Re: DT soundcard driver with special clock routing

[Petr Kulhavy <petr@barix.com>]

Hi Sylwester,

thank you for the explanation. I can follow the clock description in the 
DT and it looks like a reasonable approach.
However neither the codec or I2S seem to implement any clock provider.
How is the implementation side done? I mean someone needs to set the 
PLLs, etc.

Thanks
Petr

On 10.12.2015 18:13, Sylwester Nawrocki wrote:
> 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.
>