Hi,
I have a Radeon RX 580 connected a Dell P2415Q monitor (via DisplayPort) running kernel 4.20.6 from Arch Linux. On a 3840x2160 resolution, a 29.98 Hz refresh rate works fine. However switching to 59.98 Hz does not work and produces the following backtrace in dmesg:

[Feb 8 11:37] [drm] Failed to updateMST allocation table forpipe idx:0
[  +0,000059] WARNING: CPU: 6 PID: 1161 at drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link.c:2410 core_link_enable_stream+0x68f/0x1040 [amdgpu]
[  +0,000000] Modules linked in: fuse amdgpu nls_iso8859_1 nls_cp437 vfat fat edac_mce_amd kvm_amd kvm irqbypass chash amd_iommu_v2 gpu_sched i2c_algo_bit ttm snd_hda_codec_realtek snd_hda_codec_generic drm_kms_helper snd_hda_codec_hdmi snd_hda_intel drm crct10dif_pclmul snd_hda_codec crc32_pclmul ghash_clmulni_intel snd_hda_core snd_hwdep agpgart snd_pcm syscopyarea mousedev wmi_bmof ccp joydev input_leds aesni_intel sysfillrect snd_timer sysimgblt fb_sys_fops r8169 aes_x86_64 snd sp5100_tco crypto_simd cryptd realtek soundcore i2c_piix4 glue_helper libphy rng_core k10temp pcspkr wmi pinctrl_amd gpio_amdpt evdev mac_hid pcc_cpufreq acpi_cpufreq crypto_user ip_tables x_tables ext4 crc32c_generic crc16 mbcache jbd2 fscrypto raid1 md_mod sr_mod cdrom hid_generic usbhid hid sd_mod ahci libahci libata crc32c_intel xhci_pci scsi_mod xhci_hcd
[  +0,000032] CPU: 6 PID: 1161 Comm: gnome-shell Not tainted 4.20.6-arch1-1-ARCH #1
[  +0,000001] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./AB350 Pro4, BIOS P5.40 12/19/2018
[  +0,000040] RIP: 0010:core_link_enable_stream+0x68f/0x1040 [amdgpu]
[  +0,000001] Code: 48 8b 81 f0 02 00 00 41 0f b6 54 24 5a 44 89 de d0 ea 48 8b b8 98 00 00 00 83 e2 01 e8 0a 95 00 00 49 8b 0c 24 e9 ea fb ff ff <0f> 0b e9 2a fe ff ff 48 8b 80 80 02 00 00 48 8b 40 50 48 8b 40 30
[  +0,000001] RSP: 0018:ffffad5c0260f8c0 EFLAGS: 00010246
[  +0,000001] RAX: 0000000000000000 RBX: ffff91d0032e0188 RCX: 0000000000000000
[  +0,000001] RDX: 0000000000000005 RSI: ffffffffc0f08ab0 RDI: 0000000000000004
[  +0,000001] RBP: ffff91d03f278000 R08: 0000000000000005 R09: 0000000000000000
[  +0,000000] R10: 0000000000000001 R11: 0000000000000000 R12: ffff91d048fd5400
[  +0,000001] R13: ffff91d048fd5590 R14: ffff91d00736e800 R15: 0000000000000006
[  +0,000001] FS:  00007f0623aa5c00(0000) GS:ffff91d04eb80000(0000) knlGS:0000000000000000
[  +0,000001] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  +0,000001] CR2: 00007f06222fe000 CR3: 0000000406544000 CR4: 00000000003406e0
[  +0,000000] Call Trace:
[  +0,000047]  ? dce110_opp_program_fmt+0x52/0x200 [amdgpu]
[  +0,000041]  dce110_apply_ctx_to_hw+0x427/0x490 [amdgpu]
[  +0,000040]  dc_commit_state+0x2c3/0x520 [amdgpu]
[  +0,000043]  amdgpu_dm_atomic_commit_tail+0x2a9/0xd20 [amdgpu]
[  +0,000004]  ? preempt_count_add+0x79/0xb0
[  +0,000003]  ? _raw_spin_lock_irq+0x1a/0x40
[  +0,000001]  ? _raw_spin_unlock_irq+0x1d/0x30
[  +0,000001]  ? wait_for_common+0x113/0x190
[  +0,000001]  ? _raw_spin_unlock_irq+0x1d/0x30
[  +0,000001]  ? wait_for_common+0x113/0x190
[  +0,000007]  commit_tail+0x3d/0x70 [drm_kms_helper]
[  +0,000005]  drm_atomic_helper_commit+0x103/0x110 [drm_kms_helper]
[  +0,000005]  drm_atomic_helper_set_config+0x80/0x90 [drm_kms_helper]
[  +0,000009]  drm_mode_setcrtc+0x193/0x6c0 [drm]
[  +0,000010]  ? drm_mode_getcrtc+0x180/0x180 [drm]
[  +0,000007]  drm_ioctl_kernel+0xaf/0xf0 [drm]
[  +0,000007]  drm_ioctl+0x333/0x3e0 [drm]
[  +0,000008]  ? drm_mode_getcrtc+0x180/0x180 [drm]
[  +0,000025]  amdgpu_drm_ioctl+0x49/0x80 [amdgpu]
[  +0,000003]  do_vfs_ioctl+0xa4/0x630
[  +0,000003]  ksys_ioctl+0x60/0x90
[  +0,000002]  __x64_sys_ioctl+0x16/0x20
[  +0,000003]  do_syscall_64+0x5b/0x170
[  +0,000001]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  +0,000002] RIP: 0033:0x7f062d25580b
[  +0,000002] Code: 0f 1e fa 48 8b 05 55 b6 0c 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 0f 1f 44 00 00 f3 0f 1e fa b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 25 b6 0c 00 f7 d8 64 89 01 48
[  +0,000000] RSP: 002b:00007fff79fca318 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[  +0,000002] RAX: ffffffffffffffda RBX: 00007fff79fca350 RCX: 00007f062d25580b
[  +0,000000] RDX: 00007fff79fca350 RSI: 00000000c06864a2 RDI: 000000000000000b
[  +0,000001] RBP: 00007fff79fca350 R08: 0000000000000000 R09: 000055871fa797e0
[  +0,000001] R10: 000055871fa990f0 R11: 0000000000000246 R12: 00000000c06864a2
[  +0,000000] R13: 000000000000000b R14: 0000000000000000 R15: 000055871fa990f0
[  +0,000002] ---[ end trace bd6eba6f481fec44 ]---

Please let me know if you need any further info.