Re: [PATCH] counter: stm32-lptimer-cnt: fix the check on arr and cmp registers update

[William Breathitt Gray]

On Wed, Nov 23, 2022 at 02:36:09PM +0100, Fabrice Gasnier wrote:
> The ARR (auto reload register) and CMP (compare) registers are
> successively written. The status bits to check the update of these
> registers are polled together with regmap_read_poll_timeout().
> The condition to end the loop may become true, even if one of the register
> isn't correctly updated.
> So ensure both status bits are set before clearing them.
> 
> Fixes: d8958824cf07 ("iio: counter: Add support for STM32 LPTimer")
> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@xxxxxxxxxxx>
> ---
>  drivers/counter/stm32-lptimer-cnt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/counter/stm32-lptimer-cnt.c b/drivers/counter/stm32-lptimer-cnt.c
> index d6b80b6dfc28..8439755559b2 100644
> --- a/drivers/counter/stm32-lptimer-cnt.c
> +++ b/drivers/counter/stm32-lptimer-cnt.c
> @@ -69,7 +69,7 @@ static int stm32_lptim_set_enable_state(struct stm32_lptim_cnt *priv,
>  
>  	/* ensure CMP & ARR registers are properly written */
>  	ret = regmap_read_poll_timeout(priv->regmap, STM32_LPTIM_ISR, val,
> -				       (val & STM32_LPTIM_CMPOK_ARROK),
> +				       (val & STM32_LPTIM_CMPOK_ARROK) == STM32_LPTIM_CMPOK_ARROK,

This is a reasonable fix, but I don't like seeing so much happening in
an argument list -- it's easy to misunderstand what's going on which can
lead to further bugs the future. Pull out this condition to a dedicated
bool variable with a comment explaining why we need the equivalence
check (i.e. to ensure both status bits are set and not just one).

William Breathitt Gray

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