unique id for wayland objects

Pekka Paalanen ppaalanen at gmail.com
Tue May 22 07:51:36 UTC 2018

On Mon, 21 May 2018 23:06:12 +0800
zou lan <nancy.lan.zou at gmail.com> wrote:

> Hi pekka
> I debug the presentation time recently. I have checked each user call
> drmAtomicCommit get the corresponding pageflip event.
> But some presentation results (c2p) are a little weird. They are 1~5ms or
> -1 ~ -5 ms. some results are normal(8 ~ 20 ms).
> I used to explain the weird result is because  pageflip return last frame's
> timestamp, but I can't explain the normal results.
> I suspect whether the kernel and user have the same time start point. It's
> just a guess, I have no evidence.
> Do you have some tips about the c2p results? Thank you.

Hi Nancy,

are the c2p numbers from weston-presentation-shm demo?

The presentation-time extension has an event to tell the client which
clock domain is being used for the timestamps. This comes from DRM,
Weston attempts to enable CLOCK_MONOTONIC, but if your kernel is
ancient or otherwise weird, the default in DRM is CLOCK_REALTIME.

Check your weston log for these lines:
               presentation clock: CLOCK_MONOTONIC, id 1
               presentation clock resolution: 0.000000001 s

If the resolution is much coarser than this, you have a kernel build
configuration issue.

If all that checks out and you see the problem even with unmodified
weston, then I would suspect a driver bug, but it's hard to tell.

You may want to grab a timeline recording and post that compressed. We
can then use wesgr to visualize it:
You can start and stop the recording by typing Mod+Shift+Space, 't'. It
writes a file to weston's current working directory and you see it
mentioned in the log as well.

The c2p times should really not vary much. On an intel GPU, with 60 Hz
display, the default mode of weston-presentation-shm gives me a steady
23-24 ms c2p. This is the default repaint window setting. If I use the
low latency mode (-p), I get c2p 17-18 ms. (The intel driver can
actually send the pageflip event a little earlier than the timestamp it
carries, because it predicts the vblank when it's sure the presentation
will happen.)

I also found a crash bug in Weston just now when playing with
weston-presentation-shm, probably related to a patch I recently landed.

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