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

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Message-ID: <20190814073939.ubgzysmkmmel5h4y@pengutronix.de>
Date:   Wed, 14 Aug 2019 09:39:39 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Baolin Wang <baolin.wang@...aro.org>
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

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

> > > 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.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |