Re: [PATCH v2 2/2] ASoC: tegra: Unify ASoC machine drivers

[Jon Hunter <jonathanh@nvidia.com>]

On 24/05/2021 14:40, Dmitry Osipenko wrote:
> 24.05.2021 15:22, Jon Hunter пишет:
>>
>>
>> On 21/05/2021 20:05, Dmitry Osipenko wrote:
>>
>> ...
>>
>>>>> +unsigned int tegra_asoc_machine_mclk_rate(unsigned int srate)
>>>>> +{
>>>>> +	unsigned int mclk;
>>>>> +
>>>>> +	switch (srate) {
>>>>> +	case 64000:
>>>>> +	case 88200:
>>>>> +	case 96000:
>>>>> +		mclk = 128 * srate;
>>>>> +		break;
>>>>> +	default:
>>>>> +		mclk = 256 * srate;
>>>>> +		break;
>>>>> +	}
>>>>> +	/* FIXME: Codec only requires >= 3MHz if OSR==0 */
>>>>> +	while (mclk < 6000000)
>>>>> +		mclk *= 2;
>>>>
>>>> So this appears to be specific to the wm8903 codec or at least this is
>>>> where it came from. And given that the switch statement is not complete
>>>> in terms of the sample rates (ie. only has a subset), I am wondering if
>>>> set should keep this specific to the wm8903 codec?
>>>
>>> The RT5631 codec of Asus Transformers will re-use this function.
>>
>> OK, but does it need this FIXME part? That appears to be codec specific.
> 
> Downstream RT5631 Tegra driver has exactly the same FIXME.

What downstream branch are you referring to? I still don't think that
that is a good reason to make this 'FIXME' the standard going forward
and hence I would prefer that it is kept specific the wm8903. Otherwise
people will keep using this code without understanding if this is
needed/correct.

> Although, I now see that downstream RT5631 uses 384*srate for the
> default cases.
> 
> I also see that WM8994 driver that we have in grate-kernel for Galaxy
> Tab and SGH-I927 also re-uses that mclk_rate function.
> 
>>> IIUC, the default switch-case works properly for all rates below 64KHz,
>>> at least I haven't had any problems with it. Could you please clarify
>>> why you are saying that the switch statement appears to be incomplete?
>>
>> It looks a bit weird because less than 64kHz and greater than 96kHz we
>> use 256 and for only 64kHz, 88.2kHz and 96kHz we use 128. So it is not
>> clear to me which sample rates have actually been tested with this and
>> if this is complete or not?
>>
>> Is it intended that we use 256 for sample rates greater than 96kHz?
> 
> The 128*srate gives MCLK >6MHZ for 64/88/96, 256*srate gives MCLK >6MHZ
> for rates below 64kHZ. Looks like the goal is to get MCLK >6MHZ.

The wm8903 supports 8kHz sample rates and 256*8000 is less than 6MHz.
Yes the FIXME loop corrects this, but you could also extend the case
statement to multiply by 512 for 8kHz.

>  The WM8903 datasheet says:
> 
> "The  following  operating  frequency  limits  must  be  observed  when
>  configuring  CLK_SYS.  Failure  to  observe   these   limits   will
> result   in   degraded  noise   performance   and/or   incorrect
> ADC/DAC  functionality.
> 
> If DAC_OSR = 0 then CLK_SYS  3MHz
> If DAC_OSR = 1 then CLK_SYS  6MHz"
> 
> Where DAC_OSR is DAC Oversampling Control
> 0 = Low power (normal oversample)
> 1 = High performance (double rate)
> 
> I see that DAC_OSR=0 by default, it can be switched to 1 by userspace
> ALSA control.
> 

Yes that is all fine, but again this is specific to the wm8903.

Jon

-- 
nvpublic