[Uboot-stm32] [PATCH] pwm: stm32: Fix division by zero for 32-bit timers

[Fabrice Gasnier]

On 10/30/26 11:31, Thomas Perrot wrote:
> stm32_pwm_set_config() derives the prescaler with
> do_div(div, priv->max_arr + 1). max_arr is a u32, so on a 32-bit timer
> (max_arr = 0xFFFFFFFF, e.g. TIM5 on STM32MP15) the sum wraps to 0 and
> do_div() divides by zero. Configuring a PWM on such a timer, for
> instance for a backlight, resets the board.

Hi Thomas,

Ouch :-(

Thanks for spotting !

> 
> do_div() takes a 32-bit divisor, so widening the operand is not enough.
> Use div64_u64() with a 64-bit divisor. The prescaler then comes out as
> 0, which is right since the period fits in ARR without prescaling.
> 
> Fixes: be5523d38296 ("pwm: stm32: add driver to support pwm with timer")
> Signed-off-by: Thomas Perrot <thomas.perrot at bootlin.com>
> ---
> On a 32-bit timer such as TIM5 on STM32MP15, stm32_pwm_set_config()
> divides by max_arr + 1, which wraps to 0 in a u32. Seen when a
> backlight on TIM5 is configured from U-Boot.
> 
> The fix is ported from a change carried in ST's U-Boot v2023.10-stm32mp
> tree.

Do you mean this is applicable to ST's downstream too ?
(I don't find such fix there)

> ---
>  drivers/pwm/pwm-stm32.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c
> index a691f75e4a71..3f2b593477b1 100644
> --- a/drivers/pwm/pwm-stm32.c
> +++ b/drivers/pwm/pwm-stm32.c
> @@ -13,6 +13,7 @@
>  #include <asm/arch/timers.h>
>  #include <dm/device_compat.h>
>  #include <linux/bitfield.h>
> +#include <linux/math64.h>
>  #include <linux/time.h>
>  
>  #define CCMR_CHANNEL_SHIFT	8
> @@ -64,7 +65,7 @@ static int stm32_pwm_set_config(struct udevice *dev, uint channel,
>  	do_div(div, NSEC_PER_SEC);
>  	prd = div;
>  
> -	do_div(div, priv->max_arr + 1);
> +	div = div64_u64(div, (u64)priv->max_arr + 1);

I'm fine with the patch. Just suggesting here: div isn't recycled later.
So could directly set:
	prescaler = div64_u64(prd, (u64)priv->max_arr + 1)

>  	prescaler = div;

By doing this, the div variable may be dropped completely to end-up with simply:
	prd = (unsigned long long)priv->rate * period_ns;
	do_div(prd, NSEC_PER_SEC);

	prescaler = div64_u64(prd, (u64)priv->max_arr + 1);

Up to you to update...

Both case you can add my:
Tested-by: Fabrice Gasnier <fabrice.gasnier at foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier at foss.st.com>

Best Regards,
Fabrice

>  	if (prescaler > MAX_TIM_PSC)
>  		return -EINVAL;
> 
> ---
> base-commit: 03e45557e1f38a39a0c80fe30483130ff376cf92
> change-id: 20261029-tprrt-pwm-stm32-32bit-timer-div-eeb070ead061
> 
> Best regards,