linux-kernel - Re: [PATCH 1/2] pinctrl: core: configure pinmux from pins debug file

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Date:   Mon, 17 May 2021 15:57:13 -0700
From:   Drew Fustini <drew@...gleboard.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Dario Binacchi <dariobin@...ero.it>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH 1/2] pinctrl: core: configure pinmux from pins debug file

On Mon, May 17, 2021 at 11:02:00PM +0300, Andy Shevchenko wrote:
> On Sun, May 16, 2021 at 7:43 PM Dario Binacchi <dariobin@...ero.it> wrote:
> >
> > The MPUs of some architectures (e.g AM335x) must be in privileged
> > operating mode to write on the pinmux
> 
> pinmux is not pin configuration. You need to rethink the approach.
> 
> > registers. In such cases, where
> > writes will not work from user space, now it can be done from the pins
> > debug file if the platform driver exports the pin_dbg_set() helper among
> > the registered operations.
> 
> Drew, is it similar to what you are trying to achieve?

Yes, I would say this similar to what I was trying to accomplish: being
able to change contents of conf_<module>_<pin> register [table 9-60]
from userspace.

However, I was specifically looking to change bits 2:0 which is mux
mode. My motivation was to allow BeagleBone users to easily switch
between pin functions on the expansion headers during runtime to make
rapid prototyping with a breadboard easier (such as changing header pin
from GPIO to SPI mode). Most of the header pins have 7 different modes.

Ultimately, the solution I settled on with feedback from this list was
to create pinmux-select debugfs file that can activate desired fucntion:
6199f6becc86 ("pinctrl: pinmux: Add pinmux-select debugfs file")

Bits 6:3 are related to what this subsystem would refer to as pin conf
such as slew, input enable and bias. Thus it might make sense to expose
something like a select-pinconf file to activate pin conf settings from
userspace. This would require using 'pinconf-single' compatible.

I fixed pinctrl-single bug regarding pinconf last year so it should be
possible to use 'pinconf-single' compatible for the am33xx_pinmux node:
f46fe79ff1b6 ("pinctrl-single: fix pcs_parse_pinconf() return value")

Thanks,
Drew