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[Michiel Janssens <1412730@xxxxxxxxxxxxxxxxxx>]

Investigated the issue some more.
Seems that EDID is not always given by the monitor when asked by the OS, for instance via "Detect Monitors" via gui or via xrandr via command line.
The workaround with xorg.conf I have abandoned. It wasnt working great too. Still a lot of time X was detecting resolutions again resulting in garbled screens or non active screen, however it stayed at the right resolution.

I managed to extract the edid for U2415 (256 byte), which i have attached.
Parse-edid has issues reading the extension block, but in Xorg.0.log I never saw issues reading the edid when it was available from the monitor.
My current workaround is by using kernel commandline parameter drm_kms_helper.edid_firmware=DP-1:edid/U2415.edid
This seems to work well.

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https://bugs.launchpad.net/bugs/1412730

Title:
  DELL U2415 monitor detection issues

Status in xorg package in Ubuntu:
  New

Bug description:
  My Laptop HP Elitebook 8740w has two external screens connected.
  1 DELL U2412M connected to vga connector
  1 DELL U2415 connected to Displayport.

  I worked for about a year with a single external display, just U2412M
  on the displayport without the issue I decribe.

  The first issue:
  When booting up and with screens attatched and active, the 2415 boots in 1024x768 resolution, the 2412 boots at the same moment in 1920x1200 as it should.
  At the point where the login screen appears the 2415 goes in power save mode, most of the time.
  The 2412 displays just fine at the moment.
  After logging in, turning the 2415 off and on, sometimes helps the get the screen active (out of power save).
  When it gets active, the resoltion is 1024x768, not 1920x1200.
  In unity-control-center in display settings the 2415 is displayed as "Unknown Display" and the maximum resolution I can select is 1024x768.

  Second issue:
  After a laptop suspend the 2415 doenst alway become active again, the 2412 always.
  Turning off an on the 2415 multiple time mostly helps to get the screen active.

  Current partial workaround:
  xorg.conf with monitor section for 1920x1200 as in attatchment.
  This doesn't solve booting in 1024x768, but solves the resolution after logging in.
  unity-control-center still lists "Unknown Screen"

  Another strange behavior:
  when executing xrandr command, most of the time the 2415 stays in 1024, but after a number of executions the screen swithes to 1920x1200. At that moment xrandr outputs all possible resolutions as it should.
  When executing xrandr another time the 2415 switches back to 1024x768

  Already tried kernel v3.18.3-vivid, no change.
  Also tried Utopic live cd, no change.
  Tried Windows 8.1, no issues with both screens. (however my main OS is Ubuntu trusty, so..., just for testing)

  ProblemType: Bug
  DistroRelease: Ubuntu 14.04
  Package: xorg 1:7.7+1ubuntu8
  ProcVersionSignature: Ubuntu 3.13.0-44.73-generic 3.13.11-ckt12
  Uname: Linux 3.13.0-44-generic x86_64
  .tmp.unity.support.test.0:

  ApportVersion: 2.14.1-0ubuntu3.6
  Architecture: amd64
  CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
  CompositorRunning: compiz
  CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0'
  CompositorUnredirectFSW: true
  Date: Tue Jan 20 10:48:24 2015
  DistUpgraded: Fresh install
  DistroCodename: trusty
  DistroVariant: ubuntu
  DkmsStatus:
   vboxhost, 4.3.20, 3.13.0-40-generic, x86_64: installed
   vboxhost, 4.3.20, 3.13.0-43-generic, x86_64: installed
   vboxhost, 4.3.20, 3.13.0-44-generic, x86_64: installed
  ExtraDebuggingInterest: Yes
  GraphicsCard:
   Advanced Micro Devices, Inc. [AMD/ATI] Broadway XT [Mobility Radeon HD 5870] [1002:68a0] (prog-if 00 [VGA controller])
     Subsystem: Hewlett-Packard Company FirePro M7820 [103c:1520]
  InstallationDate: Installed on 2014-05-11 (253 days ago)
  InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
  MachineType: Hewlett-Packard HP EliteBook 8740w
  PccardctlStatus:
   Socket 0:
     3.3V
    16-bit
    PC Card
     Subdevice 0 (function 0) bound to driver "pata_pcmcia"
  ProcKernelCmdLine: BOOT_IMAGE=/@trusty/boot/vmlinuz-3.13.0-44-generic root=UUID=cfafe8f3-4766-4fc7-b6f3-857eefcd2aff ro rootflags=subvol=@trusty plymouth:debug splash quiet drm.debug=0xe radeon.audio=1
  SourcePackage: xorg
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 07/07/2014
  dmi.bios.vendor: Hewlett-Packard
  dmi.bios.version: 68CAD Ver. F.50
  dmi.board.name: 1520
  dmi.board.vendor: Hewlett-Packard
  dmi.board.version: KBC Version 33.31
  dmi.chassis.asset.tag: CNU1152Q9W
  dmi.chassis.type: 10
  dmi.chassis.vendor: Hewlett-Packard
  dmi.modalias: dmi:bvnHewlett-Packard:bvr68CADVer.F.50:bd07/07/2014:svnHewlett-Packard:pnHPEliteBook8740w:pvr:rvnHewlett-Packard:rn1520:rvrKBCVersion33.31:cvnHewlett-Packard:ct10:cvr:
  dmi.product.name: HP EliteBook 8740w
  dmi.sys.vendor: Hewlett-Packard
  version.compiz: compiz 1:0.9.11.3+14.04.20141104-0ubuntu1
  version.ia32-libs: ia32-libs N/A
  version.libdrm2: libdrm2 2.4.56-1~ubuntu1
  version.libgl1-mesa-dri: libgl1-mesa-dri 10.1.3-0ubuntu0.2
  version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
  version.libgl1-mesa-glx: libgl1-mesa-glx 10.1.3-0ubuntu0.2
  version.xserver-xorg-core: xserver-xorg-core 2:1.15.1-0ubuntu2.6
  version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.8.2-1ubuntu2
  version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.3.0-1ubuntu3.1
  version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.910-0ubuntu1.3
  version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.10-1ubuntu2
  xserver.bootTime: Tue Jan 20 09:37:08 2015
  xserver.configfile: /etc/X11/xorg.conf
  xserver.errors:

  xserver.logfile: /var/log/Xorg.0.log
  xserver.version: 2:1.15.1-0ubuntu2.6
  xserver.video_driver: radeon

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