drm/i915: xrandr causes gmch_pfit.lvds_border_bits warning or fails

[Uwe Geuder <intel-gfx-ugeuder@snkmail.com>]

[Disclaimer: I'm not familiar with this list. I tried to study FAQs but didn't
get much wiser. Please give pointers if I missed something that I should
have read before posting...]

I have a 

00:02.0 VGA compatible controller: Intel Corporation Atom Processor D4xx/D5xx/N4xx/N5xx Integrated Graphics Controller

I noticed that VGA output (external display) cannot be turned on. Originally
the problem was observed using a 3.11 distro (Ubuntu) kernel. However,
the problem is 100% reproducable using the 3.14 mainline kernel.

There are 2 problems:

1.) xrandr commands changing LVDS1 modes (except --preferred) cause
one kernel warning every time, although they correctly result in the
requested mode change.

[  117.444327] [drm:intel_pipe_config_compare] *ERROR* mismatch in gmch_pfit.lvds_border_bits (expected 32768, found 0)
[  117.444336] ------------[ cut here ]------------
[  117.444430] WARNING: CPU: 1 PID: 955 at /home/apw/COD/linux/drivers/gpu/drm/i915/intel_display.c:9530 check_crtc_state+0x285/0x360 [i915]()
[  117.444437] pipe state doesn't match!


2.) xrandr commands changing VGA1 modes do nothing visible (to me). No kernel
warning, no mode change. External display stays dark. (Tested with 2
different projectors and one oldish VGA monitor)

I don't know whether the 2 problems are related. The first one
wouldn't disturb me as such, but the 2nd one of course does.

I saw that the same error message has been discussed earlier and the advice
was to boot with drm.debug=0xe and send the full dmesg output.

I don't get the message at boot as others have reported, it comes only
for xrandr commands (well maybe others have xrandr in their desktop
initializations).

Please find my kernel log at https://gist.github.com/9958381

This is the 3.14 mainline kernel as built by the Ubuntu kernel team.
The log covers full boot and 1 xrandr command

xrandr --output VGA1 --mode 800x600 --output LVDS1 --mode 800x600

I also run strace on Xorg during xrandr, but the results didn't tell
me anything. I can provide them that helps anybody.


Uwe Geuder
Nomovok Ltd.
Tampere, Finland
uwe.gxuder@nomovok.com (humans correct 1 obvious spelling error)