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

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Message-ID: <fwaodg2ovh7j47ifwjhgeppxs3oiqht5ecbs7bmfbi7j6djejs@shwokpcmutr3>
Date: Wed, 07 Jan 2026 16:05:56 +0000
From: Sean Nyekjaer <sean@...nix.com>
To: Uwe Kleine-König <ukleinek@...nel.org>
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

Hi Uwe,

On Wed, Jan 07, 2026 at 04:54:46PM +0100, Uwe Kleine-König wrote:
> Hey Sean,
> 
> On Tue, Jan 06, 2026 at 11:30:34AM +0000, Sean Nyekjaer wrote:
> > On Tue, Jan 06, 2026 at 11:22:57AM +0100, Uwe Kleine-König wrote:
> > > 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.
> > > [...]
> > > 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.
> 
> I cannot. I have:
> 
> 	# uname -r
> 	6.11.0-rc1-00028-geb18504ca5cf-dirty
> 
> (the -dirty is only from enabling the pwm for my machine, no driver
> changes)
> 
> 	# cat /sys/kernel/debug/pwm
> 	0: platform/40001000.timer:pwm, 4 PWM devices
> 	...
> 	 pwm-3   (sysfs               ): requested enabled period: 313720 ns duty: 10000 ns polarity: normal
> 
> and pulseview/sigrok detects 3.187251% with a period of 313.8 µs.
> 
> After
> 
> 	echo inversed > /sys/class/pwm/pwmchip0/pwm3/polarity
> 
> the output changes to
> 
> 	# cat /sys/kernel/debug/pwm
> 	0: platform/40001000.timer:pwm, 4 PWM devices
> 	...
> 	 pwm-3   (sysfs               ): requested enabled period: 313720 ns duty: 10000 ns polarity: inverse
> 
> and pulseview/sigrok claims 96.812749% with a period of 313.8 µs.
> So the polarity change happend as expected.
> 
> This is on an st,stm32mp135f-dk board.
> 
> Where is the difference to your observations?
> 

Thanks for taking a look!
I'm using the PWM for a backlight. With this [0] in my dts:

[0]:
	backlight: backlight {
		compatible = "pwm-backlight";
		pwms = <&pwm4 0 125000 PWM_POLARITY_INVERTED>;
		brightness-levels = <102 235 255>;
		default-brightness-level = <80>;
		num-interpolated-steps = <100>;
		enable-gpios = <&gpiof 12 GPIO_ACTIVE_LOW>;
	status = "okay";
	};

Maybe that is doing something differently.

/Sean