linux-kernel - Re: [PATCH] pinctrl: intel: Implements gpio free function

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Date:   Thu, 4 Apr 2019 14:52:08 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        zhuchangchun <zhuchangchun@...e.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        hendychu@...yun.coma,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH] pinctrl: intel: Implements gpio free function

On Thu, Apr 04, 2019 at 12:51:35PM +0200, Enrico Weigelt, metux IT consult wrote:
> On 03.04.19 06:13, Linus Walleij wrote:
> 
> > But the chardev on the other hand will protect you from all this.> > If the program crashes, the lines will be free:ed.> > If two scripts
> try to access the same GPIO, they will be denied.
> Right, when you want this concurrency protection and cleanup stiff
> the chardev is the better choice. But I've already had several cases
> where the simplicity of the sysfs interface is a big win - all you need
> few trivial fs operations.
> 
> That's also nice for exporting in a grid, eg. via 9P (eg. nice for
> quickly building up HIL environments)
> 
> ioctls have the bad side effect that they can't be exported via
> network in a generic way - your remote fs protocol must support all of
> them - even worse: it needs to cope with overlapping ioctl-nr's that
> can have entirely different data structures depending on the actual
> device. And now try to do that w/ reasonable effort and w/o creating
> a shared memory between server and client :p
> 
> Another interesting usecase is permission handling:
> 
> a) some ioctls need special privileges (oh, how I hate all these "if
>    (!capable(CAP_SYS_ADMIN)) ..." lines in the drivers), but you wanna
>    give some unprivileged user access to them
> b) you wanna give an unprivileged user access to specific gpio's,
>    but not to all at once.
> 
> With pure filesystem based approach, you can easly define permissions
> for each filesystem object.

Also you may consider gpiod daemon and it's socket interface, for example.
Yes, complicated, but solves above problems AFAICT.

I guess the best person, missed in Cc, Bartosz, can tell more about user space
interaction.

And btw gpiod still a good to have for other even local cases:
https://github.com/brgl/libgpiod/issues/29
https://github.com/brgl/libgpiod/issues/40
...

> Yes, these usecases aren't so common for average Jon Doe, but the gpio
> subsystem is used that way, out there in the field, and it would be bad
> if that functionality would go away someday.

-- 
With Best Regards,
Andy Shevchenko