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

[Harish Jenny K N <harish_kandiga@xxxxxxxxxx>]

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@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
>

Can you please comment on this ?


Thanks,

Harish Jenny K N