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

[Pierre-Louis Bossart]

On 5/1/20 2:31 PM, Radoslaw Biernacki wrote:
> 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?
> 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?

> -static int skylake_nau8825_hw_params(struct snd_pcm_substream *substream,
> -	struct snd_pcm_hw_params *params)
> +static int skylake_nau8825_trigger(struct snd_pcm_substream *substream, int cmd)
>   {
>   	struct snd_soc_pcm_runtime *rtd = substream->private_data;
> +	struct snd_pcm_runtime *runtime = substream->runtime;
>   	struct snd_soc_dai *codec_dai = asoc_rtd_to_codec(rtd, 0);
> -	int ret;
> -
> -	ret = snd_soc_dai_set_sysclk(codec_dai,
> -			NAU8825_CLK_MCLK, 24000000, SND_SOC_CLOCK_IN);
> +	int ret = 0;
>   
> -	if (ret < 0)
> -		dev_err(rtd->dev, "snd_soc_dai_set_sysclk err = %d\n", ret);
> +	switch (cmd) {
> +	case SNDRV_PCM_TRIGGER_START:
> +		ret = snd_soc_dai_set_sysclk(codec_dai, NAU8825_CLK_FLL_FS, 0,
> +					     SND_SOC_CLOCK_IN);
> +		if (ret < 0) {
> +			dev_err(codec_dai->dev, "can't set FS clock %d\n", ret);
> +			break;
> +		}
> +		ret = snd_soc_dai_set_pll(codec_dai, 0, 0, runtime->rate,
> +					  runtime->rate * 256);
> +		if (ret < 0)
> +			dev_err(codec_dai->dev, "can't set FLL: %d\n", ret);
> +		break;
> +	case SNDRV_PCM_TRIGGER_RESUME:
> +		ret = snd_soc_dai_set_pll(codec_dai, 0, 0, runtime->rate,
> +					  runtime->rate * 256);
> +		if (ret < 0)
> +			dev_err(codec_dai->dev, "can't set FLL: %d\n", ret);
> +		msleep(20);

is there a reason why you'd need a msleep for resume and not for start?