Description:
there is no accelerated rendering on the secondary screen in dual head mode when the secondary screen is rotated.

I have intel integrated graphics:
00:02.0 VGA compatible controller: Intel Corporation 82Q35 Express Integrated Graphics Controller (rev 02)
00:02.1 Display controller: Intel Corporation 82Q35 Express Integrated Graphics Controller (rev 02)

I have two monitors as reported by xrandr:
Screen 0: minimum 320 x 200, current 2704 x 1280, maximum 4096 x 4096
VGA1 connected 1680x1050+0+0 (normal left inverted right x axis y axis) 473mm x 296mm
1280x1024 75.0 + 60.0
1680x1050 60.0*+
1680x945 60.0
1400x1050 74.9 60.0
1600x900 60.0
1440x900 75.0 59.9
1280x960 60.0
1366x768 60.0
1360x768 60.0
1280x800 74.9 59.8
1152x864 75.0
1280x768 74.9 59.9
1024x768 75.1 70.1 60.0
1024x576 60.0
800x600 72.2 75.0 60.3 56.2
848x480 60.0
640x480 72.8 75.0 60.0
720x400 70.1
DVI1 connected 1024x1280+1680+0 left (normal left inverted right x axis y axis) 376mm x 301mm
1280x1024 60.0*+ 75.0
1280x960 60.0
1280x800 74.9 59.9
1152x864 75.0
1280x768 74.9 60.0
1024x768 75.1 70.1 60.0
1024x576 60.0
800x600 72.2 75.0 60.3 56.2
848x480 60.0
640x480 72.8 75.0 60.0
720x400 70.1

graphically this looks like:
+-------------------+---------+
| + |
| + |
| VGA1 + DVI1 |
| 1680x1050 +1024x1280|
| + |
+-------------------+ |
| |
+---------+

I cannot run composite windows managers on such setups because it exceeds the virtual framebuffer size of 2048x2048, but that is not the problem since I use a non-accelerated tiling window manager.

I can un-rotate the right/DVI screen and then there is no problem, but when it is rotated rendering becomes laggy...
glxgears shows about 10 fps on the right screen while it is 60 on the left screen.
Also dragging windows and scrolling large scrollareas is slow and looks jaggedy.

Additional info:
* package version(s)
2.20.19-1 on x86_64

no special configuration in /etc/X11
xorg log shows this after the xrandr command that rotates the second screen:
[ 21599.275] (II) intel(0): Allocated new frame buffer 2752x1280 stride 11008, untiled
[ 21599.317] (II) intel(0): Allocated new frame buffer 1280x1024 stride 8192, tiled

when I set it back to non-rotated it shown only one new line:
[ 26053.553] (II) intel(0): Allocated new frame buffer 3008x1050 stride 12032, untiled
If I do that, then the problem does not occur so it is not a problem with the virtual framebuffer.

I found an ubuntu bugreport that says that this bug(presuming it's the same) was solved upstream in 2.3.1
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/174650

Steps to reproduce:
* attach two monitors
* assign the above configuration:
> xrandr --output VGA1 --mode 1680x1050 --primary
> xrandr --output DVI1 --right-of VGA1 --rotate left
* run glxgears on both screens and observe the big difference(also looks better on the non-rotated screen)