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

[Dmitry Osipenko]

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.

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 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.