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

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Message-ID: <c4792fd3-73e3-20ee-cd7a-a8d00eb877f9@foss.st.com>
Date:   Wed, 23 Nov 2022 15:56:31 +0100
From:   Fabrice Gasnier <fabrice.gasnier@...s.st.com>
To:     William Breathitt Gray <william.gray@...aro.org>
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 11/22/22 08:33, William Breathitt Gray wrote:
> 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.

Hi William,

I'm not sure to fully understand your proposal here.
Could you clarify ?

regmap_read_poll_timeout() macro requires:

 * @val: Unsigned integer variable to read the value into
 * @cond: Break condition (usually involving @val)

So do you wish I introduce a macro that abstracts the condition check ?
(val & STM32_LPTIM_CMPOK_ARROK) == STM32_LPTIM_CMPOK_ARROK


Best regards,
Fabrice

> 
> William Breathitt Gray