:41 PM, Russell King - ARM Linux wrote:
[...]
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.
That's pretty straight forward and can be done using custom rules linking
the number of channels to the sample rate. Have a look at
snd_ac97_pcm_double_rate_rules() this is pretty much the same constraint.
- Lars
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