Description:
I have a Dell XPS 13 (9350), a Dell P2715Q 27" 4K monitor, and a Kensington SD4600P USB-C dock. The setup works perfectly in Windows 10 (4k @ 60hz), but in ArchLinux 4.6.4-1, I can get it to work at 4k@29hz via HDMI, but if I connect via DisplayPort 1.2 (which supports 4k @ 60hz), it'll lock up within a few minutes after entering Gnome shell from GDM. Most of the time it will lockup within a couple of seconds. When connected via DisplayPort, I can sometimes quickly go to the Display settings applet to change the refresh rate to 29Hz instead of the defaulted 59hz, which will work.

I've resorted to connecting only via HDMI, forcing it to 29hz, but the mouse and refreshes are annoying laggy.

Pretty much the same problem is described with Ubuntu here:
http://askubuntu.com/questions/780856/ubuntu-crashes-when-setting-maximum-resolution-on-external-4k-monitor


Additional info:
* Linux 4.6.4-1-ARCH #1 SMP PREEMPT Mon Jul 11 19:12:32 CEST 2016 x86_64 GNU/Linux

* Gnome & video packages:
extra/gdm 3.20.1-2 (gnome) [installed]
extra/gnome-shell 3.20.3-1 (gnome) [installed]
extra/xf86-video-intel 1:2.99.917+676+g26f8ab5-1 (xorg-drivers xorg) [installed]

* /etc/mkinitcpio.conf MODULES:
MODULES="intel_agp i915"

* $ lspci
00:00.0 Host bridge: Intel Corporation Skylake Host Bridge/DRAM Registers (rev 09)
00:02.0 VGA compatible controller: Intel Corporation Iris Graphics 540 (rev 0a)
00:04.0 Signal processing controller: Intel Corporation Skylake Processor Thermal Subsystem (rev 09)
00:14.0 USB controller: Intel Corporation Sunrise Point-LP USB 3.0 xHCI Controller (rev 21)
00:14.2 Signal processing controller: Intel Corporation Sunrise Point-LP Thermal subsystem (rev 21)
00:15.0 Signal processing controller: Intel Corporation Sunrise Point-LP Serial IO I2C Controller #0 (rev 21)
00:15.1 Signal processing controller: Intel Corporation Sunrise Point-LP Serial IO I2C Controller #1 (rev 21)
00:16.0 Communication controller: Intel Corporation Sunrise Point-LP CSME HECI #1 (rev 21)
00:17.0 SATA controller: Intel Corporation Sunrise Point-LP SATA Controller [AHCI mode] (rev 21)
00:1c.0 PCI bridge: Intel Corporation Device 9d10 (rev f1)
00:1c.4 PCI bridge: Intel Corporation Sunrise Point-LP PCI Express Root Port #5 (rev f1)
00:1c.5 PCI bridge: Intel Corporation Sunrise Point-LP PCI Express Root Port #6 (rev f1)
00:1d.0 PCI bridge: Intel Corporation Device 9d18 (rev f1)
00:1f.0 ISA bridge: Intel Corporation Sunrise Point-LP LPC Controller (rev 21)
00:1f.2 Memory controller: Intel Corporation Sunrise Point-LP PMC (rev 21)
00:1f.3 Audio device: Intel Corporation Sunrise Point-LP HD Audio (rev 21)
00:1f.4 SMBus: Intel Corporation Sunrise Point-LP SMBus (rev 21)
01:00.0 PCI bridge: Intel Corporation DSL5110 Thunderbolt Bridge [Falcon Ridge LP 2014]
02:00.0 PCI bridge: Intel Corporation DSL5110 Thunderbolt Bridge [Falcon Ridge LP 2014]
02:01.0 PCI bridge: Intel Corporation DSL5110 Thunderbolt Bridge [Falcon Ridge LP 2014]
02:02.0 PCI bridge: Intel Corporation DSL5110 Thunderbolt Bridge [Falcon Ridge LP 2014]
05:00.0 USB controller: Intel Corporation Device 15b5
3a:00.0 Network controller: Broadcom Corporation BCM4350 802.11ac Wireless Network Adapter (rev 08)
3b:00.0 Unassigned class [ff00]: Realtek Semiconductor Co., Ltd. RTS525A PCI Express Card Reader (rev 01)
3c:00.0 Non-Volatile memory controller: Toshiba America Info Systems Device 010f (rev 01)

