Re: [Y2038] [PATCH v4 06/10] cec: add HDMI CEC framework: y2038 question

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 06 May 2015 17:58:01 Hans Verkuil wrote:
> 
> On 05/04/2015 12:14 PM, Arnd Bergmann wrote:
> > On Monday 04 May 2015 09:42:36 Hans Verkuil wrote:
> >> Ping! (Added Arnd to the CC list)
> > 
> > Hi Hans,
> > 
> > sorry I missed this the first time
> > 
> >> On 04/27/2015 09:40 AM, Hans Verkuil wrote:
> >>> Added the y2038 mailinglist since I would like to get their input for
> >>> this API.
> >>>
> >>> Y2038 experts, can you take a look at my comment in the code below?
> >>>
> >>> Thanks!
> >>
> >> Arnd, I just saw your patch series adding struct __kernel_timespec to
> >> uapi/linux/time.h. I get the feeling that it might take a few kernel
> >> cycles before we have a timespec64 available in userspace. Based on that
> >> I think this CEC API should drop the timestamps for now and wait until
> >> timespec64 becomes available before adding it.
> >>
> >> The timestamps are a nice-to-have, but not critical. So adding it later
> >> shouldn't be a problem. What is your opinion?
> > 
> > It will take a little while for the patches to make it in, I would guess
> > 4.3 at the earliest. Using your own struct works just as well and would
> > be less ambiguous.
> > 
> > However, for timestamps, I would recommend not using timespec anyway.
> > Instead, just use a single 64-bit nanosecond value from ktime_get_ns()
> > (or ktime_get_boot_ns() if you need a time that keeps ticking across
> > suspend). This is more efficient to get and simpler to use as long
> > as you don't need to convert from nanosecond to timespec.
> 
> Possibly stupid follow-up question:
> 
> is ktime_get_ns() just a different representation as ktime_get_ts64()?

Yes.

> Or is there some offset between the two? They seem to be identical based
> on a quick test, but I'd like to be certain that that's always the case.
>
> Users need to be able to relate this timestamp to a struct timespec as
> returned by V4L2 (and others).

* ktime_get_ns() uses the same timebase as ktime_get_ts64().
* ktime_get_boot_ns() uses the same timebase as ktime_get_boottime() or
  getboottime64(), which differs from the first after suspend
* ktime_get_real_ns() uses the same time as gettimeofday() in user space,
  which is always different from the other two.

	Arnd