[alsa-devel] DT soundcard driver with special clock routing

[Sylwester Nawrocki]

On 10/12/15 17:31, Petr Kulhavy wrote:
> Hi,
> 
> I'm developing a DT-based driver for an ARM SoC device with the 
> following special clock routing:
> 
> - WM8758 codec attached to the CPU DAI via I2S
> - the codec is I2S clock master
> - the CPU feeds the codec with 12MHz clock (outside of the I2S)
> - the codec converts the 12MHz  internally to 48kHz*256 or 44.1kHz *256 
> clock and provides it to I2S as MCLK
> 
> So the special set-up needed here is that the codec needs to be 
> configured to:
> a) generate the proper MCLK
> b) enable the respective GPIO pin as clock output
> 
> What is the best way of representing this set-up using device tree?
> I'm wondering if this can be done by adding a specific option in the 
> codec DT binding for enabling the clock output and then use the simple 
> audio card.
> Or do I need to write a specific soundcard driver that sets up the codec 
> registers? I would like to avoid that if possible and use existing code...
> 
> What would you recommend?

I assume when the clock enable/disable code is not there it could
be added to the codec driver, e.g. like in case of max98090.
The clock source just needs to be exposed as a common clock object.

As an example of similar setup you could take a look at
arch/arm/boot/dts/exynos4412-odroid-common.dts. The difference
was that in case of Odroid the clock was generated by a PLL inside
a SoC, then it was routed to I2S which fed the clock to the codec
through the SoC's io pin. The codec was I2S master and the I2S
IP block was actually a source of its main ("mclk") clock.
I hope this helps.

-- 
Regards,
Sylwester