Re: [PATCH v1 1/3] clk: clocking-wizard: fix clock difference detection

[Brian Masney <bmasney@redhat.com>]

Hi Colin,

On Wed, May 06, 2026 at 03:05:53PM -0500, Colin Foster wrote:
> The diff calculation didn't take into account rollover. As such, a
> target clock frequency below the requested rate would not be considered.
> 
> Before this change, bogus diffs would be used to determine the closest
> possible clock:
> 
> 8<--------
> clk-wizard-test: requesting 133312500 Hz on output 0 (clock NOT enabled)
> *** Clock wizard - Matching for rate 133312500 parent rate 99999000
> m = 33, d = 1, o = 25, freq = 131998680, diff = 18446744073708237796
> m = 34, d = 1, o = 26, freq = 130767923, diff = 18446744073707007039
> m = 35, d = 1, o = 26, freq = 134614038, diff = 1301538
> m = 36, d = 1, o = 27, freq = 133332000, diff = 19500
> 8<--------
> 
> After this change:
> 
> 8<--------
> clk-wizard-test: requesting 133312500 Hz on output 0 (clock NOT enabled)
> *** Clock wizard - Matching for rate 133312500 parent rate 99999000
> m = 33, d = 1, o = 25, freq = 131998680, diff = 1313820
> m = 35, d = 1, o = 26, freq = 134614038, diff = 1301538
> m = 36, d = 1, o = 27, freq = 133332000, diff = 19500
> 8<--------
> 
> Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
> ---
>  drivers/clk/xilinx/clk-xlnx-clock-wizard.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/clk/xilinx/clk-xlnx-clock-wizard.c b/drivers/clk/xilinx/clk-xlnx-clock-wizard.c
> index 032a688840d8..88b47b8cc387 100644
> --- a/drivers/clk/xilinx/clk-xlnx-clock-wizard.c
> +++ b/drivers/clk/xilinx/clk-xlnx-clock-wizard.c
> @@ -408,7 +408,7 @@ static int clk_wzrd_get_divisors(struct clk_hw *hw, unsigned long rate,
>  			if (o < omin || o > omax)
>  				continue;
>  			freq = DIV_ROUND_CLOSEST_ULL(vco_freq, o);
> -			diff = freq - rate;
> +			diff = abs(freq - rate);
>  
>  			if (diff < best_diff) {
>  				printk("m = %d, d = %d, o = %d, freq = %llu, diff = %llu", m, d, o, freq, diff);

Stephen didn't pick this up last development cycle and I'm assembling a
pull for him. This doesn't apply to upstream.  Please post a new
version that applies cleanly.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/clk/xilinx/clk-xlnx-clock-wizard.c#n341

Brian