linux-kernel - Re: [PATCH 4/9] gpio: regmap: add the .get

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Message-ID: <CACRpkda+=A5R4vZZQZKmF3LnGd6xMYbNomahgTW+j9aX9swBFA@mail.gmail.com>
Date: Fri, 11 Jul 2025 20:06:59 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: Ioana Ciornei <ioana.ciornei@....com>, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Bartosz Golaszewski <brgl@...ev.pl>, Shawn Guo <shawnguo@...nel.org>, Michael Walle <mwalle@...nel.org>, 
	Lee Jones <lee@...nel.org>, Frank Li <Frank.Li@....com>
Subject: Re: [PATCH 4/9] gpio: regmap: add the .get_direction() callback

On Fri, Jul 11, 2025 at 7:45 PM Andrew Lunn <andrew@...n.ch> wrote:
> On Fri, Jul 11, 2025 at 07:43:13PM +0200, Linus Walleij wrote:
> > On Wed, Jul 9, 2025 at 5:09 PM Andrew Lunn <andrew@...n.ch> wrote:
> >
> > > This is not my area, so i will deffer to the GPIO
> > > Maintainers. However, it is not clear to me what get_direction()
> > > should return.
> >
> > This callback should return the current direction as set up
> > in the hardware.
> >
> > A major usecase is that this is called when the gpiochip is
> > registered to read out all the current directions of the GPIO
> > lines, so the kernel has a clear idea of the state of the
> > hardware.
> >
> > Calling this should ideally result in a read of the status from
> > a hardware register.
>
> O.K, so completely different to what is proposed in this patch.
>
> Maybe you can suggest a better name.

If the hardware only supports one direction, then .get_direction()
should return that direction.

What the patch does is to
read the direction from the hardware and use that in the
set_direction() callback, as if all regmapped hardware in the
world had fixed direction, that's wrong.

I'd just add something custom in gpio-regmap if this is
something reoccuring in regmapped GPIO drivers.

bool is_fixed_direction(struct gpio_regmap *gpio, unsigned int offset)

or so?

Then the core can use is_fixed_direction() together
with gpio_get_direction() to check if it can do
a certain set_direction().

Pseudocode:

mydir = get_direction(line)
if (is_fixed_direction(line) && (mydir != requested_dir)
  return -ERROR;

Yours,
Linus Walleij