Can't get 8k@30 working with two active DisplayPort 1.4 -> HDMI 2.1 adapters
Brief summary of the problem:
The end goal I have is to use DisplayPort 1.4 KVM connected to 3 machines and active DisplayPort 1.4 -> HDMI 2.1 adapter (Club 3D CAC-1085) on the output to drive 8k TV.
Direct connection (without KVM) works fine, Nvidia RTX 2080 Ti also works through KVM, but RX 5500 under Linux doesn't work with 8k@30 through KVM (it does work with 4k@60 though), TV can't seem to get the proper signal, shows spinner and then says there is no signal.
Since Nvidia GPU works my intuition was that DisplayPort outputs (tried both) on RX 5500 are weaker and active repeater would help, so I got myself Club 3D CAC-1007 that I put between GPU and DP->HDMI adapter. Again, RTX 2080 Ti works even with 8k@60, RX 5500 only works at lower resolution and even then it is flaky.
I see these messages in kernel logs:
[drm:retrieve_link_cap [amdgpu]] *ERROR* retrieve_link_cap: Read receiver caps dpcd data failed.
Might be related, either way here is the full boot log where I got blank screen after GRUB: boot.log
I tried to rule-out cabling issues, so I tried 1m and 2m DisplayPort cables (several, all should be high quality), also tried several HDMI 2.1 cables for connecting to TV (including fiber optic cable). I also tried both DisplayPort connectors as I found not all of them are equally reliable on Nvidia GPU too.
The fact that Nvidia GPU can do it makes me thing AMD GPU should be capable of the same too.
The only thing I didn't try is fiber optic DisplayPort cable between GPU and KVM, primarily because those are very expensive.
Hardware description:
- CPU: AMD 5900X
- GPU:
Advanced Micro Devices, Inc. [AMD/ATI] Navi 14 [Radeon RX 5500/5500M / Pro 5500M] [1002:7340] (rev c7)
(H88-1E414-500HP) - System Memory: 128G DDR4 3600@CL16
- Display(s): Samsung QE65QN800B TV (latest from Samsung, 8k@60 capable)
- Type of Display Connection: DP 1.4 -> HDMI 2.1
System information:
- Distro name and Version: Ubuntu 22.04
- Kernel version: 5.19.10-xanmod1-x64v2
- Custom kernel: Xanmod
- AMD official driver version: N/A
- Mesa version: 22.3
git2209210600.3ce1dboibaf~j
How to reproduce the issue:
- Connect DisplayPort 1.4 active repeater to RX 5500 with 1m long DisplayPort 1.4 cable
- Connect the other end to DisplayPort 1.4 KVM with 2m DisplayPort 1.4 cable
- Connect DisplayPort 1.4 -> HDMI 2.1 active adapter to the output of KVM
- Connect output of adapter to TV box with 1m HDMI 2.1 cable