* Relevant (?) dmesg when switching refresh:
[30168.150965] ------------[ cut here ]------------
[30168.151020] WARNING: CPU: 0 PID: 4012 at drivers/gpu/drm/i915/intel_pm.c:3586 skl_update_other_pipe_wm+0x177/0x180 [i915]
[30168.151027] WARN_ON(!wm_changed)
[30168.151031] Modules linked in:
[30168.151037] snd_usb_audio snd_usbmidi_lib snd_rawmidi snd_seq_device hid_generic fuse ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 xt_addrtype iptable_filter xt_conntrack nf_nat nf_conntrack br_netfilter bridge stp llc dm_thin_pool dm_persistent_data dm_bio_prison dm_bufio libcrc32c crc32c_generic loop snd_hda_codec_hdmi i2c_designware_platform dell_led i2c_designware_core snd_soc_skl snd_soc_skl_ipc snd_soc_sst_ipc snd_soc_sst_dsp dell_laptop dell_wmi snd_hda_ext_core dell_smbios snd_soc_sst_match intel_rapl dcdbas snd_soc_core brcmfmac snd_hda_codec_realtek snd_hda_codec_generic x86_pkg_temp_thermal nls_iso8859_1 intel_powerclamp coretemp nls_cp437 snd_compress snd_pcm_dmaengine ac97_bus vfat brcmutil uvcvideo fat kvm_intel videobuf2_vmalloc snd_hda_intel
[30168.151151] videobuf2_memops videobuf2_v4l2 cfg80211 kvm snd_hda_codec videobuf2_core rtsx_pci_ms snd_hda_core snd_hwdep memstick snd_pcm snd_timer input_leds led_class snd irqbypass serio_raw pcspkr soundcore i2c_i801 shpchp videodev media idma64 joydev mei_me mousedev mei intel_lpss_pci intel_pch_thermal btusb btrtl processor_thermal_device intel_soc_dts_iosf fan thermal wmi i2c_hid battery hci_uart btbcm btqca btintel bluetooth rfkill pinctrl_sunrisepoint pinctrl_intel intel_lpss_acpi intel_lpss intel_hid evdev int3403_thermal int340x_thermal_zone mac_hid int3400_thermal sparse_keymap acpi_als acpi_thermal_rel acpi_pad ac tpm_tis kfifo_buf industrialio tpm fjes processor sch_fq_codel ip_tables x_tables ext4 crc16 jbd2 mbcache algif_skcipher af_alg hid_multitouch usbhid hid dm_crypt dm_mod rtsx_pci_sdmmc
[30168.151289] mmc_core atkbd libps2 crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd ahci libahci nvme nvme_core xhci_pci libata xhci_hcd rtsx_pci scsi_mod usbcore usb_common i8042 serio i915 video button i2c_algo_bit drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops drm intel_agp intel_gtt
[30168.151362] CPU: 0 PID: 4012 Comm: gnome-shell Tainted: G W 4.6.4-1-ARCH #1
[30168.151367] Hardware name: Dell Inc. XPS 13 9350/09JHRY, BIOS 1.4.4 06/14/2016
[30168.151373] 0000000000000286 000000004906adcf ffff88045f6d7960 ffffffff812e54c2
[30168.151384] ffff88045f6d79b0 0000000000000000 ffff88045f6d79a0 ffffffff8107a6bb
[30168.151393] 00000e0200000063 ffff88006552e000 ffff88045f6d7a2c ffff880468db9da4
[30168.151402] Call Trace:
[30168.151419] [<ffffffff812e54c2>] dump_stack+0x63/0x81
[30168.151428] [<ffffffff8107a6bb>] __warn+0xcb/0xf0
[30168.151435] [<ffffffff8107a73f>] warn_slowpath_fmt+0x5f/0x80
[30168.151473] [<ffffffffa00f7707>] skl_update_other_pipe_wm+0x177/0x180 [i915]
[30168.151510] [<ffffffffa00f7894>] skl_update_wm+0x184/0x620 [i915]
[30168.151558] [<ffffffffa0186cbf>] ? intel_ddi_enable_transcoder_func+0x17f/0x260 [i915]
[30168.151594] [<ffffffffa00fb43e>] intel_update_watermarks+0x1e/0x30 [i915]
[30168.151647] [<ffffffffa0169dc2>] haswell_crtc_enable+0x322/0x8b0 [i915]
[30168.151698] [<ffffffffa0165931>] intel_atomic_commit+0x5f1/0x15f0 [i915]
[30168.151722] [<ffffffffa0033c62>] ? drm_atomic_check_only+0x182/0x600 [drm]
[30168.151742] [<ffffffffa0034117>] drm_atomic_commit+0x37/0x60 [drm]
[30168.151758] [<ffffffffa00a37dd>] drm_atomic_helper_connector_dpms+0xed/0x1a0 [drm_kms_helper]
[30168.151786] [<ffffffffa0028840>] drm_mode_obj_set_property_ioctl+0x240/0x250 [drm]
[30168.151807] [<ffffffffa0018aa2>] drm_ioctl+0x152/0x540 [drm]
[30168.151830] [<ffffffffa0028600>] ? drm_mode_obj_get_properties_ioctl+0xa0/0xa0 [drm]
[30168.151842] [<ffffffff8124234c>] ? eventfd_read+0x5c/0x90
[30168.151854] [<ffffffff811f62b7>] ? __vfs_read+0x37/0x100
[30168.151863] [<ffffffff81209be3>] do_vfs_ioctl+0xa3/0x5d0
[30168.151871] [<ffffffff81214017>] ? __fget+0x77/0xb0
[30168.151880] [<ffffffff8120a189>] SyS_ioctl+0x79/0x90
[30168.151892] [<ffffffff815c7232>] entry_SYSCALL_64_fastpath+0x1a/0xa4
[30168.151899] ---[ end trace f61a05be43014942 ]---
[30278.075930] perf: interrupt took too long (2512 > 2500), lowering kernel.perf_event_max_sample_rate to 79500
[30384.842548] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe A FIFO underrun


Steps to reproduce:
* Connect an external 4k monitor via DisplayPort 1.2 (which supports 60hz)
* Try to log into X windows or Wayland