When using modesetting Xorg driver together with Intel GVT (i915.enable_gvt=1), the brightness control on my notebook becomes extremely slow. The slowness can happen either when pressing the brightness buttons or when using xfce4-power-manager to configure brightness, it seems it has to wait one second for every brightness change. However, if I try to write directly to /sys/class/backlight/intel_backlight/brightness there is no delay. Even something as: sudo bash -c "while true ; do echo 200 > /sys/class/backlight/intel_backlight/brightness ; sleep 0.05 ; echo 60 > /sys/class/backlight/intel_backlight/brightness ; sleep 0.05 ; done" changes the brightness very quickly, working with no problems. When using intel Xorg driver this problem is not present. When disabling Intel GVT the problem does not appear too. It is only present when combining modesetting Xorg driver and Intel GVT. System specifications: Computer model: Lenovo Ideapad 310-14ISK; Processor: Intel Core i3-6100U; Video: Intel HD Graphics 520; Architecture: amd64; Mesa: 19.3.0-devel (git-83a346cd58); Kernel version: drm-tip (58747eccea0669678be9cd532f778501024d0556); Distribution: Xubuntu 18.04.3 amd64.
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