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Date:   Fri, 18 Dec 2020 18:01:25 +0200
From:   Andy Shevchenko <>
To:     Drew Fustini <>
Cc:     "open list:GPIO SUBSYSTEM" <>,
        Linux Kernel Mailing List <>,
        Pantelis Antoniou <>,
        Pantelis Antoniou <>,
        Pantelis Antoniou <>,
        Jason Kridner <>,
        Robert Nelson <>,
        Linus Walleij <>,
        Tony Lindgren <>
Subject: Re: [RFC PATCH v2] pinctrl: add helper to expose pinctrl state in debugfs

On Fri, Dec 18, 2020 at 6:52 AM Drew Fustini <> wrote:
> [0] currently uses an out-of-tree driver called
> bone-pinmux-helper [1] developed by Pantelis Antoniou [2] back in 2013.
> The driver assists users of our BeagleBone and PocketBeagle boards in
> rapid prototyping by allowing them to change at run-time between defined
> set of pinctrl states [3] for each pin on the expansion connectors [4].
> This is achieved by exposing a 'state' file in sysfs for each pin which
> is used by our 'config-pin' utility [5].
> Our goal is to eliminate all out-of-tree drivers for
> boards and thus I have been working to replace bone-pinmux-helper with a
> new driver that could be acceptable upstream. My understanding is that
> debugfs, unlike sysfs, could be the appropriate mechanism to expose such
> functionality.

No objections here.

> I used the compatible string "pinctrl,state-helper" but would appreciate
> advice on how to best name this. Should I create a new vendor prefix?

Here is the first concern. Why does this require to be a driver with a
compatible string?

> The P9_14_pinmux entry would cause pinctrl-state-helper to be probed.
> The driver would create the corresponding pinctrl state file in debugfs
> for the pin.  Here is an example of how the state can be read and
> written from userspace:
> root@...glebone:~# cat /sys/kernel/debug/pinctrl/pinctrl_state/ocp:P9_14_pinmux/state
> default
> root@...glebone:~# echo pwm > /sys/kernel/debug/pinctrl/pinctrl_state/ocp:P9_14_pinmux/state
> root@...glebone:~# cat /sys/kernel/debug/pinctrl/pinctrl_state/ocp:P9_14_pinmux/state
> pwm
> I would very much appreciate feedback on both this general concept, and
> also specific areas in which the code should be changed to be acceptable
> upstream.

Two more concerns:
 - why is it OF only?
 - why has it been separated from pin control per device debug folder?

> [0]
> [1]
> [2]
> [3]
> [4]
> [5]

With Best Regards,
Andy Shevchenko

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