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

[Baolin Wang]

On Wed, 14 Aug 2019 at 16:49, Uwe Kleine-KÃnig
<u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> On Wed, Aug 14, 2019 at 03:52:08PM +0800, Baolin Wang wrote:
> > On 14/08/2019, Uwe Kleine-KÃnig <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> > > On Wed, Aug 14, 2019 at 03:25:53PM +0800, Baolin Wang wrote:
> > >> On Wed, 14 Aug 2019 at 15:01, Uwe Kleine-KÃnig
> > >> <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> > >> > On Wed, Aug 14, 2019 at 09:51:34AM +0800, Baolin Wang wrote:
> > >> > > On Tue, 13 Aug 2019 at 22:13, Uwe Kleine-KÃnig
> > >> > > <u.kleine-koenig@xxxxxxxxxxxxxx> 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-$n"?
>
> The binding you implemented supports "pinctrl-$n". And this is described
> somewhere in the generic pinctrl binding docs. The same applies to
> "assigned-clock-parents".
>
> That you happen to use assigned-clock-parents but not pinctrl-$n on the
> board you used to develop your driver is a detail that IMHO shouldn't
> decide about being listed in the binding doc for your PWM type.
>
> > 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
>
> I agree that if the driver assumes special pinctrl names this is worth
> mentioning. If however there is nothing special and just some generic
> stuff is used that is described in some other location then I'd chose to
> not add this redundant information to the binding document. So if I
> reviewed the patch that created
> Documentation/devicetree/bindings/mmc/sdhci-sprd.txt I would have
> suggested to drop "assigned-clocks" and "assigned-clock-parents" there,
> too.
>
> > > 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
> > ......
>
> I didn't check each of them, but I assume the same applies here, too.
> Please don't copy blindly but think before using other people's stuff as

I did not  copy blindly.

> reference. Even in the Linux kernel where reviews seem and are told to
> catch non-optimal stuff, there are places where this doesn't work. IMHO
> the key question is: Does it add value to describe "assigned-clocks" in
> the binding for your PWM device given that you're only using this
> generic and well documented construct?

I just want to remind users that they should set the parent clock for
PWMs, otherwise the PWM source clock can be not available.

>
> > > 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.
>
> Fine for me.
>
> > >> > > 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.
>
> In my eyes there is a difference between a) The way the clocks are
> implemented in the XZ SoC implies that to actually use the PWM you need
> to reparent some clock; and b) Each "sprd,ums512-pwm" device really
> needs an "assigned-clock" property, otherwise it cannot work.
>
> If you write "required" in the binding doc the semantic should be b) but
> the motivation here seems to be a). Legal questions aside someone could
> implement a PWM that has the same register layout and behaviour as the
> PWM in your SoC but with a different clock tree. Should they use a
> different compatible just because they don't need "assigned-clock"?

Fair enough, I move them to be optional.

-- 
Baolin Wang
Best Regards