Re: [PATCH 1/2] pwm: make it possible to apply pwm changes in atomic context

[Sean Young]

Hello Uwe,

On Wed, Oct 04, 2023 at 11:59:20AM +0200, Uwe Kleine-König wrote:
> On Sun, Oct 01, 2023 at 11:40:29AM +0100, Sean Young wrote:
> > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > index dc66e3405bf5..d9679ae5b2be 100644
> > --- a/drivers/pwm/core.c
> > +++ b/drivers/pwm/core.c
> > @@ -505,7 +505,7 @@ int pwm_apply_state(struct pwm_device *pwm, const struct pwm_state *state)
> >  	 * is a bad idea. So make it explicit that calling this function might
> >  	 * sleep.
> >  	 */
> > -	might_sleep();
> > +	might_sleep_if(pwm_can_sleep(pwm));
> >  
> >  	if (!pwm || !state || !state->period ||
> >  	    state->duty_cycle > state->period)
> 
> I'd like to have a mechanism to catch drivers that missed to set
> .can_sleep. The best idea I currently have for that is to disable
> preemption if IS_ENABLED(CONFIG_PWM_DEBUG) && !pwm_can_sleep(pwm) while
> .apply() is running.

If we have pwm_apply_state_atomic(), then CONFIG_DEBUG_ATOMIC_SLEEP will
catch them, but only in that code path of course.

How about using non_block_start() and non_block_end() if can_sleep is
not set?

> > diff --git a/drivers/pwm/pwm-fsl-ftm.c b/drivers/pwm/pwm-fsl-ftm.c
> > index b7c6045c5d08..b8b9392844e9 100644
> > --- a/drivers/pwm/pwm-fsl-ftm.c
> > +++ b/drivers/pwm/pwm-fsl-ftm.c
> > @@ -405,6 +405,7 @@ static int fsl_pwm_probe(struct platform_device *pdev)
> >  
> >  	fpc->soc = of_device_get_match_data(&pdev->dev);
> >  	fpc->chip.dev = &pdev->dev;
> > +	fpc->chip.can_sleep = true;
> 
> As .apply() being callable in non-sleepable context only depends on
> .apply() I think a better place for this property is in struct pwm_ops.

That makes sense.

> Also I wonder if the distinction between atomic and sleeping
> pwm_state_apply() should be more explicit. For GPIOs you have a sleeping
> variant gpiod_set_value_cansleep() that allows to immediately determine
> the intended context in the caller. This would allow that programming
> a PWM stays a preemption point (if possible/desired) even if the
> underlying hardware/driver is atomic. To not have to touch all consumer
> drivers, maybe the pair for pwm should better be
> 
> 	pwm_apply_state()
> 	pwm_apply_state_atomic()

Do we need pwm_config_atomic(), pwm_enable_atomic(), and pwm_disable_atomic()
too? These are just convenience functions, so we can probably do without them.

> instead of a "cansleep" suffix for the sleeping variant? Or maybe it's
> better to accept touching all consumer drivers to get semantics similar
> to gpio? I couldn't decide quickly what I really like better here, so
> that's your chance to comment and influence the outcome :-)

If you expect to have more parameters for pwm_apply_state() then a flags
argument makes sense.

TBH I like the pwm_apply_state_atomic() option.


Sean