Ubuntu 20.04/22搭配RX 7900XT仅识别三台显示器中的一台问题求助

Hey there, let's work through this multi-monitor issue with your RX 7900XT on Ubuntu. The root cause here is pretty straightforward: your RX 7900XT uses AMD's RDNA 3 architecture, which isn't fully supported by the default kernels in Ubuntu 20.04 (5.4) and 22.04 (5.15). That's why you're stuck with a fallback display driver (like VESA) that only detects one monitor, and seeing that Failed to get size of gamma for output default error from xrandr.

Here's how to fix it step by step:

1. Update your kernel to a version that supports RDNA 3 #

Newer kernels have full support for your GPU, so this is the first critical step:

• For Ubuntu 20.04: Use the HWE (Hardware Enablement) edge kernel, which gets you a newer supported kernel:

  sudo apt update && sudo apt install linux-generic-hwe-20.04-edge
  

• For Ubuntu 22.04: Upgrade to the HWE kernel for 22.04, which will give you a 6.x series kernel:

  sudo apt update && sudo apt install linux-generic-hwe-22.04
  

2. Install the latest open-source AMDGPU drivers #

The default drivers might still be outdated even after updating the kernel. Using the Oibaf graphics PPA will get you the latest open-source AMD drivers optimized for new GPUs:

sudo add-apt-repository ppa:oibaf/graphics-drivers
sudo apt update && sudo apt full-upgrade

3. Verify everything works after a reboot #

Once you've completed the above steps, restart your PC. Then:

• Run lspci -k | grep -A 2 -i "VGA" — look for kernel driver in use: amdgpu in the output. This confirms the proper GPU driver is loaded (instead of VESA/fbdev).
• Run xrandr again — it should now list all three of your connected monitors. You can then use Ubuntu's display settings to arrange them as needed.

4. Extra troubleshooting if issues persist #

If you still don't see all monitors:

• Check your BIOS settings: Make sure PCIe 4.0 is enabled (if your motherboard supports it) and disable integrated graphics (iGPU) if you're using a CPU with one — sometimes iGPU/dGPU conflicts can cause multi-monitor detection problems.
• Share the output of dmesg | grep amdgpu — this log will show any errors with the AMDGPU driver loading, which can help narrow down the issue.

Hope this gets all your monitors up and running smoothly!

备注：内容来源于stack exchange，提问作者jaldabaoth