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

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Date: Sun, 5 May 2024 14:25:00 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Doug Berger <opendmb@...il.com>
Cc: Linus Walleij <linus.walleij@...aro.org>, 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 Fri, May 3, 2024 at 10:21 PM Doug Berger <opendmb@...il.com> wrote:
>
> 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
>

Linus,

Please let me know if this is still a NAK, if so, I'll drop this
series from my tree at least for this release.

Bart