linux-kernel - Re: [PATCH v2 6/6] gpiolib: notify user-space about in-kernel line state changes

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Message-ID: <CAMRc=MddUUx-iDUWY53nStzt9nutRzB=EkGyaHa+e37Wm+10+A@mail.gmail.com>
Date: Mon, 14 Oct 2024 10:13:59 +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 v2 6/6] gpiolib: notify user-space about in-kernel line
 state changes

On Mon, Oct 14, 2024 at 4:24 AM Kent Gibson <warthog618@...il.com> wrote:
>
> On Thu, Oct 10, 2024 at 11:10:27AM +0200, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >
> > We currently only notify user-space about line config changes that are
> > made from user-space. Any kernel config changes are not signalled.
> >
> > Let's improve the situation by emitting the events closer to the source.
> > To that end let's call the relevant notifier chain from the functions
> > setting direction, gpiod_set_config(), gpiod_set_consumer_name() and
> > gpiod_toggle_active_low(). This covers all the options that we can
> > inform the user-space about. We ignore events which don't have
> > corresponding flags exported to user-space on purpose - otherwise the
> > user would see a config-changed event but the associated line-info would
> > remain unchanged.
> >
> > gpiod_direction_output/input() can be called from any context.
> > Fortunately, we now emit line state events using an atomic notifier
> > chain, so it's no longer an issue.
> >
> > Let's also add non-notifying wrappers around the direction setters in
> > order to not emit superfluous reconfigure events when requesting the
> > lines as the initial config should be part of the request notification.
> >
>
> So lines requested from kernel space will result in a LINE_REQUESTED and
> then a series of LINE_CHANGED_CONFIG?  Whereas for lines requested from
> userspace those will be collapsed into the one LINE_REQUESTED event?

No, why? I added the notification about the request to
gpiod_find_and_request() which is called by all the kernel getters and
it already configures all the flags without emitting events and calls
the non-notify variant of the direction setter. When a kernel driver
requests a GPIO, I only see a single event UNLESS after the
gpiod_get() call returns, it sets direction or changes config - just
like user-space.

Bart

> That's not ideal, but I realise making the rest of the kernel behave as
> per cdev would be non-trivial, so ok - it all comes out in the wash.
> And it is clearly better than nothing.
>
> Cheers,
> Kent.
>