Subject: Re: [PATCH v5 1/3] dt-bindings: pinctrl: Add bindings for pinctrl-microchip-sgpio driver

From: Lars Povlsen <[email-masked]>
Date: Fri, 9 Oct 2020 12:00:43 +0200

Linus Walleij writes:

> Hi Lars!
>
> This is overall looking fine. Except for the 3 cell business. I just can't
> wrap my head around why that is needed.
>
> On Thu, Oct 8, 2020 at 3:05 PM Lars Povlsen <[email-masked]> wrote:
>
>> +      '#gpio-cells':
>> +        const: 3
>
> So at the very least needs a description making it crystal clear why each
> cell is needed, and used for since the standard bindings are not used.
>
> +      sgpio_in2: gpio@0 {
> +        reg = <0>;
> +        compatible = "microchip,sparx5-sgpio-bank";
> +        gpio-controller;
> +        #gpio-cells = <3>;
> +        ngpios = <96>;
> +      };
>
> So here reg = 0 and the out port has reg 1. Isn't that what you also put
> in the second cell of the GPIO phandle? Then why? The driver
> can very well just parse its own reg property and fill that in.

Linus,

NO! The second cell is the second dimension - NOT the direction. As I
wrote previously, the direction is now inherent from the handle, ie. the
"reg" value of the handle.

The hardware describe a "port" and a "bit index" addressing, where the
second cell in

  gpios = <&sgpio_in2 11 0 GPIO_OUT_LOW>;

is the "bit index" - not the "reg" from the phandle.

In the example above, note

  ngpios = <96>;

As the "port" is [0; 31], this defines "bit index" to be [0; 2], so the
(input) GPIO cells will be:

p0b0, p0b1, p0b2
...
p31b0, p31b1, p31b2 

being identical to 

<&sgpio_inX 0 0 GPIO_OUT_LOW>
<&sgpio_inX 0 1 GPIO_OUT_LOW>
<&sgpio_inX 0 2 GPIO_OUT_LOW>
...
<&sgpio_inX 31 0 GPIO_OUT_LOW>
<&sgpio_inX 31 1 GPIO_OUT_LOW>
<&sgpio_inX 31 2 GPIO_OUT_LOW>

('X' being the SGPIO controller instance).

So no, there *really* is a need for a 3-cell GPIO specifier (or whatever
its called).

Hope this is clearer now...

---Lars

>
> When you obtain a phandle like that:
>
> gpios = <&sgpio_in2 11 0 GPIO_OUT_LOW>;
>
> Isn't that 0 just duplicating the "reg"? Just parse reg when you set up
> your driver state and put it as variable in the state container for your
> driver state for this particular gpio_chip. No need to get it from
> the phandle.
>
> Yours,
> Linus Walleij

-- 
Lars Povlsen,
Microchip

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