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[sanette <1322644@xxxxxxxxxxxxxxxxxx>]

I think I found what triggers the error (of course, this is just the manifestation of the error, maybe not the root cause):
from 3.13.0-17.37 a limited number of "retries" has been introduced, which fails after 7 retries; this seems to be not enough:

--- linux-3.13.0/drivers/gpu/drm/i915/intel_dp.c
+++ linux-3.13.0/drivers/gpu/drm/i915/intel_dp.c
@@ -537,6 +537,7 @@
 	uint8_t	msg[20];
 	int msg_bytes;
 	uint8_t	ack;
+	int retry;
 
 	if (WARN_ON(send_bytes > 16))
 		return -E2BIG;
@@ -548,18 +549,20 @@
 	msg[3] = send_bytes - 1;
 	memcpy(&msg[4], send, send_bytes);
 	msg_bytes = send_bytes + 4;
-	for (;;) {
+	for (retry = 0; retry < 7; retry++) {
 		ret = intel_dp_aux_ch(intel_dp, msg, msg_bytes, &ack, 1);
 		if (ret < 0)
 			return ret;
 		if ((ack & AUX_NATIVE_REPLY_MASK) == AUX_NATIVE_REPLY_ACK)
-			break;
+			return send_bytes;
 		else if ((ack & AUX_NATIVE_REPLY_MASK) == AUX_NATIVE_REPLY_DEFER)
-			udelay(100);
+			usleep_range(400, 500);
 		else
 			return -EIO;
 	}
-	return send_bytes;
+
+	DRM_ERROR("too many retries, giving up\n");
+	return -EIO;
 }

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https://bugs.launchpad.net/bugs/1322644

Title:
  [Dell XPS 13 (L321X)] External monitor does not wake up after sleep

Status in “linux” package in Ubuntu:
  Incomplete

Bug description:
  The problem started after I upgraded from kubuntu 13.10 to 14.04.
  the monitor is connected to my laptop via a VGA mini display port.
  At boot, it works correctly, but if the screen goes blank after some inactivity, then it stays black forever.

  WORKAROUND: If I unplug the VGA adapter, and plug it back, it works
  again.

  ProblemType: Bug
  DistroRelease: Ubuntu 14.04
  Package: xorg 1:7.7+1ubuntu8
  ProcVersionSignature: Ubuntu 3.13.0-24.47-generic 3.13.9
  Uname: Linux 3.13.0-24-generic x86_64
  .tmp.unity.support.test.0:

  ApportVersion: 2.14.1-0ubuntu3.2
  Architecture: amd64
  CompizPlugins: [core,composite,opengl,compiztoolbox,decor,vpswitch,snap,mousepoll,resize,place,move,wall,grid,regex,imgpng,session,gnomecompat,animation,fade,unitymtgrabhandles,workarounds,scale,expo,ezoom,unityshell]
  CompositorRunning: kwin
  CurrentDesktop: KDE
  Date: Fri May 23 16:02:09 2014
  DistUpgraded: 2014-05-09 13:04:30,209 DEBUG enabling apt cron job
  DistributionChannelDescriptor:
   # This is a distribution channel descriptor
   # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
   canonical-oem-sputnik-20120802-0
  DistroCodename: trusty
  DistroVariant: kubuntu
  DkmsStatus:
   fwts-efi-runtime-dkms, 14.03.01, 3.11.0-20-generic, x86_64: installed
   fwts-efi-runtime-dkms, 14.03.01, 3.13.0-24-generic, x86_64: installed
   vboxhost, 4.3.6, 3.11.0-20-generic, x86_64: installed
   vboxhost, 4.3.6, 3.13.0-24-generic, x86_64: installed
  GraphicsCard:
   Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller [8086:0116] (rev 09) (prog-if 00 [VGA controller])
     Subsystem: Dell Device [1028:052e]
  InstallationDate: Installed on 2012-11-13 (555 days ago)
  InstallationMedia: Ubuntu 12.04 "Precise" - Failed to find casper uuid.conf in 'binary/casper/initrd.img' LIVE/INSTALL Binary 20120802-21:13
  LightdmGreeterLog:
   QInotifyFileSystemWatcherEngine::addPaths: inotify_add_watch failed: Aucun fichier ou dossier de ce type
   QFileSystemWatcher: failed to add paths: /var/lib/lightdm/.config/ibus/bus
   Bus::open: Can not get ibus-daemon's address.
   IBusInputContext::createInputContext: no connection to ibus-daemon
  LightdmGreeterLogOld:
   QInotifyFileSystemWatcherEngine::addPaths: inotify_add_watch failed: Aucun fichier ou dossier de ce type
   QFileSystemWatcher: failed to add paths: /var/lib/lightdm/.config/ibus/bus
   Bus::open: Can not get ibus-daemon's address.
   IBusInputContext::createInputContext: no connection to ibus-daemon
  MachineType: Dell Inc. Dell System XPS L321X
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.13.0-24-generic root=UUID=f18a56b7-1429-4ce3-b6e3-65edacff16a3 ro quiet splash crashkernel=384M-:128M vt.handoff=7
  SourcePackage: xorg
  UpgradeStatus: Upgraded to trusty on 2014-05-09 (14 days ago)
  dmi.bios.date: 07/02/2012
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: A06
  dmi.board.name: 085X6F
  dmi.board.vendor: Dell Inc.
  dmi.board.version: A00
  dmi.chassis.type: 8
  dmi.chassis.vendor: Dell Inc.
  dmi.chassis.version: 0.1
  dmi.modalias: dmi:bvnDellInc.:bvrA06:bd07/02/2012:svnDellInc.:pnDellSystemXPSL321X:pvr:rvnDellInc.:rn085X6F:rvrA00:cvnDellInc.:ct8:cvr0.1:
  dmi.product.name: Dell System XPS L321X
  dmi.sys.vendor: Dell Inc.
  version.compiz: compiz 1:0.9.11+14.04.20140423-0ubuntu1
  version.ia32-libs: ia32-libs 20090808ubuntu36
  version.libdrm2: libdrm2 2.4.52-1
  version.libgl1-mesa-dri: libgl1-mesa-dri 10.1.0-4ubuntu5
  version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
  version.libgl1-mesa-glx: libgl1-mesa-glx 10.1.0-4ubuntu5
  version.xserver-xorg-core: xserver-xorg-core 2:1.15.1-0ubuntu2
  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
  version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.10-1ubuntu2
  xserver.bootTime: Fri May 23 06:57:33 2014
  xserver.configfile: None
  xserver.errors: Failed to load module "displaylink" (module does not exist, 0)
  xserver.logfile: /var/log/Xorg.0.log
  xserver.outputs:
   product id                                4920
   vendor                                     CMN
  xserver.version: 2:1.15.1-0ubuntu2

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