Re: [PATCH 2/2] soc: qcom: geni: geni_se_clk_freq_match() should always accept multiples

[Matthias Kaehlcke <mka@chromium.org>]

On Thu, Aug 30, 2018 at 11:36:12AM -0700, Douglas Anderson wrote:
> The geni_se_clk_freq_match() has some strange semantics.  Specifically
> it is defined with two modes:
> 1. It can find a clock that's an exact multiple of the requested rate
> 2. If can find a non-exact match but it can't handle multiples then

nit: s/If/It/

> 
> ...but callers should always be able to handle a clock that is a
> multiple of the requested clock so mode #2 doesn't really make sense.
> Let's change the semantics so that the non-exact match can also accept
> multiples and then change the code to handle that.
> 
> The only caller of this code is the unlanded SPI driver [1] which
> currently passes "exact = True", thus it should be safe to change the
> semantics in this way.  ...and, in fact, the SPI driver should likely
> be modified to pass "exact = False" (with the new semantics) since
> that will allow it to work with SPI devices that request a clock rate
> that doesn't exactly match a rate we can make.
> 
> [1] https://lkml.kernel.org/r/1535107336-2214-1-git-send-email-dkota@codeaurora.org
> 
> Fixes: eddac5af0654 ("soc: qcom: Add GENI based QUP Wrapper driver")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> 
>  drivers/soc/qcom/qcom-geni-se.c | 37 ++++++++++++++++++++-------------
>  1 file changed, 22 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/soc/qcom/qcom-geni-se.c b/drivers/soc/qcom/qcom-geni-se.c
> index 1b19b8428c4a..092b32a315c3 100644
> --- a/drivers/soc/qcom/qcom-geni-se.c
> +++ b/drivers/soc/qcom/qcom-geni-se.c
> @@ -544,16 +544,17 @@ EXPORT_SYMBOL(geni_se_clk_tbl_get);
>   * @se:		Pointer to the concerned serial engine.
>   * @req_freq:	Requested clock frequency.
>   * @index:	Index of the resultant frequency in the table.
> - * @res_freq:	Resultant frequency which matches or is closer to the
> - *		requested frequency.
> + * @res_freq:	Resultant frequency of the source clock.
>   * @exact:	Flag to indicate exact multiple requirement of the requested
>   *		frequency.
>   *
> - * This function is called by the protocol drivers to determine the matching
> - * or exact multiple of the requested frequency, as provided by the serial
> - * engine clock in order to meet the performance requirements. If there is
> - * no matching or exact multiple of the requested frequency found, then it
> - * selects the closest floor frequency, if exact flag is not set.
> + * This function is called by the protocol drivers to determine the best match
> + * of the requested frequency as provided by the serial engine clock in order
> + * to meet the performance requirements.
> + *
> + * If we return success:
> + * - if @exact is true  then @res_freq / <an_integer> == @req_freq
> + * - if @exact is false then @res_freq / <an_integer> <= @req_freq
>   *
>   * Return: 0 on success, standard Linux error codes on failure.
>   */
> @@ -564,6 +565,9 @@ int geni_se_clk_freq_match(struct geni_se *se, unsigned long req_freq,
>  	unsigned long *tbl;
>  	int num_clk_levels;
>  	int i;
> +	unsigned long best_delta;
> +	unsigned long new_delta;
> +	unsigned int divider;
>  
>  	num_clk_levels = geni_se_clk_tbl_get(se, &tbl);
>  	if (num_clk_levels < 0)
> @@ -572,18 +576,21 @@ int geni_se_clk_freq_match(struct geni_se *se, unsigned long req_freq,
>  	if (num_clk_levels == 0)
>  		return -EINVAL;
>  
> -	*res_freq = 0;
> +	best_delta = 0;
>  	for (i = 0; i < num_clk_levels; i++) {
> -		if (!(tbl[i] % req_freq)) {
> +		divider = DIV_ROUND_UP(tbl[i], req_freq);
> +		new_delta = req_freq - tbl[i] / divider;
> +		if (!best_delta || new_delta < best_delta) {

nit: if you assigned best_delta to ULONG_MAX above you could omit the
check for !best_delta here, better to read IMO.

> +			/* We have a new best! */
>  			*index = i;
>  			*res_freq = tbl[i];
> -			return 0;
> -		}
>  
> -		if (!(*res_freq) || ((tbl[i] > *res_freq) &&
> -				     (tbl[i] < req_freq))) {
> -			*index = i;
> -			*res_freq = tbl[i];
> +			/* If the new best is exact then we're done */
> +			if (new_delta == 0)
> +				return 0;
> +
> +			/* Record how close we got */
> +			best_delta = new_delta;
>  		}
>  	}
>  

Looks good to me assuming that the statement "callers should always be
able to handle a clock that is a multiple of the requested clock" is
correct.

Reviewed-by: Matthias Kaehlcke <mka@chromium.org>