[alsa-devel] [RFC PATCH] ASoC: pcm512x: enable set_sysclk callback

[Mark Brown]

On Mon, Jun 04, 2018 at 08:48:40AM -0500, Pierre-Louis Bossart wrote:

> What I find odd is the selection between the two static clocks. I have
> nothing against the clock API, I just find that the Pi solution isn't
> terribly elegant. The machine driver selects the 44.1 or 48kHz oscillator by
> configuring GPIO using registers exposed by the codec driver, the clock API
> implementation does nothing related to hardware, so we have three entities
> involved to flip 2 GPIOs.

That just sounds like a not great implementation decision in that
platform code rather than a fundamental problem.

> And even if I use this solution, I still don't know how to tell the codec
> driver which sclk to use in an ACPI environment (this is to enable HiFiberry
> cards on Up/Up^2). At the moment the code does
> pcm512x->sclk = devm_clk_get(dev, NULL);
> and I don't have a moral equivalent for ACPI. I can add information for the
> codec driver to get from the DSDT, I'd need however a agreed solution based
> on _DSD or some property, and the thread with AMD on their 'mclk_name' topic
> for DA7219 didn't go anywhere, did it?

For systems out in the wild now you're going to need to have a bit of
platform glue code anyway so I'm not sure there's that big an overhead
(other than the data tables, but that seems idiomatic for ACPI).  It's
a bit of work to get something set up but I'd not expect it to be that
bad, but perhaps I'm missing something there.

> Bottom line there is an 'easy' solution for this specific case and a more
> complicated one but potentially more generic based on the clock API - hence
> the RFC.

As far as I can see the two aren't that far off - creating a fixed
frequency clock based on DMI data shouldn't be that much harder than
doing a set_sysclk().
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