Re: [RFC PATCH 2/7] dt-bindings: pwm: document the PWM polarity flag

[Uwe Kleine-König]

Hello Thierry,

On Tue, Mar 17, 2020 at 06:43:44PM +0100, Thierry Reding wrote:
> On Tue, Mar 17, 2020 at 02:32:26PM +0200, Oleksandr Suvorov wrote:
> > Add the description of PWM_POLARITY_NORMAL flag.
> > 
> > Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@xxxxxxxxxxx>
> > ---
> > 
> >  Documentation/devicetree/bindings/pwm/pwm.txt | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/pwm/pwm.txt b/Documentation/devicetree/bindings/pwm/pwm.txt
> > index 084886bd721e..440c6b9a6a4e 100644
> > --- a/Documentation/devicetree/bindings/pwm/pwm.txt
> > +++ b/Documentation/devicetree/bindings/pwm/pwm.txt
> > @@ -46,6 +46,7 @@ period in nanoseconds.
> >  Optionally, the pwm-specifier can encode a number of flags (defined in
> >  <dt-bindings/pwm/pwm.h>) in a third cell:
> >  - PWM_POLARITY_INVERTED: invert the PWM signal polarity
> > +- PWM_POLARITY_NORMAL: don't invert the PWM signal polarity
> 
> This doesn't make sense. PWM_POLARITY_NORMAL is not part of the DT ABI.

"is not part of the DT ABI" is hardly a good reason. If it's sensible to
be used, it is sensible to define it.

(And as far as I understand it, the term PWM_POLARITY_INVERTED isn't
part of the DT ABI either. Only the value 1 has a meaning (for some PWM
controlers).)

> The third cell of the specifier is a bitmask of flags.
> 
> PWM_POLARITY_NORMAL is an enumeration value that evaluates to 0, so it
> makes absolutely no sense as a flag.

Using 0 or PWM_POLARITY_NORMAL doesn't have an effect on the compiled
device tree, that's true. But having the term PWM_POLARITY_NORMAL (in
contrast to a plain 0) in a dts file is useful in my eyes for human
readers.

> PWM signals are considered to be "normal" by default, so no flag is
> necessary to specify that.

GPIOs are considered to be active high by default, still there is
GPIO_ACTIVE_HIGH (which also evaluates to 0). Also there is
IRQ_TYPE_NONE.

Best regards
Uwe

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