linux-kernel - Re: [libgpiod v1.6.x][PATCH] tests: mockup: unbind mockup devices before unloading the module

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Message-ID: <CAMRc=MfMu5kq8pM-Wcqz7uST5+etkuvYnTchyrP6NsHZJ32XRw@mail.gmail.com>
Date:   Thu, 24 Aug 2023 09:39:54 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Kent Gibson <warthog618@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [libgpiod v1.6.x][PATCH] tests: mockup: unbind mockup devices
 before unloading the module

On Thu, Aug 24, 2023 at 9:36 AM Kent Gibson <warthog618@...il.com> wrote:
>
> On Thu, Aug 24, 2023 at 09:32:06AM +0200, Bartosz Golaszewski wrote:
> > On Tue, Aug 22, 2023 at 9:08 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> > >
> > > On Tue, Aug 22, 2023 at 2:21 AM Kent Gibson <warthog618@...il.com> wrote:
> > > >
> > > > On Mon, Aug 21, 2023 at 05:33:39PM +0200, Bartosz Golaszewski wrote:
> > > > > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > > > >
> > > > > gpio-mockup relies on the GPIO devices being registered in module's __init
> > > > > function and them being unregistered in __exit. This works with the GPIO
> > > > > subsystem as it only takes a reference to the underlying owner module when
> > > > > a GPIO descriptor is requested and not when the GPIO device is
> > > > > instantiated.
> > > > >
> > > > > This behavior may change in the future in the kernel so make the behavior
> > > > > of libgpiomockup more correct and have it unbind all mockup devices over
> > > > > sysfs before unloading the module.
> > > > >
> > > >
> > > > Never knew that unbinding was even an option.
> > > > Maybe update gpio-mockup's documentation?
> > > >
> > >
> > > Yeah, I might once we agree on that reference counting patch.
> > >
> > > > Just clarifying what the potential impact of the existing libgpiomockup
> > > > behaviour and future kernel behaviour is - the kernel may log errors but
> > > > otherwise correctly handle userspace unloading behaving badly?
> > > > So this patch is pre-emptory noise reduction?
> > > >
> > >
> > > No, it's a bug-fix-in-advance. gpio-mockup will fail to unload (until
> > > we unbind all devices anyway) if we couple the module's reference with
> > > struct gpio_device. So will every driver that registers devices from
> > > its module_init() function and tears them down in module_exit(). But
> > > these drivers are wrong to do so in the first place and unloading them
> > > sound like a rare thing to do anyway, so I'm willing to give it a try.
> > >
> > > Bartosz
> >
> > So what do you think Kent? Does it make sense to have it in v1.6? I
> > would need to make a new bugfix release but I have something else
> > queued anyway.
> >
>
> If the plan is to change the kernel such that it will no longer unload
> modules with bound devices then the patch totally makes sense.
>
> Cheers,
> Kent.

Linus has not commented on that yet and there's a thing I need to
check first (hopefully today) but I think it will make more sense.
This patch on the other hand is not incorrect even if the behavior
doesn't change. I will queue it.

Bart