Thank you, so something fairly low-level has to be wrong.
Focusing on the alt mode message which also appears in [1], I followed the suggestion of enabling DRM debug logging. I toggled the CORE, DRIVER and KMS categories (echo 0x7 >1 /sys/module/drm/parameters/debug) and filtered out the messages that appeared repeatedly all the time irrespective of plugging an external display on/off (supposedly related to driving my laptop display). There were quite a few, so for future reference the command was
% sudo dmesg -wP | grep -vE "VRR packet update|PSR (SU|FFU) dirty rect|OBJ ID: [0-9]+|amdgpu_ih_process|CP EOP|drm_ioctl"
with this setup, the output when plugging in the HDMI display is as follows:
[43800.454342] usb 7-1: USB disconnect, device number 18
[43801.767212] ucsi_acpi USBC000:00: unknown error 256
[43801.767230] ucsi_acpi USBC000:00: GET_CABLE_PROPERTY failed (-5)
[43802.055281] ucsi_acpi USBC000:00: unknown error 256
[43802.055290] ucsi_acpi USBC000:00: con4: failed to register partner alt modes (-5)
[43802.663102] amdgpu 0000:c1:00.0: [drm:detect_link_and_local_sink [amdgpu]] DP Alt mode state on HPD: 0
[43802.723964] usb 7-1: new full-speed USB device number 19 using xhci_hcd
[43802.897808] usb 7-1: New USB device found, idVendor=32ac, idProduct=0002, bcdDevice= 0.00
[43802.897820] usb 7-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[43802.897826] usb 7-1: Product: HDMI Expansion Card
[43802.897831] usb 7-1: Manufacturer: Framework
[43802.897835] usb 7-1: SerialNumber: 11AD1D00A49C4010191D0B00
[43802.903400] amdgpu 0000:c1:00.0: [drm] Alt mode has timed out after 239 ms
[43802.903405] amdgpu 0000:c1:00.0: [drm:link_detect [amdgpu]] link_detect: link_index=3 is_local_sink_detect_success=0 pre_link_type=0 link_type=1
[43802.962447] hid-generic 0003:32AC:0002.002B: hiddev97,hidraw2: USB HID v1.11 Device [Framework HDMI Expansion Card] on usb-0000:c3:00.4-1/input1
[43804.235542] ucsi_acpi USBC000:00: unknown error 256
[43804.235552] ucsi_acpi USBC000:00: con4: failed to register partner alt modes (-5)
[43804.428590] ucsi_acpi USBC000:00: unknown error 256
[43804.428596] ucsi_acpi USBC000:00: con4: failed to register partner alt modes (-5)
Same with the DP display. However, I noticed that when unplugging there is a hotplug event generated
[45212.165756] amdgpu 0000:c1:00.0: [drm:drm_sysfs_connector_hotplug_event] [CONNECTOR:112:DP-3] generating connector hotplug event
[45212.165779] amdgpu 0000:c1:00.0: [drm:drm_client_dev_hotplug] fbdev: ret=0
[45212.170344] amdgpu 0000:c1:00.0: [drm:drm_helper_probe_single_connector_modes] [CONNECTOR:112:DP-3]
[45212.170348] amdgpu 0000:c1:00.0: [drm:drm_helper_probe_single_connector_modes] [CONNECTOR:112:DP-3] disconnected
There is no such event when plugging in either of the displays, I’m not sure whether that’s normal.
I could only find a single driver issue [2] with this message and it looks unrelated, plus it’s fixed anyway.