[alsa-devel] ASoC : question about custom rate

[Lars-Peter Clausen]

On 09/10/2014 01:45 AM, Matt Flax wrote:
> On 09/09/14 20:44, Lars-Peter Clausen wrote:
>> On 09/09/2014 10:43 AM, Matt Flax wrote:
>>> Hi there,
>>>
>>> I have an ASIC running on an ASoC with a custom rate @ 18750 Hz.
>>> I would like ALSA to report the correct rate.
>>>
>>> If I do nothing, then the following reports 16000 Hz :
>>> cat /proc/asound/card0/pcm0c/sub0/hw_params
>>>
>>> I have verified that the hardware is running at the expected 18750 Hz.
>>> I am wondering what/where is the correct place to setup the actual rate ?
>>>
>>> I am trying to set the rates at startup like so but ALSA will not work with
>>> the driver when I do this :
>>>
>>> static int implant_startup(struct snd_pcm_substream *substream, struct
>>> snd_soc_dai *dai)
>>> {
>>>      snd_pcm_hw_constraint_minmax(substream->runtime,
>>> SNDRV_PCM_HW_PARAM_RATE, 18750, 18750);
>>
>>
>> This is the correct approach. But it is quite likely that your I2S driver
>> will set additional constraints (e.g. just 16k, 32k, etc.) that results in
>> an empty list for the rate constraints. If the I2S driver works fine maybe
>> change the constraints to only set a minimum and maximum rate instead of
>> specific rates.
>
> Yes, you are right, the i2s driver uses the macro SNDRV_PCM_RATE_8000_96000.
>
> I have used your advice to test by manually setting the i2s rates to
> SNDRV_PCM_RATE_CONTINUOUS in the i2s driver's src code.
> Now the rate is reported correctly in software. Thank you.
>
> Is there a way to access the underlying i2s driver from within my codec's
> startup file so that I don't need to alter the core driver source code ?

No, that would be a layering violation. If the I2S driver supports more than 
just the currently advertised rates it should be updated.

- Lars