Re: [PATCH 2/3] gpio: of: support gpio-ranges for multiple gpiochip devices

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On 5/3/2024 1:25 AM, Linus Walleij wrote:
Hi Dough,

thanks for your patch!
Thanks for your review!


I'm a bit confused here:
"Communication is hard" and I may be confused about your confusion, but hopefully we can work it out.


On Wed, Apr 24, 2024 at 8:51 PM Doug Berger <opendmb@xxxxxxxxx> wrote:


+               /* Ignore ranges outside of this GPIO chip */
+               if (pinspec.args[0] >= (chip->offset + chip->ngpio))
+                       continue;
+               if (pinspec.args[0] + pinspec.args[2] <= chip->offset)
+                       continue;

Here pinspec.args[0] and [2] comes directly from the device tree.

The documentation in Documentation/devicetree/bindings/gpio/gpio.txt
says:

2.2) Ordinary (numerical) GPIO ranges
-------------------------------------

It is useful to represent which GPIOs correspond to which pins on which pin
controllers. The gpio-ranges property described below represents this with
a discrete set of ranges mapping pins from the pin controller local number space
to pins in the GPIO controller local number space.

The format is: <[pin controller phandle], [GPIO controller offset],
                 [pin controller offset], [number of pins]>;

The GPIO controller offset pertains to the GPIO controller node containing the
range definition.
I think we are in agreement here. For extra clarity, I will add that in my understanding pinspec.args[0] corresponds to [GPIO controller offset] and pinspec.args[2] corresponds to [number of pins].


So I do not understand how pinspec[0] and [2] can ever be compared
to something involving chip->offset which is a Linux-specific offset.

It rather looks like you are trying to accomodate the Linux numberspace
in the ranges, which it was explicitly designed to avoid.
The struct gpio_chip documentation in include/linux/gpio/driver.h says:

> * @offset: when multiple gpio chips belong to the same device this
> *	can be used as offset within the device so friendly names can
> *	be properly assigned.

It is my understanding that this value represents the offset of a gpiochip relative to the GPIO controller device defined by the GPIO controller node in device tree. This puts it in the same number space as [GPIO controller offset]. I believe it was introduced for the specific purpose of translating [GPIO controller offset] values into Linux-specific offsets, which is why it is being reused for that purpose in this patch.

For GPIO Controllers that contain a single gpiochip the 'offset' member is 0 and the device tree node offsets can be applied directly to the gpiochip. However, when a GPIO Controller contains multiple gpiochips, the device tree node offsets must be translated to each individual gpiochip.


I just don't get it.

So NACK until I understand what is going on here.

Yours,
Linus Walleij
I hope it makes sense now, but if not please help me understand what I may be missing.

Thanks,
    Doug





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