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

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Message-Id: <DBBKJ05VNSDG.30MNWDWT9JAEC@kernel.org>
Date: Mon, 14 Jul 2025 08:36:03 +0200
From: "Michael Walle" <mwalle@...nel.org>
To: "Linus Walleij" <linus.walleij@...aro.org>, "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>, "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 8:06 PM CEST, Linus Walleij wrote:
> 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;

You don't need a .is_fixed_direction(). You can deduce that if only
.get_direction() is set in the gpio-regmap config.

mydir = get_direction(line)
if (!config->set_direction && mydir != requested_dir)
  return -ERROR;

That or either Andrew's idea of setting a bitmap within the
gpio-regmap config which already tells the gpio-regmap core and then
amend gpio_regmap_get_direction() to return that fixed direction if
that bitmap is not NULL.

I'm fine with both.

-michael

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