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

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Date:   Tue, 22 Nov 2022 02:33:52 -0500
From:   William Breathitt Gray <william.gray@...aro.org>
To:     Fabrice Gasnier <fabrice.gasnier@...s.st.com>
Cc:     jic23@...nel.org, alexandre.torgue@...s.st.com,
        olivier.moysan@...s.st.com, linux-iio@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] counter: stm32-lptimer-cnt: fix the check on arr and cmp
 registers update

On Tue, Nov 22, 2022 at 02:27:50AM -0500, William Breathitt Gray wrote:
> 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@...s.st.com>
> > ---
> >  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

Alternatively, you could pull out just (val & STM32_LPTIM_CMPOK_ARROK)
to a separate variable and keep the equivalence condition inline if you
think it'll be clearer that way.

William Breathitt Gray

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