linux-kernel - Re: [RESEND PATCH v6 6/7] gpiolib: add new ioctl() for monitoring changes in line info

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Date:   Thu, 20 Feb 2020 16:03:10 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Arnd Bergmann <arnd@...db.de>, Kent Gibson <warthog618@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [RESEND PATCH v6 6/7] gpiolib: add new ioctl() for monitoring
 changes in line info

On Wed, Feb 12, 2020 at 12:00 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> > On Tue, Feb 11, 2020 at 10:19 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> > > From: Bartosz Golaszewski <bgolaszewski@...libre.com>

> > A question:
> >
> > Bartosz, since you know about possible impacts on userspace,
> > since this code use the preferred ktime_get_ns() rather than
> > ktime_get_ns_real(), what happens if we just patch the other
> > event timestamp to use ktime_get_ns() instead, so we use the
> > same everywhere?
> >
> > If it's fine I'd like to just toss in a patch for that as well.
> >
>
> Arnd pointed out it would be an incompatible ABI change[1].

Yeah, I was thinking more about this specific answer from Arnd:

> "It is an incompatible ABI change, the question here is whether anyone
> actually cares. If nothing relies on the timestamps being in
> CLOCK_REALTIME domain, then it can be changed, the question
> is just how you want to prove that this is the case."

So the question is if userspace really cares.

What happens with libgpiod or users of it? Are they assuming
the weirdness of CLOCK_REALTIME, or are they simply assuming
something that is monotonic increasing and just lucky that they
didn't run into anything jumping backwards in time even though
they *could*.

I think I'll propose a change and see what people say.

> However - I asked Khouloud who's working on v2 of the line event
> interface to use ktime_get_ns().

That's great!

Yours,
Linus Walleij