Performance regression with Linux 5.7-rc1 on Iris Plus 655 and 4K screen (Bisected)
Starting with Linux 5.7-rc1 on my NUC8i7bek with an Iris Plus 655, I noticed a rather severe performance regression where a number of things (notably scrolling in a maximized Firefox window and window minimize/maximize/restore animations) that used to be perfectly smooth 60FPS in 5.6 now drop a lot of frames and/or run at 30FPS. I only noticed this on the system with a 4K screen; I was not able to reproduce on a system with a 1080p screen. I bisected and found that https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/gpu/drm/i915?id=98479ada421a8fd2123b98efd398a6f1379307ab introduces the issue.
System information (from the affected system):
GPU: Intel Corporation Iris Plus Graphics 655
uname -m: x86_64
uname -r: 5.7.0-050700rc1-lowlatency
Distro: Kubuntu 20.04
Machine: Intel NUC8i7bek
Display connector: HDMI 2.0 via integrated LSPCON
Nothing relevant shows in dmesg or any other log.