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Date:   Tue, 18 May 2021 11:38:38 +0200 (CEST)
From:   Dario Binacchi <dariobin@libero.it>
To:     Drew Fustini <drew@beagleboard.org>,
        Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>
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Subject: Re: [PATCH 1/2] pinctrl: core: configure pinmux from pins debug
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Hi,

> Il 18/05/2021 00:57 Drew Fustini <drew@beagleboard.org> ha scritto:
> 
>  
> 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@libero.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")
> 

In the kernel version 4.1.6 that I am using on my custom board, I have fixed
the commit f07512e615dd ("pinctrl/pinconfig: add debug interface"). However, 
this feature was later removed (https://lore.kernel.org/patchwork/patch/1033755/). 
Do you think it is better to bring that functionality back to life or the submitted
patch could be fine too?

Thanks and regards,
Dario

> Thanks,
> Drew