linux-kernel - Re: [PATCH 5/5] gpiolib: notify user-space about in-kernel line state changes

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Message-ID: <CAMRc=MdU5+AC4PyPjuXuG_S7R59OJ-DaaCdX2fZfoCcs5BveJg@mail.gmail.com>
Date: Sat, 5 Oct 2024 11:42:34 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Kent Gibson <warthog618@...il.com>
Cc: Linus Walleij <linus.walleij@...aro.org>, linux-gpio@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH 5/5] gpiolib: notify user-space about in-kernel line state changes

On Sat, Oct 5, 2024 at 9:46 AM Kent Gibson <warthog618@...il.com> wrote:
>
> On Fri, Oct 04, 2024 at 04:43:26PM +0200, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >
> > There is a problem with gpiod_direction_output/input(), namely the fact
> > that they can be called both from sleeping as well as atomic context. We
> > cannot call the blocking notifier from atomic and we cannot switch to
> > atomic notifier because the pinctrl functions we call higher up the stack
> > take a mutex. Let's instead use a workqueue and schedule a task to emit
> > the event from process context on the unbound system queue for minimal
> > latencies.
> >
>
> So now there is a race between the state of the desc changing and the
> notified reading it?
>

Theoretically? Well, yes... In practice I don't think this would
matter. But I understand the concern and won't insist if it's a
deal-breaker for you.

Ideally we'd switch to an atomic notifier but I don't have a good idea
on how to handle pinctrl_gpio_can_use_line(). It digs deep into the
pinctrl code and it's all synchronized with a mutex. Unlike GPIO, it
doesn't make any sense to spend days converting pinctrl to SRCU for a
single corner-case.

I wanted to use in_atomic() to determine whether we can emit the event
immediately or (if we're in interrupt or with a spinlock taken) we
should use a workqueue as a fallback but checkpatch.pl is very adamant
about it being an error (in capital reds).

Bart