[amdgpu, regression, 5.5.18, raven] hang on reboot
starting with 5.5.18 Ryzen 5 system (Lenovo e485) soft locks up on shutdown/reboot:
[ 2340.719306] watchdog: BUG: soft lockup - CPU#2 stuck for 22s! [systemd-shutdow:1]
[ 2340.720296] Modules linked in: 8021q garp stp mrp llc bnep xfs wmi_bmof btusb btrtl btintel uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_v4l2 kvm_amd bluetooth videobuf2_common kvm snd_hda_codec_conexant rtwpci rtw88 snd_hda_codec_generic irqbypass snd_hda_codec_hdmi mac80211 snd_hda_intel snd_intel_dspcfg snd_hda_codec i2c_piix4 snd_hwdep libarc4 snd_hda_core snd_pcm cfg80211 snd_timer tpm_crb ucsi_acpi typec_ucsi wmi typec thinkpad_acpi tpm_tis i2c_scmi tpm_tis_core btrfs xor zstd_decompress zstd_compress amdgpu crct10dif_pclmul crc32_pclmul crc32c_intel raid6_pq mfd_core amd_iommu_v2 gpu_sched ttm sdhci_pci iosf_mbi cqhci
[ 2340.725596] CPU: 2 PID: 1 Comm: systemd-shutdow Tainted: G T 5.5.18-gentoo #1
[ 2340.725596] Hardware name: LENOVO 20KUCTO1WW/20KUCTO1WW, BIOS R0UET76W (1.56 ) 11/05/2019
[ 2340.729587] RIP: 0010:amdgpu_mm_rreg+0x101/0x130 [amdgpu]
[ 2340.729587] Code: 48 8b 7d 08 48 83 c5 18 44 89 e9 44 89 e2 89 de e8 04 19 a3 f4 48 8b 45 00 48 85 c0 75 e2 eb 95 48 03 85 d0 05 00 00 44 8b 28 <eb> 84 48 8b 1c 24 4c 8b 6c 24 18 44 89 e6 4c 8b 74 24 20 4c 8b 64
[ 2340.733592] RSP: 0018:ffffb9a900057cc0 EFLAGS: 00000286 ORIG_RAX: ffffffffffffff13
[ 2340.733592] RAX: ffffb9a9011bb184 RBX: 0000000000000000 RCX: 0000000000000002
[ 2340.737570] RDX: 0000000000000000 RSI: 000000000000ec61 RDI: ffff99c432620000
[ 2340.741587] RBP: ffff99c432620000 R08: 0000000000000000 R09: 0000000000000002
[ 2340.741587] R10: 00000000000000f0 R11: 0000000000006000 R12: 000000000000ec61
[ 2340.745583] R13: 00000000ffffffff R14: ffff99c432620450 R15: 000000000000327b
[ 2340.745583] FS: 00007f6d0936ae00(0000) GS:ffff99c43f280000(0000) knlGS:0000000000000000
[ 2340.749597] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2340.749597] CR2: 00007f6d0a0c6660 CR3: 000000043a01c000 CR4: 00000000003406e0
[ 2340.753593] Call Trace:
[ 2340.753593] gfx_v9_0_rlc_stop+0xcb/0x190 [amdgpu]
[ 2340.757596] gfx_v9_0_hw_fini+0x95/0xce0 [amdgpu]
[ 2340.761596] amdgpu_device_ip_suspend_phase2+0x9a/0x150 [amdgpu]
[ 2340.761596] amdgpu_device_ip_suspend+0x46/0x70 [amdgpu]
[ 2340.765595] amdgpu_pci_shutdown+0x32/0x40 [amdgpu]
[ 2340.765595] pci_device_shutdown+0x45/0x90
[ 2340.769596] device_shutdown+0x155/0x1b0
[ 2340.769596] kernel_restart+0x11/0x30
[ 2340.773593] __do_sys_reboot+0x21f/0x260
[ 2340.777598] do_syscall_64+0x55/0x180
[ 2340.777598] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 2340.781596] RIP: 0033:0x7f6d09ff7947
[ 2340.781596] Code: 44 00 00 48 83 ec 18 89 fa be 69 19 12 28 bf ad de e1 fe 64 48 8b 04 25 28 00 00 00 48 89 44 24 08 31 c0 b8 a9 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 21 48 8b 4c 24 08 64 48 33 0c 25 28 00 00 00
[ 2340.785593] RSP: 002b:00007ffc162d7160 EFLAGS: 00000246 ORIG_RAX: 00000000000000a9
[ 2340.785593] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f6d09ff7947
[ 2340.789594] RDX: 0000000001234567 RSI: 0000000028121969 RDI: 00000000fee1dead
[ 2340.793598] RBP: 0000000000000000 R08: 0000000000000000 R09: 00007ffc162d6590
[ 2340.793598] R10: 00007f6d0936aca0 R11: 0000000000000246 R12: 0000000000000000
[ 2340.797596] R13: 0000000000000000 R14: 0000000000000006 R15: 0000000000000000