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

[Harish Jenny K N <harish_kandiga@xxxxxxxxxx>]

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@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
>>
> 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
>

Can we please finalize this ?


Sorry for the repeated emails.

Am I missing something here ? I am not getting replies.



Thanks,

Harish Jenny K N