linux-kernel - Re: [PATCH 1/2] dt-bindings: pwm: brcm,bcm7038: Document the 'open-drain' property

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Message-ID: <7hdopmy4owoletyq274c2hzxtm7xuxaejakwlas477ax2tfzcu@yrbcxxup5krz>
Date: Thu, 28 Nov 2024 12:35:27 +0100
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: Florian Fainelli <florian.fainelli@...adcom.com>
Cc: Florian Fainelli <f.fainelli@...il.com>, Rob Herring <robh@...nel.org>, 
	linux-kernel@...r.kernel.org, Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, 
	Conor Dooley <conor+dt@...nel.org>, 
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>, Thierry Reding <thierry.reding@...il.com>, 
	"open list:PWM SUBSYSTEM" <linux-pwm@...r.kernel.org>, 
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" <devicetree@...r.kernel.org>, 
	"moderated list:BROADCOM BCM7XXX ARM ARCHITECTURE" <linux-arm-kernel@...ts.infradead.org>, justin.chen@...adcom.com, 
	Linus Walleij <linus.walleij@...aro.org>, linux-gpio@...r.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: pwm: brcm,bcm7038: Document the
 'open-drain' property

Hello Florian,

[adding Linus and linux-gpio to Cc:]

On Tue, Oct 29, 2024 at 09:03:57AM -0700, Florian Fainelli wrote:
> On 10/29/24 03:44, Uwe Kleine-König wrote:
> > On Tue, Oct 15, 2024 at 10:07:10AM -0700, Florian Fainelli wrote:
> > > On 10/15/24 09:32, Rob Herring wrote:
> > > > Another thing to consider is for any PWM controller with more than
> > > > 1 output, you might want this to be per output and therefore should be
> > > > a flag in the cells.
> > > 
> > > Yes, that is a good point, this controller has two channels, so it seems
> > > like increasing the #pwm-cells might be the way to go.
> > 
> > So the idea is something like:
> > 
> > diff --git a/include/dt-bindings/pwm/pwm.h b/include/dt-bindings/pwm/pwm.h
> > index ab9a077e3c7d..d18b006a7399 100644
> > --- a/include/dt-bindings/pwm/pwm.h
> > +++ b/include/dt-bindings/pwm/pwm.h
> > @@ -11,5 +11,6 @@
> >   #define _DT_BINDINGS_PWM_PWM_H
> >   #define PWM_POLARITY_INVERTED			(1 << 0)
> > +#define PWM_OUTPUT_OPEN_DRAIN			(1 << 1)
> >   #endif
> > 
> > and then add support for that to the core and drivers? There is some
> > intersection with pinctrl (depending on hardware). I wonder if
> > abstracting this somehow using the typical pinctrl properties would be a
> > saner option??
> 
> But what if the pin is not managed by a pinctrl provider?

Then create one? If that's the PWM itself that is the pinctrl device it
would look as follows:

	pwm@...08000 {
		compatible = "brcm,bcm7038-pwm";
		pinctrl-0 = <&pwm_pins>;
		reg = <0xf0408000 0x28>;
		#pwm-cells = <2>;
		#pinctrl-cells = <0>;
		clocks = <&upg_fixed>;
		
		pinctrl {
			pwm_pins: pwm-pins {
				pins = "A", "B";
				drive-open-drain;
			};
		};
	};

Maybe this is difficult if there is a pinctrl that configures the output
as "PWM" and then there is that additional register in the PWM IP to
make this pin open drain? One could just use

	pinctrl-0 = <&pwm_pins>, <&system_pinctrl_pwm>;

then. Not entirely sure this is overengineered, but the dt
representation would be nice (IMHO). Thoughts?

> I have started
> going the route of implementing the PWM_OUTPUT_OPEN_DRAIN bit as an
> additional specifier in the #pwm-cells, but I am not sure to what extent
> this should be allowed to be changed at runtime.

I would not expect that the open-drainness needs to change at runtime.

Best regards
Uwe

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