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

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Message-ID: <ZR_hJ05h5O6SpM_D@orome.fritz.box>
Date:   Fri, 6 Oct 2023 12:27:51 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Sean Young <sean@...s.org>, Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Vladimir Zapolskiy <vz@...ia.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Daire McNamara <daire.mcnamara@...rochip.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Fabrice Gasnier <fabrice.gasnier@...s.st.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        linux-pwm@...r.kernel.org,
        Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] pwm: make it possible to apply pwm changes in atomic
 context

On Wed, Oct 04, 2023 at 11:59:20AM +0200, Uwe Kleine-König wrote:
> Hello Sean,
> 
> 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.
> 
> > 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.

What about drivers for devices that can be either sleeping or not? There
are things like regmap which can abstract those differences away, so you
could have a driver that works on both types of devices, so setting this
in ops wouldn't be correct all the time. I think can_sleep needs to be
per-chip rather than per-driver.

Thierry

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