Created attachment 126674 [details]
Output of dmesg with the DRM debugging turned on
After upgrading from kernel 4.7.4 to 4.8-rc7, several frames are dropped (resulting in jerky graphics and a jerky mouse cursor) when the mouse passes over a boundary about a hundred pixels from the right side of my left monitor (DP-1). Sometimes, especially if the system is under heavy load, a FIFO underrun on one of the monitors will occur as well.
My system is a Lenovo m900 with a "Intel(R) Core(TM) i5-6500T CPU @ 2.50GHz" processor. It is running kernel 4.8.0-040800rc7-lowlatency (as downloaded from http://kernel.ubuntu.com/~kernel-ppa/mainline/) on Kubuntu 16.10 amd64. The system has two monitors, both Dell U2312HMs connected by DisplayPort. The left monitor is connected to DP-1 and the right monitor to DP-2. I have also tried connecting the monitors using DisplayPort-DVI adapters, but this does not affect the issue.
I bisected the issue and found that it was introduced with commit 9909113cc48a7ce6e772573e3cc82a3f03ffa8ef (https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/gpu/drm/i915?id=9909113cc48a7ce6e772573e3cc82a3f03ffa8ef). However, reverting the commit isn't a possibility because that causes severe problems with mouse cursor updates (a cursor trail getting stuck on the screen and often flickering rapidly.)
Created attachment 126675 [details]
Output of dmesg without the DRM debugging enabled, showing a FIFO underrun
I have tested a drm-intel-next build from yesterday and cannot reproduce the issue there, so it seems that the fix just needs to be applied to Linus's kernel tree.
This still happens with 4.8-rc8.
(In reply to Michael Marley from comment #3)
> This still happens with 4.8-rc8.
This should be landing more in incoming 4.9
How about now with latest kernel?
Sorry, I forgot to post right after it happened, but this was fixed in a 4.8 point release (4.8.5, maybe?) and stays fixed in 4.9-rc8. Thanks!