[alsa-devel] [PATCH 2/2] ASoC: simple-card: Add support for samplerate and samplewidth constraints

[Lars-Peter Clausen]

On 03/03/2015 01:00 PM, Jyri Sarha wrote:
> On 03/03/2015 01:30 PM, Mark Brown wrote:
>> On Tue, Mar 03, 2015 at 12:09:14PM +0200, Jyri Sarha wrote:
>>> On 03/02/2015 09:58 PM, Lars-Peter Clausen wrote:
>>
>>>> Can you include a description why this is needed and how and when it is
>>>> supposed to be used?
>>
>>> Would this addition do:
>>> ------------------------------------------------------------
>>> These constraints help to disable the sample-format and sample-rate
>>> combinations that do not properly work on a specific HW.
>>> ------------------------------------------------------------
>>
>> Not entirely...
>>
>>> The reason why we need these is coming from limitations in McASP clock
>>> generation. With a simple divider one can only produce certain bit-clocks.
>>> With those bit-clocks we can only play/capture some sample-rate and
>>> sample-width combinations accurately.
>>
>>> The McASP driver could try to set the constraints automatically. However,
>>> since the constraint code can not select sample-width and sample-rate
>>> combinations there is a compromise to be made between them. Making such
>>> compromises automatically does not usually work that well.
>>
>> ...this is more the point.  Perhaps the constraints language needs
>> improvement here?
>>
>
> Improving constraint functionality would certainly help, however the way
> that code works is beyond my understanding and I do not believe such an
> improvement would be coming from anybody else any time soon either.

Restricting the available sample formats based on the sample rate and vice 
versa is possible with the current constraint framework. Take a look at what 
Peter Rosin recently did to the pcm512x driver. Your restrictions sound very 
similar to what he did.

>
>>> In our case these properties could of course be added to McASP driver, but
>>> then again I would expect that there is a wider need for this kind of
>>> functionality. And it may not always be clear if either end of the link
>>> alone is responsible for less than perfect operation.
>>
>> The trouble with this sort of interface is that it's a quick and dirty
>> way for people to bodge around things rather than actually fixing them
>> properly.  Of course sometimes fixing things properly is really hard and
>> that means we want a temporary bodge but having to put them in DT is
>> really unfortunate.
>>
>
> I agree with that. However, the simple-card binding goes already now quite a
> bit beyond just describing the hardware. The binding for instance decides
> the configuration that is going to be used over the dai-link. These
> constraints could be seen as an extension to that configuration.
>
> I am wondering if there would be some better way to select the dai-link
> configuration than writing it to DT or creating a custom machine driver for
> each setup.
>
> But about this patch. Should I just give it up, or would you be willing to
> apply it if I improve the description more and add a warning against using
> these properties to work around driver bugs to the binding document?

Well, your description is basically saying that you want to use this to work 
around a driver bug, so...