Re: [RFC PATCH 7/7] dt-bindings: motion: Add motion-simple-pwm bindings

Maud Spierings
2025-03-03T10:53:26Z

On 2/28/25 4:18 PM, Uwe Kleine-König wrote:
> Hey David,
>
> On Fri, Feb 28, 2025 at 11:09:31AM +0100, David Jander wrote:
>> On Fri, 28 Feb 2025 10:37:48 +0100
>> Krzysztof Kozlowski <krzk@kernel.org> wrote:
>>
>>> On 28/02/2025 10:22, David Jander wrote:
>>>>
>>>>>> +
>>>>>> + motion,pwm-inverted:
>>>>>> + $ref: /schemas/types.yaml#/definitions/flag
>>>>> And PWM flag does not work?
>>>> I have seen PWM controllers that don't seem to support the
>>>> PWM_POLARITY_INVERTED flag and those where it just doesn't work. Should all
>>>
>>> Shouldn't the controllers be fixed? Or let's rephrase the question: why
>>> only this PWM consumer needs this property and none of others need it?
>> CCing Uwe Kleine-Koenig and linux-pwm mailing list.
>>
>> I know that at least in kernel 6.11 the pwm-stm32.c PWM driver doesn't
>> properly invert the PWM signal when specifying PWM_POLARITY_INVERTED. I agree
>> this is a probably bug that needs fixing if still present in 6.14-rc. Besides
>> that, if linux-pwm agrees that every single PWM driver _must_ properly support
>> this flag, I will drop this consumer flag an start fixing broken PWM drivers
>> that I encounter. I agree that it makes more sense this way, but I wanted to
>> be sure.
> Some hardwares cannot support PWM_POLARITY_INVERTED. Affected drivers
> include:
>
> pwm-adp5585
> pwm-ntxec
> pwm-raspberrypi-poe
> pwm-rz-mtu3 (software limitation only)
> pwm-sunplus
> pwm-twl-led (not completely sure, that one is strange)
>
> . ISTR that there is a driver that does only support inverted polarity,
> but I don't find it. For an overview I recommend reading through the
> output of:

The only one that I know of is the opencores pwm driver that the
starfive jh71xx uses, I remember talking with you there. That one does
still need a proper review I believe:
https://lore.kernel.org/all/20250106103540.10079-1-william.qiu@starfivetech.com/

It is kind of in a limbo right now

>
> for f in drivers/pwm/pwm-*; do
> echo $f;
> sed -rn '/Limitations:/,/\*\/?$/p' $f;
> echo;
> done | less
>
> . (Note not all drivers have commentary in the right format to unveil
> their limitations.)
>
> For most use-cases you can just do
>
> .duty_cycle = .period - .duty_cycle
>
> instead of inverting polarity, but there is no abstraction in the PWM
> bindings for that and also no helpers in the PWM framework. The problem
> is more or less ignored, so if you have a device with
>
> pwms = <&pwm0 0 PWM_POLARITY_INVERTED>;
>
> and the PWM chip in question doesn't support that, the pwm API functions
> will fail. So the system designer better makes sure that the PWM
> hardware can cope with the needed polarity.
>
> Best regards
> Uwe