Comment 0 for bug 1597267

Running Ubuntu 16.04 on a Lenovo ThinkPad X260 with Intel Skylake HD Graphics 520, connecting an external monitor to the DisplayPort connector causes the system to quickly hang. The system stops responding to network pings, cannot switch consoles, and needs a hard reboot.

The external monitor does work when using a text console, and does not always hang in the LightDM login screen, but starting up a desktop session will always hang.

This originally happened using a ThinkPad dock, and I tested it further using an external DP monitor. This happens on 4.4.0-28-generic as well as mainline 4.6.3-040603-generic and 4.7.0-040700rc5-generic; they all hang in a similar fashion.

I have attached a journalctl log from running 4.4.0-28-generic and connecting the DP monitor while in a text console, before switching to the graphical console and logging in. The system hangs after a series of:

kernel: WARNING: CPU: 3 PID: 2319 at /home/kernel/COD/linux/drivers/gpu/drm/i915/intel_pm.c:3647 skl_update_other_pipe_wm+0x15d/0x170 [i915]
2016-06-29T12:54:52+0300 tehobari kernel: WARNING: CPU: 3 PID: 4418 at /home/kernel/COD/linux/drivers/gpu/drm/i915/intel_display.c:13957 skl_max_scale.part.108+0x69/0x70 [i915]
kernel: [drm:gen8_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun

The first warnings come immediately when connecting, and others when switching consoles and logging in.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.4.0-28-generic 4.4.0-28.47
ProcVersionSignature: Ubuntu 4.4.0-28.47-generic 4.4.13
Uname: Linux 4.4.0-28-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: terom 1903 F.... pulseaudio
CurrentDesktop: Unity
Date: Wed Jun 29 13:03:18 2016
InstallationDate: Installed on 2016-06-26 (2 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
MachineType: LENOVO 20F6007RGE
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.4.0-28-generic root=UUID=6636a3bb-262e-4ab2-ae81-d9388e3f7684 ro rootflags=subvol=@ quiet splash
RelatedPackageVersions:
 linux-restricted-modules-4.4.0-28-generic N/A
 linux-backports-modules-4.4.0-28-generic N/A
 linux-firmware 1.157.1
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 06/01/2016
dmi.bios.vendor: LENOVO
dmi.bios.version: R02ET48W (1.21 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20F6007RGE
dmi.board.vendor: LENOVO
dmi.board.version: SDK0J40705 WIN
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.modalias: dmi:bvnLENOVO:bvrR02ET48W(1.21):bd06/01/2016:svnLENOVO:pn20F6007RGE:pvrThinkPadX260:rvnLENOVO:rn20F6007RGE:rvrSDK0J40705WIN:cvnLENOVO:ct10:cvrNone:
dmi.product.name: 20F6007RGE
dmi.product.version: ThinkPad X260
dmi.sys.vendor: LENOVO