External displays blank, kernel errors from i915/drm driver

[Maketsi]

My two external displayport connected displays worked fine until last thursday. After bringing my laptop to the same HP dock station three days later, they stayed blank no matter what.
Displays are seen by X (other of them was powered off during the bug report) and detected correctly, but there is no picture.
Displays worked with the current kernel 4.4.0-92-generic on thursday.
Dmesg log is full of kernel errors from i915 driver, multiplied by me switching the monitors on and off while troubleshooting the issue.

$ dmesg | fgrep WARNING
[ 2.860874] WARNING: CPU: 0 PID: 1 at /build/linux-hVVhWi/linux-4.4.0/arch/x86/mm/ioremap.c:198 __ioremap_caller+0x2c5/0x380()
[ 109.067969] WARNING: CPU: 2 PID: 78 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_dp_mst.c:493 intel_dp_destroy_mst_connector+0xe7/0xf0 [i915]()
[ 109.384569] WARNING: CPU: 2 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 110.235437] WARNING: CPU: 2 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 142.534674] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 196.127732] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 211.875798] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 273.426829] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 273.728180] WARNING: CPU: 2 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 274.583122] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 290.154952] WARNING: CPU: 2 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 301.498593] WARNING: CPU: 0 PID: 172 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 326.124417] WARNING: CPU: 1 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 355.932899] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 356.032234] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 384.356500] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 384.458966] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 448.277935] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 448.370419] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()

When enabling OR disabling the external display via Displays applet, the following error happens:

[ 1782.969654] WARNING: CPU: 3 PID: 1488 at /build/linux-hVVhWi/linux-4.4.0/drivers/gpu/drm/i915/intel_display.c:12746 intel_modeset_check_state+0x563/0x8a0 [i915]()
[ 1782.969657] encoder's enabled state mismatch (expected 1, found 0)
[ 1782.969658] Modules linked in: vmnet(OE) vmw_vsock_vmci_transport vsock vmw_vmci vmmon(OE) rfcomm bnep binfmt_misc arc4 hp_wmi sparse_keymap intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel iwlmvm kvm snd_hda_codec_hdmi mac80211 irqbypass snd_hda_codec_idt snd_hda_codec_generic joydev snd_hda_intel serio_raw snd_hda_codec snd_usb_audio snd_hda_core iwlwifi snd_usbmidi_lib snd_hwdep lpc_ich btusb btrtl input_leds snd_pcm btbcm rtsx_pci_ms snd_seq_midi btintel cfg80211 snd_seq_midi_event memstick bluetooth snd_rawmidi snd_seq snd_seq_device snd_timer snd mei_me soundcore mei shpchp hp_accel 8250_fintek lis3lv02d input_polldev mac_hid tpm_infineon hp_wireless ip6t_REJECT nf_reject_ipv6 nf_log_ipv6 xt_hl ip6t_rt nf_conntrack_ipv6 nf_defrag_ipv6 ipt_REJECT nf_reject_ipv4 nf_log_ipv4
[ 1782.969708] nf_log_common xt_LOG xt_limit xt_tcpudp xt_addrtype nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack ip6table_filter ip6_tables nf_conntrack_netbios_ns nf_conntrack_broadcast nf_nat_ftp nf_nat nf_conntrack_ftp nf_conntrack iptable_filter ip_tables x_tables parport_pc ppdev lp parport autofs4 drbg ansi_cprng algif_skcipher af_alg dm_crypt hid_generic 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 psmouse sysfillrect sysimgblt fb_sys_fops ahci e1000e libahci drm rtsx_pci ptp pps_core wmi video fjes
[ 1782.969750] CPU: 3 PID: 1488 Comm: Xorg Tainted: G W OE 4.4.0-92-generic #115-Ubuntu
[ 1782.969752] Hardware name: Hewlett-Packard HP EliteBook 840 G1/198F, BIOS L71 Ver. 01.35 01/26/2016
[ 1782.969754] 0000000000000286 e65bd334fa328b0d ffff8804285e7b08 ffffffff813f9c83
[ 1782.969757] ffff8804285e7b50 ffffffffc036aab8 ffff8804285e7b40 ffffffff81081312
[ 1782.969760] ffff88042601c318 ffff88042629dd00 ffff88042601c348 ffff88042601c330
[ 1782.969763] Call Trace:
[ 1782.969769] [<ffffffff813f9c83>] dump_stack+0x63/0x90
[ 1782.969775] [<ffffffff81081312>] warn_slowpath_common+0x82/0xc0
[ 1782.969778] [<ffffffff810813ac>] warn_slowpath_fmt+0x5c/0x80
[ 1782.969810] [<ffffffffc02fb643>] intel_modeset_check_state+0x563/0x8a0 [i915]
[ 1782.969838] [<ffffffffc0306e0b>] intel_atomic_commit+0x4bb/0x6f0 [i915]
[ 1782.969864] [<ffffffffc0070157>] drm_atomic_commit+0x37/0x60 [drm]
[ 1782.969875] [<ffffffffc01a9dc6>] drm_atomic_helper_set_config+0x76/0xb0 [drm_kms_helper]
[ 1782.969892] [<ffffffffc005eef2>] drm_mode_set_config_internal+0x62/0x100 [drm]
[ 1782.969909] [<ffffffffc006354c>] drm_mode_setcrtc+0x3cc/0x4f0 [drm]
[ 1782.969922] [<ffffffffc0054802>] drm_ioctl+0x152/0x540 [drm]
[ 1782.969937] [<ffffffffc0063180>] ? drm_mode_setplane+0x1b0/0x1b0 [drm]
[ 1782.969941] [<ffffffff812233cf>] do_vfs_ioctl+0x29f/0x490
[ 1782.969945] [<ffffffff8108e671>] ? __set_task_blocked+0x41/0xa0
[ 1782.969949] [<ffffffff81091006>] ? __set_current_blocked+0x36/0x60
[ 1782.969951] [<ffffffff81223639>] SyS_ioctl+0x79/0x90
[ 1782.969954] [<ffffffff810912be>] ? SyS_rt_sigprocmask+0x8e/0xc0
[ 1782.969958] [<ffffffff81841ff2>] entry_SYSCALL_64_fastpath+0x16/0x71
[ 1782.970010] ---[ end trace c8776109d781c4b1 ]---

