[PATCH V2] ASoC: Intel: boards: Use FS as nau8825 sysclk in nau88125_* machine

[Pierre-Louis Bossart]

>>> This single fix address two issues on machines with nau88125:
>>> 1) Audio distortion, due to lack of required clock rate on MCLK line
>>> 2) Loud audible "pops" on headphones if there is no sysclk during nau8825
>>>      playback power up sequence
>>>
>>> Explanation for:
>>> 1) Due to Skylake HW limitation, MCLK pin can only output 24MHz clk
>>>      rate (it can be only connected to XTAL parent clk). The BCLK pin
>>>      can be driven by dividers and therefore FW is able to set it to rate
>>>      required by chosen audio format. According to nau8825 datasheet, 256*FS
>>>      sysclk gives the best audio quality and the only way to achieve this
>>>      (taking into account the above limitations) its to regenerate the MCLK
>>>      from BCLK on nau8825 side by FFL. Without required clk rate, audio is
>>>      distorted by added harmonics.
>>
>> The BCLK is going to be a multiple of 50 * Fs due to clocking
>> restrictions. Can the codec regenerate a good-enough sysclk from this?
> 
> According to Intel, silicon has a limitation, on SKL/KBL only clk_id =
> SKL_XTAL, .name = "xtal" is available for IO domain.
> As mentioned in the commit:
> MCLK is generated by using 24MHz Xtal directly or applying a divider
> (so no way of achieving the rate required by audio format).
> BCLK/FS is generated from 24MHz and uses dividers and additional
> padding bits are used to match the clock source.
> Next gen silicon has the possibility of using additional clock sources.
> 
> Summing up, using MCLK from SKL to NAU88L25 is not an option.
> The only option we found is to use BCLK and regen the required clock
> rate by FLL on the NAU88l25 side.

Right, this 24 MHz is a recurring problem.
But what I was asking was if the NAU8825 is fine working with e.g. a 
2.4MHz bit clock. i.e. with 25 bit slots or padding at the end of the frame?

> 
>>>
>>> 2) Currently Skylake does not output MCLK/FS when the back-end DAI op
>>>      hw_param is called, so we cannot switch to MCLK/FS in hw_param.  This
>>>      patch reduces pop by letting nau8825 keep using its internal VCO clock
>>>      during widget power up sequence, until SNDRV_PCM_TRIGGER_START when
>>>      MCLK/FS is available. Once device resumes, the system will only enable
>>>      power sequence for playback without doing hardware parameter, audio
>>>      format, and PLL configure. In the mean time, the jack detecion sequence
>>>      has changed PLL parameters and switched to internal clock. Thus, the
>>>      playback signal distorted without correct PLL parameters.  That is why
>>>      we need to configure the PLL again in SNDRV_PCM_TRIGGER_RESUME case.
>>
>> IIRC the FS can be controlled with the clk_ api with the Skylake driver,
>> as done for some KBL platforms. Or is this not supported by the firmware
>> used by this machine?
> 
> According to Ben, SKL had limitations in FW for managing the clk's
> back in the days.
> Can you point to the other driver you mention so we can cross check?

There are two KBL drivers that control the SSP clocks from the machine 
driver, but indeed I don't know if this would work on Firmware, it'd be 
a question for Lech/Cezary.

kbl_rt5663_max98927.c:          ret = clk_prepare_enable(priv->mclk);
kbl_rt5663_max98927.c:          ret = clk_prepare_enable(priv->sclk);
kbl_rt5663_rt5514_max98927.c:           ret = 
clk_prepare_enable(priv->mclk);
kbl_rt5663_rt5514_max98927.c:           ret = 
clk_prepare_enable(priv->sclk);
kbl_rt5663_rt5514_max98927.c:                   ret = 
clk_prepare_enable(priv->mclk);