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

[Harish Jenny K N <harish_kandiga@mentor.com>]

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 ?

Linus has also mentioned in another patchset "[PATCH v2] Input: tsc2007 - use GPIO descriptor" that

he is in favor of introducing explicit inverters in device tree.


Please consider this and let us know your inputs.



Thanks,

Harish Jenny K N