linux-kernel - Re: [PATCH] pwm: stm32: handle polarity change when PWM is enabled

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Message-ID: <kemjjoyrhqglqq4p2j6kygspevq2mdbiujtnksw4rkdapoqcfy@zte2c7fhqvn3>
Date: Tue, 6 Jan 2026 11:22:57 +0100
From: Uwe Kleine-König <ukleinek@...nel.org>
To: Sean Nyekjaer <sean@...nix.com>
Cc: 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, linux-stm32@...md-mailman.stormreply.com, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] pwm: stm32: handle polarity change when PWM is enabled

Hello Sean,

On Tue, Jan 06, 2026 at 08:01:57AM +0100, Sean Nyekjaer wrote:
> After commit 7346e7a058a2 ("pwm: stm32: Always do lazy disabling"),
> polarity changes are ignored. Updates to the TIMx_CCER CCxP bits are
> ignored by the hardware when the master output is enabled via the
> TIMx_BDTR MOE bit.
> 
> Handle polarity changes by temporarily disabling the PWM when required,
> in line with apply() implementations used by other PWM drivers.
> 
> Fixes: 7346e7a058a2 ("pwm: stm32: Always do lazy disabling")
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Nyekjaer <sean@...nix.com>
> ---
> This patch is only applicable for stable tree's <= 6.12
> How to explicitly state that and what is the procedure?

I haven't checked in detail yet but I wonder if the problem also exists
in newer kernels. Also I think that changing the polarity with the
hardware on happend before 7346e7a058a2; in that case you blamed the
wrong commit.

So even if we decide to apply a small targetted fix for the issue you
report to stable without an equivalent commit in mainline (due to the
rework the driver saw in v6.13-rc1~157^2~9^2~3 ("pwm: stm32:
Implementation of the waveform callbacks")), I refuse to do that if the
problem still exists in mainline.

> ---
>  drivers/pwm/pwm-stm32.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c
> index eb24054f9729734da21eb96f2e37af03339e3440..d5f79e87a0653e1710d46e6bf9268a59638943fe 100644
> --- a/drivers/pwm/pwm-stm32.c
> +++ b/drivers/pwm/pwm-stm32.c
> @@ -452,15 +452,23 @@ static int stm32_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  
>  	enabled = pwm->state.enabled;
>  
> +	if (state->polarity != pwm->state.polarity) {
> +		if (enabled) {
> +			stm32_pwm_disable(priv, pwm->hwpwm);
> +			enabled = false;
> +		}
> +
> +		ret = stm32_pwm_set_polarity(priv, pwm->hwpwm, state->polarity);
> +		if (ret)
> +			return ret;
> +	}
> +
>  	if (!state->enabled) {
>  		if (enabled)
>  			stm32_pwm_disable(priv, pwm->hwpwm);
>  		return 0;
>  	}
>  
> -	if (state->polarity != pwm->state.polarity)
> -		stm32_pwm_set_polarity(priv, pwm->hwpwm, state->polarity);
> -
>  	ret = stm32_pwm_config(priv, pwm->hwpwm,
>  			       state->duty_cycle, state->period);
>  	if (ret)

I would prefer the following change:

diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c
index eb24054f9729..5f118c20f1ca 100644
--- a/drivers/pwm/pwm-stm32.c
+++ b/drivers/pwm/pwm-stm32.c
@@ -452,12 +452,16 @@ static int stm32_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	enabled = pwm->state.enabled;
 
-	if (!state->enabled) {
+	/* The hardware must be disabled to honor polarity changes. */
+	if (!state->enabled || state->polarity != pwm->state.polarity) {
 		if (enabled)
 			stm32_pwm_disable(priv, pwm->hwpwm);
-		return 0;
+		enabled = false;
 	}
 
+	if (!state->enabled)
+		return 0;
+
 	if (state->polarity != pwm->state.polarity)
 		stm32_pwm_set_polarity(priv, pwm->hwpwm, state->polarity);
 
Maybe it's just me, but I think the resulting code is simpler with this
hunk.

I have hardware using this driver, will set it up later this week for
testing.

Best regards
Uwe

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