Re: [PATCH V4 2/2] gpio: inverter: document the inverter bindings

[Rob Herring <robh+dt@kernel.org>]

On Wed, Sep 4, 2019 at 5:58 AM Harish Jenny K N
<harish_kandiga@mentor.com> wrote:
>
> Hi Rob, Hi Linus,
>
>
> On 30/08/19 10:51 AM, Harish Jenny K N wrote:
> > Hi Rob,
> >
> >
> > On 27/08/19 1:17 PM, Harish Jenny K N wrote:
> >> Hi Rob,
> >>
> >>
> >> On 19/08/19 3:06 PM, Harish Jenny K N wrote:
> >>> Hi Rob,
> >>>
> >>>
> >>> On 10/08/19 2:21 PM, Linus Walleij wrote:
> >>>> On Fri, Aug 9, 2019 at 4:08 PM Rob Herring <robh+dt@kernel.org> wrote:
> >>>>> On Mon, Aug 5, 2019 at 5:15 AM Linus Walleij <linus.walleij@linaro.org> wrote:
> >>>>>> There is some level of ambition here which is inherently a bit fuzzy
> >>>>>> around the edges. ("How long is the coast of Britain?" comes to mind.)
> >>>>>>
> >>>>>> Surely the intention of device tree is not to recreate the schematic
> >>>>>> in all detail. What we want is a model of the hardware that will
> >>>>>> suffice for the operating system usecases.
> >>>>>>
> >>>>>> But sometimes the DTS files will become confusing: why is this
> >>>>>> component using GPIO_ACTIVE_LOW when another system
> >>>>>> doesn't have that flag? If there is an explicit inverter, the
> >>>>>> DTS gets more readable for a human.
> >>>>>>
> >>>>>> But arguable that is case for adding inverters as syntactic
> >>>>>> sugar in the DTS compiler instead...
> >>>>> If you really want something more explicit, then add a new GPIO
> >>>>> 'inverted' flag. Then a device can always have the same HIGH/LOW flag.
> >>>>> That also solves the abstract it for userspace problem.
> >>>> I think there are some intricate ontologies at work here.
> >>>>
> >>>> Consider this example: a GPIO is controlling a chip select
> >>>> regulator, say Acme Foo. The chip select
> >>>> has a pin named CSN. We know from convention that the
> >>>> "N" at the end of that pin name means "negative" i.e. active
> >>>> low, and that is how the electronics engineers think about
> >>>> that chip select line: it activates the IC when
> >>>> the line goes low.
> >>>>
> >>>> The regulator subsystem and I think all subsystems in the
> >>>> Linux kernel say the consumer pin should be named and
> >>>> tagged after the datsheet of the regulator.
> >>>>
> >>>> So it has for example:
> >>>>
> >>>> foo {
> >>>>     compatible = "acme,foo";
> >>>>     cs-gpios = <&gpio0 6 GPIO_ACTIVE_LOW>;
> >>>> };
> >>>>
> >>>> (It would be inappropriate to name it "csn-gpios" since
> >>>> we have an established flag for active low. But it is another
> >>>> of these syntactic choices where people likely do mistakes.)
> >>>>
> >>>> I think it would be appropriate for the DT binding to say
> >>>> that this flag must always be GPIO_ACTIVE_LOW since
> >>>> the bindings are seen from the component point of view,
> >>>> and thus this is always active low.
> >>>>
> >>>> It would even be reasonable for a yaml schema to enfore
> >>>> this, if it could. It is defined as active low after all.
> >>>>
> >>>> Now if someone adds an inverter on that line between
> >>>> gpio0 and Acme Foo it looks like this:
> >>>>
> >>>> foo {
> >>>>     compatible = "acme,foo";
> >>>>     cs-gpios = <&gpio0 6 GPIO_ACTIVE_HIGH>;
> >>>> };
> >>>>
> >>>> And now we get cognitive dissonance or whatever I should
> >>>> call it: someone reading this DTS sheet and the data
> >>>> sheet for the component Acme Foo to troubleshoot
> >>>> this will be confused: this component has CS active
> >>>> low and still it is specified as active high? Unless they
> >>>> also look at the schematic or the board and find the
> >>>> inverter things are pretty muddy and they will likely curse
> >>>> and solve the situation with the usual trial-and-error,
> >>>> inserting some random cursewords as a comment.
> >>>>
> >>>> With an intermediate inverter node, the cs-gpios
> >>>> can go back to GPIO_ACTIVE_LOW and follow
> >>>> the bindings:
> >>>>
> >>>> inv0: inverter {
> >>>>     compatible = "gpio-inverter";
> >>>>     gpio-controller;
> >>>>     #gpio-cells = <1>;
> >>>>     inverted-gpios = <&gpio0 6 GPIO_ACTIVE_HIGH>;
> >>>> };
> >>>>
> >>>> foo {
> >>>>     compatible = "acme,foo";
> >>>>     cs-gpios = <&inv0 0 GPIO_ACTIVE_LOW>;
> >>>> };
> >>>>
> >>>> And now Acme Foo bindings can keep enforcing cs-gpios
> >>>> to always be tagged GPIO_ACTIVE_LOW.
> >>> Can you please review/let us know your opinion on this ? I think the idea here is to also isolate the changes to a separate consumer driver and avoid getting inversions inside gpiolib.
> >>>
> >>>
> >>> Thanks.
> >>>
> >>>
> >>> Regards,
> >>>
> >>> Harish Jenny K N
> >>>
> >> Can you please comment on this ?
> >>
> >>
> >> Thanks,
> >>
> >> Harish Jenny K N
> >>
> > Friendly Reminder.
> >
> > can we please finalize this ?

I think I have made my position clear and don't really have more to
add. I'm simply not convinced of the need for this. An inverter is not
a GPIO controller. You can't set/get or do any control. It is already
possible to invert GPIO lines in DT by changing the flags and it has
been this way for decades.

Rob