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: xorg 1:7.7+13ubuntu3
ProcVersionSignature: Ubuntu 4.4.0-92.115-generic 4.4.76
Uname: Linux 4.4.0-92-generic x86_64
.tmp.unity_support_test.0:

ApportVersion: 2.20.1-0ubuntu2.10
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: Mon Aug 21 09:31:31 2017
DistUpgraded: 2016-08-17 12:45:04,777 ERROR got error from PostInstallScript ./xorg_fix_proprietary.py (g-exec-error-quark: Failed to execute child process "./xorg_fix_proprietary.py" (No such file or directory) (8))
DistroCodename: xenial
DistroVariant: ubuntu
ExtraDebuggingInterest: Yes
GraphicsCard:
 Intel Corporation Haswell-ULT Integrated Graphics Controller [8086:0a16] (rev 09) (prog-if 00 [VGA controller])
   Subsystem: Hewlett-Packard Company Haswell-ULT Integrated Graphics Controller [103c:198f]
InstallationDate: Installed on 2016-02-22 (545 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
MachineType: Hewlett-Packard HP EliteBook 840 G1
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.4.0-92-generic root=/dev/mapper/ubuntu--vg-root ro quiet splash vt.handoff=7
SourcePackage: xorg
Symptom: display
UpgradeStatus: Upgraded to xenial on 2016-08-17 (368 days ago)
dmi.bios.date: 01/26/2016
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: L71 Ver. 01.35
dmi.board.name: 198F
dmi.board.vendor: Hewlett-Packard
dmi.board.version: KBC Version 15.57
dmi.chassis.asset.tag: CNU415CJ8V
dmi.chassis.type: 10
dmi.chassis.vendor: Hewlett-Packard
dmi.modalias: dmi:bvnHewlett-Packard:bvrL71Ver.01.35:bd01/26/2016:svnHewlett-Packard:pnHPEliteBook840G1:pvrA3009DD10303:rvnHewlett-Packard:rn198F:rvrKBCVersion15.57:cvnHewlett-Packard:ct10:cvr:
dmi.product.name: HP EliteBook 840 G1
dmi.product.version: A3009DD10303
dmi.sys.vendor: Hewlett-Packard
version.compiz: compiz 1:0.9.12.2+16.04.20160823-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.76-1~ubuntu16.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 17.0.7-0ubuntu0.16.04.1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 17.0.7-0ubuntu0.16.04.1
version.xserver-xorg-core: xserver-xorg-core 2:1.18.4-0ubuntu0.3
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.10.1-1ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.7.0-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20160325-1ubuntu1.2
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.12-1build2
xserver.bootTime: Mon Aug 21 09:22:50 2017
xserver.configfile: default
xserver.errors: intel(0): page flipping failed, on CRTC:26 (pipe=0), disabling synchronous page flips
xserver.logfile: /var/log/Xorg.0.log
xserver.outputs:
 product id 4669
 vendor AUO
xserver.version: 2:1.18.4-0ubuntu0.3

[Maketsi (maketsi)]

Continued the battle for additional 1.5 hours.
I verified that both displays work fine when connected directly to the laptop (1 DP) without the HP ultraslim docking station (2 DPs) between.

After powering off the docking station and rebooting the system for Nth time, I finally got the pictures back. It's worth noting that the password prompt for disk decryption was not shown on external displays, nor the booting sequence. X11 login prompt was the first image.

It seems this was somehow affected by the docking station. Resolved for now, but the ultimate cause stays unknown.

[penalvch (penalvch)]

Maketsi, closing as per your comment https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1712005/comments/2 noting this is resolved.