Hi Uwe,

On Tue, Jan 06, 2026 at 11:22:57AM +0100, Uwe Kleine-König wrote:

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@vger.kernel.org Signed-off-by: Sean Nyekjaer sean@geanix.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.

For your reference i bisected to that 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.

I have tried to boot stable/master 6.19.0-rc4, my backlight is on! In stm32_pwm_write_waveform() TIMx_CCER is set before MOE is set.

...

---

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.

Fine with me, I just looked at the other PWM drivers and copy/pasted from them :)

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

Very cool, looking forward to hear if you can re-produce.

Best regards Uwe

/Sean