[PATCH 1/2] pwm: atmel-tcb: Fix race condition and convert to guards

[Nicolas Ferre]

On 10/07/2024 at 17:31, Uwe Kleine-König wrote:
> Hello Nicolas,
> 
> On Wed, Jul 10, 2024 at 04:17:09PM +0200, Nicolas Ferre wrote:
>> On 09/07/2024 at 12:18, Uwe Kleine-König wrote:
>>> The hardware only supports a single period length for both PWM outputs. So
>>> atmel_tcb_pwm_config() checks the configuration of the other output if it's
>>> compatible with the currently requested setting. The register values are
>>> then actually updated in atmel_tcb_pwm_enable(). To make this race free
>>> the lock must be held during the whole process, so grab the lock in
>>> .apply() instead of individually in atmel_tcb_pwm_disable() and
>>> atmel_tcb_pwm_enable() which then also covers atmel_tcb_pwm_config().
>>>
>>> To simplify handling, use the guard helper to let the compiler care for
>>> unlocking. Otherwise unlocking would be more difficult as there is more
>>> than one exit path in atmel_tcb_pwm_apply().
>>>
>>> Fixes: 9421bade0765 ("pwm: atmel: add Timer Counter Block PWM driver")
>>> Signed-off-by: Uwe Kleine-König<u.kleine-koenig at baylibre.com>
>> Acked-by: Nicolas Ferre<nicolas.ferre at microchip.com>
>>
>> And I like the conversion to the "guard" lock helper.
> I hesitated a bit to add it because it will make backporting to stable
> harder. But I guess we will just not backport it, the problem doesn't
> seem to matter in practise given that it was found by looking at code
> and not hit in real life more more than 11 years after its introduction.

Fair indeed: +1.

> Best regards and thanks for your Acks,
> Uwe