Re: [PATCH v3 1/4] uinput: Add ioctl for using monotonic/ boot times

[Dmitry Torokhov]

On Mon, Dec 04, 2017 at 01:38:01PM -0800, Deepa Dinamani wrote:
> On Mon, Dec 4, 2017 at 1:18 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > On Mon, Dec 4, 2017 at 9:36 PM, Deepa Dinamani <deepa.kernel@xxxxxxxxx> wrote:
> >> On Mon, Dec 4, 2017 at 6:21 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> >>> On Mon, Dec 4, 2017 at 1:55 AM, Deepa Dinamani <deepa.kernel@xxxxxxxxx> 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.
> >>>>
> >>>> Arnd Bergmann suggested deprecating real time and using
> >>>> monotonic or other timers for all input_event times as a
> >>>> solution to both the above problems.
> >>>>
> >>>> Add a new ioctl to let the user dictate the kind of time
> >>>> to be used for input events. This is similar to the evdev
> >>>> implementation of the feature. Realtime is still the
> >>>> default time. This is to maintain backward compatibility.
> >>>>
> >>>> The structure to maintain input events will be changed
> >>>> in a different patch.
> >>>
> >>> Based on Peter's comment from when you first posted this,
> >>> https://patchwork.kernel.org/patch/9381209/, I tried to follow
> >>> the code path again, to see if we can come up with a way
> >>> to avoid introducing a new ioctl.
> >>>
> >>> There is one idea I had now: The two events we
> >>> get (upload and erase) are both triggered from evdev,
> >>> which gets called from user space through the EVIOCSFF
> >>> and EVIOCRMFF ioctls. This device already sets the
> >>> clock domain. Would it make sense to send the event
> >>> to the uinput owner using the same clock domain that
> >>> was set by the evdev owner, or are these two separate
> >>> by definition?
> >>
> >> uinput and evdev are two separate drivers. One is to write events to a
> >> virtual device and the other is to read from any input device.
> >> I considered both of these separate as the two events.
> >> Let me know if you guys prefer something else.
> >
> > Ok
> >
> >> We could also do away with this patch and just say we extend time till
> >> 2106 as we change struct input_event if we are okay with using
> >> realtime only for uinput.
> >
> > Another option might be to use monotonic times unconditionally
> > in uinput. The DRM drivers actually did this conversion successfully:
> > they changed the timestamps from real-time to monotonic-time and
> > added a module parameter to revert back to the old behavior. In
> > the end (after a few years) it turned out that nothing relied on real
> > time anyway, so I sent a patch to kill off the option.
> >
> > It seems rather likely that this is in the same category: the user
> > space reading the events either doesn't access the timestamps
> > at all, or is only interested in relative times, not absolute ones.
> >
> > The one program that I found that reads from /dev/uinput
> > is this one: http://svn.navi.cx/misc/trunk/inputpipe/src/
> > Here, all input_events get relayed between two machines,
> > so an input device on one can be used on the other across
> > a socket. The input_event data that we get from uinput gets
> > either interpreted (ignoring the timestamp) or pushed into
> > the evdev interface on the other machine, which then ignores
> > the timestamp in the kernel and creates a new stamp instead.
> 
> Right. I considered using just monotonic times before.
> I decided against it as John did not change the default times to
> monotonic times in a80b83b7b8.
> But, if nobody really cares about the actual timestamps and only
> relative timestamps matter, this might be a better option.

The timestamps in the kernel->userspace path in uinput are only for
force feedback control messages; userspace is not supposed to analyze
them but simply execute the instructions as they come in. I think we
should switch to the monotonic time and see if someone screams at us.
Then we can either add an option or see if there are other means of
resolving the issue.

Thanks.

-- 
Dmitry