Re: [PATCH v2 2/5] clk: rockchip: pll: fix rounding of negative k in RK3588 frac PLL

Hi Alexey,

On 7/23/2026 5:04 PM, Alexey Charkov wrote:
> Current code uses magical constants when rounding up the magnitude of
> negative fractional PLL component k. This leads to overshooting the
> requested rate when the calculated fractional part has less than 0.3 in
> its decimal part due to failure to round up the fractional part.
> 
> Use a proper rounding up function to avoid overshooting the requested
> rate and make the calculation more readable.
> 
> Fixes: 6bfb37e70209 ("clk: rockchip: rk3588: fix up the frac pll calculation")
> Reviewed-by: Quentin Schulz <[email protected]>
> Signed-off-by: Alexey Charkov <[email protected]>
> ---
>  drivers/clk/rockchip/clk_pll.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/clk/rockchip/clk_pll.c b/drivers/clk/rockchip/clk_pll.c
> index d0df3b8fb49d..69d2d182dcb5 100644
> --- a/drivers/clk/rockchip/clk_pll.c
> +++ b/drivers/clk/rockchip/clk_pll.c
> @@ -11,6 +11,7 @@
>  #include <asm/arch-rockchip/hardware.h>
>  #include <div64.h>
>  #include <linux/delay.h>
> +#include <linux/math64.h>
>  
>  static struct rockchip_pll_rate_table rockchip_auto_table;
>  
> @@ -177,7 +178,10 @@ rockchip_rk3588_pll_k_get(u32 m, u32 p, u32 s, u64 fin_hz, u64 fvco)
>  	k = ffrac * 65536 / fref;
>  	if (k > 32767) {
>  		ffrac = ((m + 1) * fref) - fvco;
> -		k = ((ffrac * 65536 * 10 / fref) + 7) / 10;
> +		/*
> +		 * Round up to avoid overshooting requested rate for negative k
> +		 */
> +		k = DIV64_U64_ROUND_UP(ffrac * 65536, fref);

Use of DIV64_U64_ROUND_UP() seem to cause build errors for Rockchip 32-bit
tagets, see CI job at [1].

[1] https://git.u-boot-project.org/u-boot/contributors/kwiboo/u-boot/-/jobs/54629#L4016

Regards,
Jonas

>  		if (k > 32767)
>  			k = 0;
>  		else
>