linux-kernel - Re: [PATCH v2 0/6] gpio: cdev: bail out of poll() if the device goes down

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Message-ID: <CAMRc=McXpjqFDfkGMaRqDnLs+sRGYrBK3+5Qsjq9HNYwOHaUgg@mail.gmail.com>
Date:   Fri, 18 Aug 2023 21:16:56 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Kent Gibson <warthog618@...il.com>, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 0/6] gpio: cdev: bail out of poll() if the device goes down

On Fri, Aug 18, 2023 at 3:36 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
>
> On Fri, Aug 18, 2023 at 3:29 PM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> >
> > On Fri, Aug 18, 2023 at 02:06:21PM +0200, Bartosz Golaszewski wrote:
> > > On Fri, Aug 18, 2023 at 12:34 PM Andy Shevchenko
> > > <andriy.shevchenko@...ux.intel.com> wrote:
> > > >
> > > > On Thu, Aug 17, 2023 at 08:49:52PM +0200, Bartosz Golaszewski wrote:
> > > > > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > > > >
> > > > > Wake up all three wake queues (the one associated with the character
> > > > > device file, the one for V1 line events and the V2 line request one)
> > > > > when the underlying GPIO device is unregistered. This way we won't get
> > > > > stuck in poll() after the chip is gone as user-space will be forced to
> > > > > go back into a new system call and will see that gdev->chip is NULL.
> > > >
> > > > Why can't you use the global device unbind notifications and filter out
> > > > what you are interested in?
> > >
> > > There's no truly global device unbind notification - only per-bus.
> > > GPIO devices can reside on any bus, there are no limitations and so
> > > we'd have to subscribe to all of them.
> >
> > We have, but it requires a bit of code patching.
> > Look at device_platform_notify()/device_platform_notify_remove().
> >
> > I noticed, btw, that platform_notify() and Co is a dead code :-)
> > Maybe it can be converted to a list and a manager of that list,
> > so specific cases can utilize it.
> >
>
> That's not going to happen anytime soon and I doubt Greg would like
> the idea without more users interested in receiving these events. :(
>
> This GPIO notifier is an implementation detail - once we do have
> global notifiers, we can switch to using them instead.
>
> Bart

Seems to me the whole platform_notify() thing was meant for a
different purpose. There's no trace for it ever being used for
notifying users about driver bind and unbind events.

It appears to me too that adding a global notifier would be as simple
as extending the functionality of bus_notify() with a second, global
notifier chain. But I won't be the one to propose this to Greg KH. :)

Bart