Re: [PATCH v2 1/8] iio: adc: stm32-dfsdm: make spi_master_freq more accurate

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 21 Mar 2019 17:47:22 +0100
Fabrice Gasnier <fabrice.gasnier@st.com> wrote:

> Current ckout divider may be set to a value that makes ckout to exceed
> spi-max-frequency. Rather use lower value (e.g. round up divider when
> ckout isn't accurate).
> 
> Also when the SPI clock isn't accurate, 'spi_master_freq' is filled in
> with expected frequency. Use computed value instead to be more accurate:
> - e.g. source clock / (CKOUTDIV + 1)
> 
> Enforce checks on the divider: ckoutdiv range can be from 1-255 to provide
> divider of 2-256.
> 
> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Applied to the togreg branch of iio.git and pushed out as testing for the
autobuilders to play with it.

Thanks,

Jonathan

> ---
> Changes in v2:
> - rework the way to determine ckoutdiv bitfield: deal with divider directly
>   to explicitly handle the rounding, the range constraints of 2-256 and the
>   divider = ckoutdiv + 1.
> ---
>  drivers/iio/adc/stm32-dfsdm-core.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/iio/adc/stm32-dfsdm-core.c b/drivers/iio/adc/stm32-dfsdm-core.c
> index bf089f5..472b809 100644
> --- a/drivers/iio/adc/stm32-dfsdm-core.c
> +++ b/drivers/iio/adc/stm32-dfsdm-core.c
> @@ -199,7 +199,7 @@ static int stm32_dfsdm_parse_of(struct platform_device *pdev,
>  {
>  	struct device_node *node = pdev->dev.of_node;
>  	struct resource *res;
> -	unsigned long clk_freq;
> +	unsigned long clk_freq, divider;
>  	unsigned int spi_freq, rem;
>  	int ret;
>  
> @@ -243,13 +243,20 @@ static int stm32_dfsdm_parse_of(struct platform_device *pdev,
>  		return 0;
>  	}
>  
> -	priv->spi_clk_out_div = div_u64_rem(clk_freq, spi_freq, &rem) - 1;
> -	if (!priv->spi_clk_out_div) {
> -		/* spi_clk_out_div == 0 means ckout is OFF */
> +	divider = div_u64_rem(clk_freq, spi_freq, &rem);
> +	/* Round up divider when ckout isn't precise, not to exceed spi_freq */
> +	if (rem)
> +		divider++;
> +
> +	/* programmable divider is in range of [2:256] */
> +	if (divider < 2 || divider > 256) {
>  		dev_err(&pdev->dev, "spi-max-frequency not achievable\n");
>  		return -EINVAL;
>  	}
> -	priv->dfsdm.spi_master_freq = spi_freq;
> +
> +	/* SPI clock output divider is: divider = CKOUTDIV + 1 */
> +	priv->spi_clk_out_div = divider - 1;
> +	priv->dfsdm.spi_master_freq = clk_freq / (priv->spi_clk_out_div + 1);
>  
>  	if (rem) {
>  		dev_warn(&pdev->dev, "SPI clock not accurate\n");