linux-kernel - Re: [PATCH v2 1/2] dt-bindings: pwm: sprd: Add Spreadtrum PWM documentation

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Date:   Wed, 14 Aug 2019 15:52:08 +0800
From:   Baolin Wang <baolin.wang@...aro.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Orson Zhai <orsonzhai@...il.com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-pwm@...r.kernel.org, DTML <devicetree@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] dt-bindings: pwm: sprd: Add Spreadtrum PWM documentation

Hi Uwe,

On 14/08/2019, Uwe Kleine-König <u.kleine-koenig@...gutronix.de> wrote:
> On Wed, Aug 14, 2019 at 03:25:53PM +0800, Baolin Wang wrote:
>> Hi Uwe,
>>
>> On Wed, 14 Aug 2019 at 15:01, Uwe Kleine-König
>> <u.kleine-koenig@...gutronix.de> wrote:
>> >
>> > Hello Baolin,
>> >
>> > On Wed, Aug 14, 2019 at 09:51:34AM +0800, Baolin Wang wrote:
>> > > On Tue, 13 Aug 2019 at 22:13, Uwe Kleine-König
>> > > <u.kleine-koenig@...gutronix.de> wrote:
>> > > > On Tue, Aug 13, 2019 at 09:46:40PM +0800, Baolin Wang wrote:
>> > > > > +- assigned-clock-parents: The phandle of the parent clock of PWM
>> > > > > clock.
>> > > >
>> > > > I'm not sure you need to point out assigned-clocks and
>> > > > assigned-clock-parents as this is general clk stuff. Also I wonder
>> > > > if
>> > > > these should be "required properties".
>> > >
>> > > I think I should describe any properties used by PWM node, like
>> > > 'clocks' and 'clock-names' properties, though they are common clock
>> > > properties.
>> >
>> > Then you might want to describe also "status", "assigned-clock-rates",
>> > "pinctrl-$n", "pinctrl-names", "power-domains", "power-domain-names"
>> > and
>> > probably another dozen I'm not aware of.
>>
>> We usually do not describe 'status', but if your device node used
>> "pinctrl-$n", "pinctrl-names" ... common properties, yes, you should
>> describe them to let users know what is the purpose of these
>> properties. That's also asked by DT maintainer Rob.
>
> Does this convince you that you should also describe "pinctrl-$n" and
> the others? If not, why not? We also usually don't describe

Our PWM device node did not use "pinctrl-$n" things, why I should
describe "pinctrl-$n"?
If some devices use pinctrl, yes, they should describe what is the
purpose of pinctrl, see:
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/Documentation/devicetree/bindings/mmc/sdhci-sprd.txt

> assigned-clock-parents. For me these are all in the same catagory:

Lots of dt bindings describe 'assigned-clock-parents',:
./Documentation/devicetree/bindings/display/msm/dsi.txt
./Documentation/devicetree/bindings/display/msm/dsi.txt
./Documentation/devicetree/bindings/display/mediatek/mediatek,hdmi.txt
./Documentation/devicetree/bindings/rtc/st,stm32-rtc.txt
./Documentation/devicetree/bindings/rtc/st,stm32-rtc.txt
./Documentation/devicetree/bindings/rtc/st,stm32-rtc.txt
./Documentation/devicetree/bindings/pci/rockchip-pcie-host.txt
./Documentation/devicetree/bindings/sound/mt2701-afe-pcm.txt
./Documentation/devicetree/bindings/sound/brcm,cygnus-audio.txt
./Documentation/devicetree/bindings/sound/brcm,cygnus-audio.txt
......

> Common properties supported for each devicetree node that represents a
> device. The only difference is that on your board you make use of some
> but not some others.

Fine, let's decide this by PWM maintainer or DT maintainer Rob.

>
>> > > Yes, they are required. Thanks for your comments.
>> >
>> > required in which sense? Why can a Spreadtrum PWM not work when the
>> > clock parents are unspecified?
>>
>> On some Spreadtrum platforms, the default source clock of PWM may not
>> be enabled, so we should force users to select one available source
>> clock for PWM output clock.
>
> Sounds like a bug in the clk tree of your SoC that shouldn't affect how
> the PWM is described in the device tree. After all a parent of a clock
> is supposed to become enabled when the clock gets enabled.

That's not a bug, that's like a MUX, the default source clock of PWM
can be disabled, since we usually do not use the default source clock.
Then we can select a available source clock by the MUX.

-- 
Baolin Wang
Best Regards