[PATCH 2/3] gpio: of: support gpio-ranges for multiple gpiochip devices
Doug Berger
opendmb at gmail.com
Fri May 3 13:21:25 PDT 2024
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 at gmail.com> 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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