Re: [PATCH 2/2] clk: meson: mpll: use 64bit math in rate_from_params

[Jerome Brunet <jbrunet@baylibre.com>]

On Sat, 2017-04-01 at 15:02 +0200, Martin Blumenstingl wrote:
> On Meson8b the MPLL parent clock (fixed_pll) has a rate of 2550MHz.
> Multiplying this with SDM_DEN results in a value greater than 32bits.
> This is not a problem on the 64bit Meson GX SoCs, but it may result in
> undefined behavior on the older 32bit Meson8b SoC.
> 
> While rate_from_params was only introduced recently to make the math
> reusable from _round_rate and _recalc_rate the original bug exists much
> longer.

Again, thanks for testing and fixing this Martin !

> 
> Fixes: 1c50da4f27 ("clk: meson: add mpll support")
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
> ---
>  drivers/clk/meson/clk-mpll.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/clk/meson/clk-mpll.c b/drivers/clk/meson/clk-mpll.c
> index 551aa2a5b291..4306ad833a4f 100644
> --- a/drivers/clk/meson/clk-mpll.c
> +++ b/drivers/clk/meson/clk-mpll.c
> @@ -62,6 +62,7 @@
>   */
>  
>  #include <linux/clk-provider.h>
> +#include <linux/math64.h>
>  #include "clkc.h"
>  
>  #define SDM_DEN 16384
> @@ -81,7 +82,7 @@ static unsigned long rate_from_params(unsigned long
> parent_rate,
>  	if (divisor == 0)
>  		return 0;
>  	else
> -		return (parent_rate * SDM_DEN) / divisor;
> +		return mul_u64_u32_div(parent_rate, SDM_DEN, divisor);

Just casting parent_rate to u64 should be enough, don't you think ?
Actually, I was thinking of copying a pattern that is used a lot in CCF and use
DIV_ROUND_UP_ULL((u64)foo, bar). See clk-divider.c.

Would this be ok with you ?

>  }
>  
>  static void params_from_rate(unsigned long requested_rate,