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

From: Matthias Kaehlcke
Date: Wed Sep 05 2018 - 20:23:32 EST


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@xxxxxxxxxxxxxx
>
> Fixes: eddac5af0654 ("soc: qcom: Add GENI based QUP Wrapper driver")
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
>
> 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@xxxxxxxxxxxx>