linux-kernel - Re: [PATCH v2] gpiolib: notify user-space about line state changes triggered by kernel

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Message-ID: <CAMRc=MfsWAe-bY62s8iBu+iAy76Wn7pxRnx-3iaOT7ym-FUstQ@mail.gmail.com>
Date:   Thu, 24 Aug 2023 11:32:07 +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: [PATCH v2] gpiolib: notify user-space about line state changes
 triggered by kernel

On Thu, Aug 24, 2023 at 11:27 AM Kent Gibson <warthog618@...il.com> wrote:
>
> On Thu, Aug 24, 2023 at 11:11:18AM +0200, Bartosz Golaszewski wrote:
> > On Thu, Aug 24, 2023 at 11:08 AM Kent Gibson <warthog618@...il.com> wrote:
> > >
> > > On Thu, Aug 24, 2023 at 10:55:44AM +0200, Bartosz Golaszewski wrote:
> > > > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > > >
> > > > We currently only emit CHANGED_CONFIG events when the user-space changes
> > > > GPIO config. We won't be notified if changes come from in-kernel. Let's
> > > > call the notifier chain whenever kernel users change direction or any of
> > > > the active-low, debounce or consumer name settings. We don't notify the
> > > > user-space about the persistence as the uAPI has no notion of it.
> > > >
> > > > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > > > ---
> > > > v1 -> v2:
> > > > - use the gpiod_line_state_notify() helper
> > > > - reorder the code in gpiod_set_debounce() for better readability
> > > >
> > > >  drivers/gpio/gpiolib.c | 16 ++++++++++++++--
> > > >  1 file changed, 14 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> > > > index 40a0022ea719..1cb7731550ca 100644
> > > > --- a/drivers/gpio/gpiolib.c
> > > > +++ b/drivers/gpio/gpiolib.c
> > > > @@ -2439,6 +2439,7 @@ int gpiod_direction_input(struct gpio_desc *desc)
> > > >       }
> > > >       if (ret == 0) {
> > > >               clear_bit(FLAG_IS_OUT, &desc->flags);
> > > > +             gpiod_line_state_notify(desc, GPIO_V2_LINE_CHANGED_CONFIG);
> > > >               ret = gpio_set_bias(desc);
> > > >       }
> > > >
> > >
> > > gpiod_direction_input() is called by cdev when a line is requested.
> > > So requesting a line now creates two events - REQUESTED and
> > > HANGED_CONFIG? Even worse - it calls gpiod_direction_input() first, so we
> > > will get CHANGED_CONFIG then REQUESTED??
> > >
> > > And a config change from cdev can call this and then generate a
> > > CHANGED_CONFIG event itself, so again double events.
> > >
> > > Same for output and probably debounce too (that one is a bit more
> > > convoluted).
> >
> > Ah, should have tested it with user-space too...
> >
> > Back to the drawing board I guess. May be the reason why we're not
> > doing it in the first place yet.
> >
>
> I think we were only looking at it from the userspace PoV, so what
> constitutes a "change" is only well defined when driven from userspace.
> This might be difficult to generalise without redefining what
> "changed" means, or providing cdev with a separate gpiolib API so you
> can tell whether the change is actually part of a cdev request, or ...
>
> So, yeah - more thought required.
>
> Got a use case that requires this, or just a nice to have?
>

I'm extending the virtual consumer driver and wanted to test from
user-spaces things like changing direction and other settings. I
figured it would make more sense to notify about all changes no matter
the source.

Bart