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

[Harish Jenny K N <harish_kandiga@xxxxxxxxxx>]

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@xxxxxxxxxx> wrote:
>> On Mon, Aug 5, 2019 at 5:15 AM Linus Walleij <linus.walleij@xxxxxxxxxx> 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