linux-kernel - Re: [PATCH v3 2/2] pwm: Add support for pwm nexus dt bindings

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Message-ID: <6js6k6xz3vuqshq2pfwqifby4t5q54ftztxxw2rau4j23xx2y5@u5xubi6v3uil>
Date: Wed, 5 Feb 2025 17:29:30 +0100
From: Uwe Kleine-König <ukleinek@...nel.org>
To: Herve Codina <herve.codina@...tlin.com>
Cc: Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, linux-pwm@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Luca Ceresoli <luca.ceresoli@...tlin.com>, Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v3 2/2] pwm: Add support for pwm nexus dt bindings

Hello Hervé,

On Wed, Feb 05, 2025 at 02:37:37PM +0100, Herve Codina wrote:
> On Wed, 5 Feb 2025 12:38:32 +0100
> Uwe Kleine-König <ukleinek@...nel.org> wrote:
> > Does this also work if &soc_pwm2 has #pwm-cells = <2>? Would I need just
> > 
> > 	pwm-map = <0 0 0 &soc_pwm1 1 0 0>,
> > 		  <1 0 0 &soc_pwm2 4 0>,
> > 		  <2 0 0 &soc_pwm1 3 0 0>;
> 
> Yes, exactly.
> 
> > 
> > then and
> > 
> > 	pwms = <&connector 1 57000 0>;
> > 
> > would then have the same effect as
> > 
> > 	pwms = <&soc_pwm2 4 57000>
> 
> Yes, the last 0 (or any other values) in pwms = <&connector 1 57000 0> is
> simply dropped in the translation (#pwm-cells = 3 in connector nexus to
> #pwm-cells = 2 in soc_pwm1 node).
> 
> In more generic terms, it works in translation from #pwm-cells = N to
> #pwm-cells = M by simply dropping the last N-M values.
> 
> Also note that even if values are dropped, you need to have them set when
> you point the nexus node because #pwm-cells = 3 is set in the connector
> node and need to be fixed and usable for all the entries in the
> pwm-map table.

Makes sense.

> > and the 0 is dropped then? Could I adapt the mapping that the effect is
> > 
> > 	pwms = <&soc_pwm2 57000 0>
> 
> In this one, I think you miss the PWM number
> 
> If I read correctly this line you ask for the PWM 57000 from the soc_pwm2
> controller. This doesn't make sense :)

Some pwm chip devices with only a single output line use this. The first
paramter is the default period (which is passed in the 2nd parameter
normally) and the 2nd paramter are flags (normally the 3rd parameter).
Back then the rationale was that for such hardware, the line index is
zero always anyhow, and so could better be skipped.

Compare of_pwm_xlate_with_flags() to of_pwm_single_xlate(). pwm-pxa is
the single offender using the latter. Thinking about that, it's easy
enough to fix without breaking compatibility. I'll tackle that.

So for a PWM on pxa `<&soc_pwm2 57000 0>` works fine.

Best regards
Uwe

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