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

[Mark Brown]

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?

> 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.
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