linux-kernel - Re: [PATCH v9 07/11] dt-bindings: pwm: pwm-mediatek: add a property "num-pwms"

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Message-ID: <20190923133626.GA4671@ulmo>
Date:   Mon, 23 Sep 2019 15:36:26 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Sam Shih <sam.shih@...iatek.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>, linux-pwm@...r.kernel.org,
        Ryder Lee <ryder.lee@...iatek.com>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        John Crispin <john@...ozen.org>,
        Matthias Brugger <matthias.bgg@...il.com>
Subject: Re: [PATCH v9 07/11] dt-bindings: pwm: pwm-mediatek: add a property
 "num-pwms"

On Mon, Sep 23, 2019 at 11:20:57AM +0800, Sam Shih wrote:
> On Sat, 2019-09-21 at 02:21 +0200, Thierry Reding wrote:
> > On Fri, Sep 20, 2019 at 06:49:07AM +0800, Sam Shih wrote:
> > > From: Ryder Lee <ryder.lee@...iatek.com>
> > > 
> > > This adds a property "num-pwms" in example so that we could
> > > specify the number of PWM channels via device tree.
> > > 
> > > Signed-off-by: Ryder Lee <ryder.lee@...iatek.com>
> > > Signed-off-by: Sam Shih <sam.shih@...iatek.com>
> > > Reviewed-by: Matthias Brugger <matthias.bgg@...il.com>
> > > Acked-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> > > ---
> > > Changes since v6:
> > > Follow reviewers's comments:
> > > - The subject should indicate this is for Mediatek
> > > 
> > > Changes since v5:
> > > - Add an Acked-by tag
> > > - This file is original v4 patch 5/10
> > > (https://patchwork.kernel.org/patch/11102577/)
> > > 
> > > ---
> > >  Documentation/devicetree/bindings/pwm/pwm-mediatek.txt | 7 ++++---
> > >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > You failed to address Rob's questions repeatedly and I agree with him
> > that you can just as easily derive the number of PWMs from the specific
> > compatible string. I won't be applying this and none of the patches that
> > depend on it.
> > 
> 
> Hi, 
> 
> Thanks for getting back to me.
> 
> New pwm driver (patch 04/11 : "pwm: mediatek: allocate the clks array
> dynamically") can support different variants with different number of
> PWMs by the new property <num-pwms>
> 
> For example:
> 1. Use "num-pwms" = <2> and assign clocks pwm1, pwm2 for mt7622
> 2. Use "num-pwms" = <6> and assign clocks pwm1, pwm2, pwm3, pwm4, pwm5,
> pwm6 for mt7622.
> 
> If we just as easily derive the number of PWMs from the specific
> compatible string in this document:
> 
>    - "pwm1-6": the six per PWM clocks for mt7622
>    
> This looks like all "pwm1", "pwm2", "pwm3", "pwm4", "pwm5", "pwm6" is
> required property in DT, It doesn't make sense.

I don't understand. Why doesn't that make sense? If your hardware block
has 6 PWMs and each can be driven by its own clock, then you need to
provide references for each of those clocks, otherwise you won't be able
to use them.

>    
> So we removed those descriptions and added  
> 
>    - "pwm1-N": the PWM clocks for each channel 
>    
>    
> But the max number of clocks from the compatible string are still
> important information that should be provide in this document.
> 
> 
> What do you think of this?
> 
>    - "pwm1-N": per PWM clocks for mt2712, the max number of PWM channels
> is 8
> 
>    - "pwm1-N": per PWM clocks for mt7622, the max number of PWM channels
> is 6
> 
>    - "pwm1-N": per PWM clocks for mt7623, the max number of PWM channels
> is 5

That's what's in the bindings already, isn't it?

	 - clocks: phandle and clock specifier of the PWM reference clock.
	 - clock-names: must contain the following, except for MT7628 which
			has no clocks
	   - "top": the top clock generator
	   - "main": clock used by the PWM core
	   - "pwm1-8": the eight per PWM clocks for mt2712
	   - "pwm1-6": the six per PWM clocks for mt7622
	   - "pwm1-5": the five per PWM clocks for mt7623

Note that the description of the "clocks" property isn't quite accurate.
It should be something like:

	- clocks: One phandle and clock specifier for each entry in the
	          "clock-names" property.

In the above you clearly describe which PWMs you have to specify for
each generation of the hardware block.

> 
>    where N starting from 1 to the maximum number of PWM channels
>    - num-pwms: the number of PWM channels.

That's redundant information. The specific number of PWMs in already
implied by the compatible string, so you don't need to duplicate that
information here.

Thierry

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