linux-kernel - Re: [Y2038] [PATCH v4 1/4] uinput: Use monotonic times for uinput timestamps.

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Message-ID: <1513288412.18523.285.camel@codethink.co.uk>
Date:   Thu, 14 Dec 2017 21:53:32 +0000
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Deepa Dinamani <deepa.kernel@...il.com>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        Peter Hutterer <peter.hutterer@...-t.net>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [Y2038] [PATCH v4 1/4] uinput: Use monotonic times for uinput
 timestamps.

On Thu, 2017-12-14 at 13:44 -0800, Deepa Dinamani wrote:
> On Thu, Dec 14, 2017 at 1:18 PM, Ben Hutchings
> > <ben.hutchings@...ethink.co.uk> wrote:
> > On Thu, 2017-12-14 at 21:17 +0000, Ben Hutchings wrote:
> > > On Thu, 2017-12-07 at 10:13 -0800, Deepa Dinamani wrote:
> > > > struct timeval which is part of struct input_event to
> > > > maintain the event times is not y2038 safe.
> > > > 
> > > > Real time timestamps are also not ideal for input_event
> > > > as this time can go backwards as noted in the patch
> > > > a80b83b7b8 by John Stultz.
> > > > 
> > > > The patch switches the timestamps to use monotonic time
> > > > from realtime time. This is assuming no one is using
> > > > absolute times from these timestamps.
> > > 
> > > Why is this change not opt-in, as for evdev?  I assume there were
> > > compatibility reasons for not changing evdev's clock by default, so I
> > > would expect them to apply to uinput as well.  (But I'm also prepared
> > > to believe that user-space is now generally compatible with and would
> > > prefer monotonic time from all input devices.)
> > 
> > Never mind, I've gone back and seen Arnd's comments about compatibility
> > on v3.  It might be worth copying those into the commit message though.
> 
> Commit message already talks about this assumption?:
> 
> The patch switches the timestamps to use monotonic time
> from realtime time. This is assuming no one is using
> absolute times from these timestamps.

Yes, but Arnd did a bit of code research to check that assumption.
A commit message that says "we checked and it appears that no user-
space depends on this" looks better than "I assume that no user-space
depends on this".

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.