Hi first post here...

Just wanted to chime in and confirm same bug on same machine:
Thinkpad x260 
Processor: Intel® Core™ i7-6500U CPU @ 2.50GHz × 4
Graphics: Gallium 0.4 on llvmpipe (LLVM 3.8, 256 bits)

In order to try and fix the bug I have added the repo:
http://ppa.launchpad.net/oibaf/graphics-drivers/ubuntu
Before this change, the system reported Graphics: Intel Skylake HD Graphics 520


I'm running a 1 day old Ubuntu 16.04.
I have experienced this crash on the stock kernel, 4.6.9 and 4.6.3

My steps to reproduce:
* plug in HDMI to a 4K external display and set display to full resolution 
(this will only have a frame rate of 30Hz due to a limitation in HDMI standard)
* turn off laptop monitor
* plug in DP to *the same* external display
* Set new configuration where HDMI if off and DP is on. (this will be at 60Hz)
* screen goes blank on both HDMI and DP source. reverts to last configuration 
because of crash.

I believe it is the combination of 4K display in full resolution *and*
60hz. This would be why the crash is only seen with DP at full
resolution.

last crash report with kernel 4.6.3:
Jul 06 19:32:14 emil-workpad kernel: ------------[ cut here ]------------
Jul 06 19:32:14 emil-workpad kernel: WARNING: CPU: 0 PID: 986 at 
/home/kernel/COD/linux/drivers/gpu/drm/i915/intel_display.c:13938 
skl_max_scale.part.99+0x6d/0x80 [i915]
Jul 06 19:32:14 emil-workpad kernel: WARN_ON_ONCE(!crtc_clock || cdclk < 
crtc_clock)
Jul 06 19:32:14 emil-workpad kernel: Modules linked in:
Jul 06 19:32:14 emil-workpad kernel:  rfcomm bnep arc4 snd_soc_skl 
snd_soc_skl_ipc snd_hda_codec_hdmi snd_soc_sst_ipc snd_soc_sst_dsp 
snd_hda_ext_core snd_hda_codec_realtek snd_hda_codec_generic snd_soc_sst_match 
intel_rapl snd_soc_core x86_pkg_temp_thermal snd_compress inte
Jul 06 19:32:14 emil-workpad kernel:  hid_lenovo usbhid hid rtsx_pci_sdmmc 
crct10dif_pclmul crc32_pclmul ghash_clmulni_intel i915 aesni_intel aes_x86_64 
lrw gf128mul glue_helper ablk_helper cryptd i2c_algo_bit drm_kms_helper 
syscopyarea sysfillrect e1000e sysimgblt psmouse f
Jul 06 19:32:14 emil-workpad kernel: CPU: 0 PID: 986 Comm: Xorg Tainted: G      
  W       4.6.3-040603-generic #201606241434
Jul 06 19:32:14 emil-workpad kernel: Hardware name: LENOVO 
20F6006XMD/20F6006XMD, BIOS R02ET44W (1.17 ) 01/25/2016
Jul 06 19:32:14 emil-workpad kernel:  0000000000000286 00000000eeed4400 
ffff8802148b7a50 ffffffff813f3373
Jul 06 19:32:14 emil-workpad kernel:  ffff8802148b7aa0 0000000000000000 
ffff8802148b7a90 ffffffff8108293b
Jul 06 19:32:14 emil-workpad kernel:  00003672c02dada0 ffff8802183dba80 
ffff880035c4e800 ffff8800b29cb000
Jul 06 19:32:14 emil-workpad kernel: Call Trace:
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffff813f3373>] dump_stack+0x63/0x90
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffff8108293b>] __warn+0xcb/0xf0
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffff810829bf>] 
warn_slowpath_fmt+0x5f/0x80
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc02a846d>] 
skl_max_scale.part.99+0x6d/0x80 [i915]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc02a8540>] 
intel_check_primary_plane+0xc0/0xe0 [i915]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc02cf66a>] ? 
intel_ddi_compute_config+0x5a/0xc0 [i915]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc029a3fe>] 
intel_plane_atomic_check+0x12e/0x1f0 [i915]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc017eff8>] 
drm_atomic_helper_check_planes+0x48/0x1d0 [drm_kms_helper]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc02b4d02>] 
intel_atomic_check+0x172/0x830 [i915]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc0084e11>] 
drm_atomic_check_only+0x181/0x600 [drm]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc0084657>] ? 
drm_atomic_set_crtc_for_connector+0x57/0xe0 [drm]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc00852a7>] 
drm_atomic_commit+0x17/0x60 [drm]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc018103a>] 
drm_atomic_helper_set_config+0x7a/0xb0 [drm_kms_helper]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc0074b52>] 
drm_mode_set_config_internal+0x62/0x100 [drm]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc0078af0>] 
drm_mode_setcrtc+0x3e0/0x500 [drm]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc0069aa2>] 
drm_ioctl+0x152/0x540 [drm]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffffc0078710>] ? 
drm_mode_setplane+0x1b0/0x1b0 [drm]
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffff812379f1>] 
do_vfs_ioctl+0xa1/0x5b0
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffff81090961>] ? 
__set_task_blocked+0x41/0xa0
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffff810932e6>] ? 
__set_current_blocked+0x36/0x60
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffff81237f79>] SyS_ioctl+0x79/0x90
Jul 06 19:32:14 emil-workpad kernel:  [<ffffffff81845ab6>] 
entry_SYSCALL_64_fastpath+0x1e/0xa8
Jul 06 19:32:14 emil-workpad kernel: ---[ end trace dce1bcd4b56b9a53 ]---
Jul 06 19:32:14 emil-workpad kernel: [drm:intel_cpu_fifo_underrun_irq_handler 
[i915]] *ERROR* CPU pipe A FIFO underrun

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https://bugs.launchpad.net/bugs/1597267

Title:
  ThinkPad X260 connecting external DisplayPort hangs system

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