https://bugs.kde.org/show_bug.cgi?id=525454
Bug ID: 525454
Summary: High CPU usage due to pixel clock overflow
Classification: Plasma
Product: kwin
Version First unspecified
Reported In:
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: X11 Integration
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
On X11, `kwin_x11` can peg one core at 100% when a 3840x2160 @ 240 Hz display
is connected. `kwin_x11 --replace` helps only temporarily.
STEPS TO REPRODUCE
1. Use an XRandR mode with `dot_clock=2332000000`, `htotal=4000`,
`vtotal=2429`.
2. Trigger off-screen rendering of a window thumbnail.
3. Inspect DBus `/KWin supportInformation` and KWin CPU usage.
OBSERVED RESULT
The output refresh rate is `4 294 765 262` mHz and KWin's main thread spins at
100% CPU.
EXPECTED RESULT
The refresh rate is about `240 000` mHz and KWin remains idle.
SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 6.7.4
KDE Frameworks Version: 6.29.0
Qt Version: 6.11.2
Kernel Version: 7.1.9-arch1-2 (64-bit)
Graphics Platform: X11
Processors: 16 × AMD Ryzen 7 7840HS w/ Radeon 780M Graphics
Memory: 96 GiB of RAM (92.0 GiB usable)
Graphics Processor: AMD Radeon 780M Graphics
ADDITIONAL INFORMATION
I plan to submit a merge request that fixes this as it's due to a simple
integer overflow. `doUpdateOutputs()` converts XCB's unsigned 32-bit
`dot_clock` to `int`, changing `2332000000` to `-1962967296`. The resulting
refresh rate wraps to `4294765262` mHz. Then `1000000 /
output()->refreshRate()` creates a 0 ms off-screen frame-callback timer.
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