linux-kernel - Re: [PATCH v2 0/2] gpio: create the /sys/class/gpio mount point with GPIO

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Message-ID: <CAMRc=MeLkwRhgdtmmjxiJFvHZNCS55FR=hY3Qc0Gsb_nA725SQ@mail.gmail.com>
Date: Wed, 16 Oct 2024 10:49:57 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Linus Walleij <linus.walleij@...aro.org>, 
	Kent Gibson <warthog618@...il.com>, linux-kernel@...r.kernel.org, 
	linux-gpio@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 0/2] gpio: create the /sys/class/gpio mount point with
 GPIO_SYSFS disabled

On Wed, Oct 16, 2024 at 9:02 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> >
> > The existence of the sysfs API in the kernel makes us stick to some
> > sub-optimal design decisions (like the global GPIO numberspace) that
> > we'll be able to entirely remove once sysfs is gone. We want people to
> > use the GPIO character device and if it takes a layer emulating the
> > old API on top of it to make them switch then it's still better than
> > keeping the API in the kernel.
>
> How are you going to emulate the "global numberspace" in usersapce if
> the kernel isn't exposing that?  Why can't you just do it in the same
> way that you would in userspace here?
>

In the new kernel API, users don't care about the GPIO numbers. Even
the GPIO drivers only care about hardware offsets within the chip they
control. But we still use the numbers for the sake of the sysfs
interface, users of the legacy in-kernel API and some really old
drivers that used to set a hard-coded base GPIO number. For most part
the dynamic base assignment algorithm can be simply moved to
user-space. For those few instances where the base needs to be a
specific value, the FUSE program will take an argument allowing to
specify it.

> Again, the issue is "do not remove apis that userspace relies on".
> That's all.  I'm going to add another one called "do not mount any
> filesystem at /sys/devices/class/ as that is insane" as well :)
>

I know you're not being 100% serious but I think it's worth mentioning
that the mounting is not done from the kernel. You can't really impose
the second one on user-space.

If user-space decides to go "mount -t configfs none
/sys/bus/platform/devices/" or "mount -t devtmpfs none /proc", then
AFAIK there's nothing you can do to stop it.

Bart