Re: [PATCH] iio: adc: stm32-adc: adapt clock duty cycle for proper operation

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 6 Nov 2020 17:57:26 +0100
Fabrice Gasnier <fabrice.gasnier@st.com> wrote:

> For proper operation, STM32 ADC should be used with a clock duty cycle
> of 50%, in the range of 49% to 51%. Depending on the clock tree, divider
> can be used in case clock duty cycle is out of this range.
> In case clk_get_scaled_duty_cycle() returns an error, kindly apply a
> divider by default (don't make the probe fail).
> 
> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Hi Fabrice,

This sounds like it's a fix for a situation in which the device is not
currently working?  If so, please let me know a fixes tag.

Thanks,

Jonathan

> ---
>  drivers/iio/adc/stm32-adc-core.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/adc/stm32-adc-core.c b/drivers/iio/adc/stm32-adc-core.c
> index cd870c0..d64a9e8 100644
> --- a/drivers/iio/adc/stm32-adc-core.c
> +++ b/drivers/iio/adc/stm32-adc-core.c
> @@ -202,7 +202,7 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
>  {
>  	u32 ckmode, presc, val;
>  	unsigned long rate;
> -	int i, div;
> +	int i, div, duty;
>  
>  	/* stm32h7 bus clock is common for all ADC instances (mandatory) */
>  	if (!priv->bclk) {
> @@ -226,6 +226,11 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
>  			return -EINVAL;
>  		}
>  
> +		/* If duty is an error, kindly use at least /2 divider */
> +		duty = clk_get_scaled_duty_cycle(priv->aclk, 100);
> +		if (duty < 0)
> +			dev_warn(&pdev->dev, "adc clock duty: %d\n", duty);
> +
>  		for (i = 0; i < ARRAY_SIZE(stm32h7_adc_ckmodes_spec); i++) {
>  			ckmode = stm32h7_adc_ckmodes_spec[i].ckmode;
>  			presc = stm32h7_adc_ckmodes_spec[i].presc;
> @@ -234,6 +239,13 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
>  			if (ckmode)
>  				continue;
>  
> +			/*
> +			 * For proper operation, clock duty cycle range is 49%
> +			 * to 51%. Apply at least /2 prescaler otherwise.
> +			 */
> +			if (div == 1 && (duty < 49 || duty > 51))
> +				continue;
> +
>  			if ((rate / div) <= priv->max_clk_rate)
>  				goto out;
>  		}
> @@ -246,6 +258,10 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
>  		return -EINVAL;
>  	}
>  
> +	duty = clk_get_scaled_duty_cycle(priv->bclk, 100);
> +	if (duty < 0)
> +		dev_warn(&pdev->dev, "bus clock duty: %d\n", duty);
> +
>  	for (i = 0; i < ARRAY_SIZE(stm32h7_adc_ckmodes_spec); i++) {
>  		ckmode = stm32h7_adc_ckmodes_spec[i].ckmode;
>  		presc = stm32h7_adc_ckmodes_spec[i].presc;
> @@ -254,6 +270,9 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
>  		if (!ckmode)
>  			continue;
>  
> +		if (div == 1 && (duty < 49 || duty > 51))
> +			continue;
> +
>  		if ((rate / div) <= priv->max_clk_rate)
>  			goto out;
>  	}