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

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Date:   Wed, 4 Oct 2023 11:59:20 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Sean Young <sean@...s.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        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

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.

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()

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 :-)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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