https://bugs.kde.org/show_bug.cgi?id=525229
Bug ID: 525229
Summary: Hardware cursor updates are not coalesced to the
vblank cadence, causing ~40% frame-to-frame cursor
speed variation
Classification: Plasma
Product: kwin
Version First 6.6.6
Reported In:
Platform: Kubuntu
OS: Linux
Status: REPORTED
Severity: minor
Priority: NOR
Component: wayland-generic
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
Created attachment 195796
--> https://bugs.kde.org/attachment.cgi?id=195796&action=edit
Full bug report
On an idle desktop, KWin's DRM hardware-cursor path submits atomic commits at a
rate decoupled from the display refresh — measured at 173.3 flip-completions/s
against a 159.96 Hz panel, i.e. 8.3% more completions than the panel can
physically display. Because the commit cadence is not locked to vblank, the
number of pointer input samples represented in each displayed frame alternates
between 2 and 3, so the cursor advances by visibly different distances on
consecutive frames at a constant hand speed.
Measured per-displayed-frame cursor travel variation: 39.5% (detrended).
The stutter is plainly visible in slow-motion video and is eliminated entirely
by KWIN_FORCE_SW_CURSOR=1, which moves the cursor onto the normal,
vblank-locked paint path.
The NVIDIA driver is not responsible: nv_drm_atomic_commit returns in a median
of 34 us (max 155 us, zero calls exceeding one frame), so it accepts everything
KWin asks of it well within budget.
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