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

On Tue, Mar 03, 2015 at 12:09:14PM +0200, Jyri Sarha wrote:

On 03/02/2015 09:58 PM, Lars-Peter Clausen wrote:

quoted

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.