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

[Arnd Bergmann <arnd@arndb.de>]

On Mon, Dec 4, 2017 at 9:36 PM, Deepa Dinamani <deepa.kernel@gmail.com> wrote:
> On Mon, Dec 4, 2017 at 6:21 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>> On Mon, Dec 4, 2017 at 1:55 AM, Deepa Dinamani <deepa.kernel@gmail.com> 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.

       Arnd