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

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Message-ID: <45b7742c-9cde-4238-9c2c-c75dfbe9d8f3@gmail.com>
Date: Fri, 3 May 2024 13:21:25 -0700
From: Doug Berger <opendmb@...il.com>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: Bartosz Golaszewski <brgl@...ev.pl>, Phil Elwell <phil@...pberrypi.com>,
 Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>,
 Florian Fainelli <florian.fainelli@...adcom.com>,
 bcm-kernel-feedback-list@...adcom.com, linux-gpio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] gpio: of: support gpio-ranges for multiple gpiochip
 devices

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@...il.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