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[Launchpad Bug Tracker <1403275@xxxxxxxxxxxxxxxxxx>]

Status changed to 'Confirmed' because the bug affects multiple users.

** Changed in: xorg (Ubuntu)
       Status: New => Confirmed

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https://bugs.launchpad.net/bugs/1403275

Title:
  i915 display driver causes crash back to login window

Status in xorg package in Ubuntu:
  Confirmed

Bug description:
  I've just installed Ubuntu 14.04 server on Dell Latitude E7440 and
  then installed Cinnamon 2.4.5 on top of it. The laptop has an external
  monitor connected through Display Port and a docking station.

  On the first day the system worked perfectly.

  On the second day, the system booted to the login screen (leaving the
  external screen empty but otherwise seemed to be under X11's control),
  it then let me type my username and password but then X exits, shows a
  bit of VT1 with errors then gets back to the login page.

  I found the following error in /var/log/kern.log:

  [drm:intel_dp_complete_link_train] ERROR failed to train DP, aborting

  I found a page which claims to solve this error for 12.04 in the
  second comment in
  https://www.libreoffice.org/bugzilla/show_bug.cgi?id=73694#c2.

  I found that the file linux-
  source-3.13.0/drivers/gpu/drm/i915/intel_dp.c, which is included in
  the tar file /usr/src/linux-source-3.13.0.tar.bz2 (also copied in
  /usr/src/linux-source-3.13.0/linux-source-3.13.0.tar.bz2), from the
  kernel source package "linux-source-3.13.0", contains the pre-patch
  lines mentioned in the fix above.

  Would you please consider including this fix in the Ubuntu kernel?

  Thanks,

  --Amos

  For completeness, the patch pointed to from the link above is:

  Link: https://www.libreoffice.org/bugzilla/attachment.cgi?id=92231

  diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
  index 8f17f8fbd0b1..5631cfb874db 100644
  --- a/drivers/gpu/drm/i915/intel_dp.c
  +++ b/drivers/gpu/drm/i915/intel_dp.c
  @@ -847,8 +847,8 @@ intel_dp_compute_config(struct intel_encoder *encoder,
   		mode_rate = intel_dp_link_required(adjusted_mode->crtc_clock,
   						   bpp);
   
  -		for (clock = 0; clock <= max_clock; clock++) {
  -			for (lane_count = 1; lane_count <= max_lane_count; lane_count <<= 1) {
  +		for (lane_count = 1; lane_count <= max_lane_count; lane_count <<= 1) {
  +			for (clock = 0; clock <= max_clock; clock++) {
   				link_clock = drm_dp_bw_code_to_link_rate(bws[clock]);
   				link_avail = intel_dp_max_data_rate(link_clock,
   								    lane_count);

  ProblemType: Bug
  DistroRelease: Ubuntu 14.04
  Package: xorg 1:7.7+1ubuntu8
  ProcVersionSignature: Ubuntu 3.13.0-43.72-generic 3.13.11.11
  Uname: Linux 3.13.0-43-generic x86_64
  ApportVersion: 2.14.1-0ubuntu3.6
  Architecture: amd64
  CurrentDesktop: X-Cinnamon
  Date: Wed Dec 17 12:57:10 2014
  InstallationDate: Installed on 2014-12-15 (1 days ago)
  InstallationMedia: Ubuntu-Server 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.3)
  SourcePackage: xorg
  Symptom: display
  Title: Xorg crash
  UpgradeStatus: No upgrade log present (probably fresh install)

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