Created attachment 16941 [details]
dmesg log after change mode

source:
xserver - master tip commit 26e7e69ab893d1f2b35213250ada40ec90944f62
drm - modesetting-101 03bf1fba67413f381d2a548fe08bd634a48fcc48
xf86-video-intel - intel-kernelmode 95fb4ac2ea326ee67f472e33ba4202942f246df1

The display becomes a mess after change the display mode.

Reproduce steps:
1, modprobe drm debug=1
2, modprobe i915 modeset=1
3, X &
4, xterm &
5, xrandr --output VGA0 --mode 800x600

My platform: Intel Q35

Output of xrandr:
nian@nian-dev:~$ xrandr -q
Screen 0: minimum 320 x 200, current 1280 x 1024, maximum 2048 x 2048
VGA0 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 380mm x 300mm
   1280x1024      60.0*+   75.0     60.0*
   1600x1200      60.0
   1600x1024      60.0
   1400x1050      60.0
   1280x960       60.0
   1152x864       75.0
   1024x768       75.1     75.0     70.1     60.0
   832x624        74.6
   800x600        72.2     75.0     60.3
   640x480        75.0     72.8     75.0     60.0     59.9
   720x400        70.1

nian@nian-dev:~$ xrandr --output VGA0 --mode 800x600
resize called 800 600
old front size 00800000, new front size 001e8000
old front offset 02800000, new front offset 03010000
FRONTBUFFER HANDLE CHANGING 0x82eace8
fb id is 42
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  149 (RANDR)
  Minor opcode of failed request:  7 ()
  Serial number of failed request:  16
  Current serial number in output stream:  17