For the upstream kernel within 14.04.5 the problem is happening again.
In the kernel log I found issues with the i915 driver:
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many full retries, give up
[drm:intel_wait_ddi_buf_idle [i915]] *ERROR* Timeout waiting for DDI BUF B idle bit
For the upstream kernel within 14.04.5 the problem is happening again.
In the kernel log I found issues with the i915 driver:
[drm:intel_ dp_link_ training_ clock_recovery [i915]] *ERROR* too many full retries, give up wait_ddi_ buf_idle [i915]] *ERROR* Timeout waiting for DDI BUF B idle bit
[drm:intel_
kernel: [599006.771813] WARNING: CPU: 0 PID: 914 at /build/ linux-lts- xenial- Hu9lgy/ linux-lts- xenial- 4.4.0/drivers/ gpu/drm/ i915/intel_ dp_mst. c:493 intel_dp_ destroy_ mst_connector+ 0x netbios_ ns nf_conntrack_ broadcast ipt_MASQUERADE nf_nat_ma codec_realtek snd_hda_codec_hdmi snd_hda_ codec_generic snd_usb_audio snd_usbmidi_lib snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd temp_thermal intel_powerclamp coretemp uvcvideo videobuf2_vmalloc kvm_intel videobuf2_memops iwlmvm snd_seq snd_pcm videobuf2_v4l2 mac80211 kvm vid destroy_ connector_ work [drm_kms_helper] 38c>] dump_stack+ 0x63/0x87 776>] warn_slowpath_ common+ 0x86/0xc0 7fc>] warn_slowpath_ fmt+0x4c/ 0x50 354>] ? drm_atomic_ helper_ set_config+ 0x94/0xb0 [drm_kms_helper] 76d>] ? drm_atomic_ state_free+ 0x4d/0x70 [drm] f7d>] intel_dp_ destroy_ mst_connector+ 0xcd/0xd0 [i915] 69b>] drm_dp_ destroy_ connector_ work+0xab/ 0x150 [drm_kms_helper] 970>] process_ one_work+ 0x150/0x3f0 0ea>] worker_ thread+ 0x11a/0x470 fd0>] ? rescuer_ thread+ 0x310/0x310 849>] kthread+0xc9/0xe0 780>] ? kthread_ park+0x60/ 0x60 70f>] ret_from_ fork+0x3f/ 0x70 780>] ? kthread_ park+0x60/ 0x60 mst_disable_ dp [i915]] *ERROR* failed to update payload -22 mst_pre_ enable_ dp [i915]] *ERROR* can't find connector
cd/0xd0 [i915]()
kernel: [599006.771816] Disabling mst crtc failed with -22
kernel: [599006.771817] Modules linked in: xfrm_user xfrm_algo xt_addrtype br_netfilter aufs xt_nat xt_mark nf_conntrack_
squerade_ipv4 xt_tcpudp ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_na
t_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw iptab
le_filter ip_tables x_tables arc4 cmac hp_wmi sparse_keymap snd_hda_
_seq_midi intel_rapl snd_seq_midi_event snd_rawmidi x86_pkg_
eobuf2_core irqbypass v4l2_common videodev iwlwifi joydev snd_seq_device serio_raw media hid_plantronics snd_timer input_leds cfg80211 rtsx_pci_ms btusb btrtl memstick btbcm snd btintel rfcomm bnep soundcore
bluetooth shpchp lpc_ich mei_me mei 8250_fintek mac_hid tpm_infineon hp_accel lis3lv02d hp_wireless input_polldev parport_pc ppdev lp parport drbg ansi_cprng dm_crypt hid_generic usbhid hid crct10dif_pclmul
crc32_pclmul rtsx_pci_sdmmc i915 aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd i2c_algo_bit drm_kms_helper psmouse syscopyarea sysfillrect sysimgblt e1000e fb_sys_fops ahci drm libahci
ptp rtsx_pci pps_core wmi video fjes
kernel: [599006.771937] CPU: 0 PID: 914 Comm: kworker/0:2 Tainted: G W 4.4.0-34-generic #53~14.04.1-Ubuntu
kernel: [599006.771939] Hardware name: Hewlett-Packard HP EliteBook Folio 9480m/22DA, BIOS M85 Ver. 01.01 07/03/2014
kernel: [599006.771951] Workqueue: events drm_dp_
kernel: [599006.771954] 0000000000000000 ffff88024495fce8 ffffffff813d438c ffff88024495fd30
kernel: [599006.771958] ffffffffc030c7c0 ffff88024495fd20 ffffffff8107d776 ffff880241726000
kernel: [599006.771961] ffff8802408c0800 ffff880241471a50 ffff880241471650 ffff880241725810
kernel: [599006.771965] Call Trace:
kernel: [599006.771972] [<ffffffff813d4
kernel: [599006.771978] [<ffffffff8107d
kernel: [599006.771981] [<ffffffff8107d
kernel: [599006.771991] [<ffffffffc017c
kernel: [599006.772017] [<ffffffffc007d
kernel: [599006.772052] [<ffffffffc02c1
kernel: [599006.772062] [<ffffffffc0177
kernel: [599006.772067] [<ffffffff81095
kernel: [599006.772071] [<ffffffff81096
kernel: [599006.772074] [<ffffffff81095
kernel: [599006.772078] [<ffffffff8109b
kernel: [599006.772081] [<ffffffff8109b
kernel: [599006.772085] [<ffffffff817f7
kernel: [599006.772088] [<ffffffff8109b
kernel: [599006.772091] ---[ end trace 3af0cc086a51172a ]---
kernel: [599007.257246] [drm:intel_
kernel: [599007.491996] [drm:intel_