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

[Jyri Sarha]

On 03/03/2015 05:31 PM, Lars-Peter Clausen wrote:
> 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.
>

Interesting. It indeed looks like the rule functionality could do what I 
want. I'll look into than. Thanks!

>>
>>>> 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...

Calling missing feature a bug is a bit harsh, but now that it seems 
there is a better to deal with this, I'll look into that.

Best regards,
Jyri