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[Michiel Janssens <1515776@xxxxxxxxxxxxxxxxxx>]

Before reporting upstream I tested some further.
Narrowed it down a little bit:
The issue only happens when displayport is connected to Dell U2415 (wether or not connected to an extra monitor (vga)).
When connected to displayport only on Dell U2412M the issue does not appear.
So the issue is related to U2415 with displayport, or a DP v1.2 capable monitor.
U2412M is only DP v1.1 capable.
Still no workaround. Daisy chaining is not posible because the graphics card is only DP v1.1 capable.
I will report upstream now.

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https://bugs.launchpad.net/bugs/1515776

Title:
  1002:68a0 After suspend-resume graphics are sluggish and input lags
  with displayport connected

Status in linux package in Ubuntu:
  Triaged

Bug description:
  This bug could be related to 1412730
  https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1412730

  Description of the issue:
  Booting up with displayport connected to Dell U2415 monitor, graphics and input is responsive, also after logging in.
  However after resuming from suspend-to-ram, graphics are sluggish and input has lag.
  Examples:
  - transition from unity greeter login locked status to unlocked status: 6 seconds (normal 1 second)
  - transition from clicking dash and getting window: 2-3 seconds (normal <1 second)
  - starting gedit: 4 seconds (normal <1 second)
  - mouse lags, sometimes the pointer is still on the left side of the screen, while it should be on the right
  - input from keyboard lags, about 1-2 seconds (normal no lag)

  When unplugging the Displayport cable:
  all lag is gone

  When plugging displayport back in again:
  lag is back, same as before

  This issue only happens after suspend-resume.
  After a reboot, response of graphics and input is normal
  So two options to get rid of the lag: unplug DP or reboot.

  Further investigation:
  Did a mainline kernel bisect:
  From 3.13.11-ckt27 (good) to v4.3-wily (bad)
  First bad is v4.0-rc1-vivid
  Did a kernel commit bisect:
  From 3.19 (good) to 4.0-rc1 (bad)
  first bad commit: [e55bca26188e45f209597abf986c87cc5a49894a] radeon/audio: enable DP audio

  Commit bisect log is attached

  My daily baseline is 14.04.3 with HWE-Vivid.
  But because in the current HWE-Vivid this issue isn't present I'm reporting it via Wily as you can see in the logs.
  I repeated testing of last good and first bad kernels on both 14.04.3 with HWE-Vivid and 15.10.
  Exactly the same behaviour.
  I mention this because if this bug wont be fixed, it will still be there in the upcoming HWE-Wily also.
  And if thats the case, I won't be able to suspend my laptop without this issue.

  ProblemType: Bug
  DistroRelease: Ubuntu 15.10
  Package: linux-image-4.2.0-18-generic 4.2.0-18.22
  ProcVersionSignature: Ubuntu 4.2.0-18.22-generic 4.2.3
  Uname: Linux 4.2.0-18-generic x86_64
  ApportVersion: 2.19.1-0ubuntu4
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  michiel    1788 F.... pulseaudio
   /dev/snd/controlC1:  michiel    1788 F.... pulseaudio
  CurrentDesktop: Unity
  Date: Thu Nov 12 22:08:27 2015
  HibernationDevice: RESUME=UUID=d0de679a-8c90-4a25-a84a-8d3c25eb0a10
  InstallationDate: Installed on 2015-10-31 (12 days ago)
  InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
  MachineType: Hewlett-Packard HP EliteBook 8740w
  ProcFB: 0 radeondrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/@wily/boot/vmlinuz-4.2.0-18-generic root=UUID=f16430cd-a447-4cf2-8f8b-ce233e03d462 ro rootflags=subvol=@wily quiet splash vt.handoff=7
  RelatedPackageVersions:
   linux-restricted-modules-4.2.0-18-generic N/A
   linux-backports-modules-4.2.0-18-generic  N/A
   linux-firmware                            1.149.1
  SourcePackage: linux
  UdevLog: Error: [Errno 2] No such file or directory: '/var/log/udev'
  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

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