BT2020_RGB 10bpc is not supported

[flibitijibibo]

Your system information

• Steam client version: 1707518866
• SteamOS version: 3.5.15 20240209.1
• Dock firmware version: 0.13.15.121
• Opted into Steam client beta?: Yes
• Opted into SteamOS beta?: Yes, Preview channel
• Have you checked for updates in Settings > System?: Yes

Please describe your issue in as much detail as possible:

Currently, Steam Deck OLED does not support outputting HDR10 for external displays. From what I can tell it just does 8-bit HDR, regardless of what the display is capable of.

I was able to verify that AMD hardware should support this when using the current HDR kernel patchset; this was tested on Fedora running embedded gamescope.

Steps for reproducing this issue:

0. Enable sudo access and sshd service if you haven't already
1. Shut down Deck if powered on
2. Plug Deck into dock, connect dock to HDR10 display via DisplayPort
3. Power on Deck, boot to game mode home screen
4. Connect via ssh
5. Check the current color format:

sudo cat /sys/kernel/debug/dri/0/crtc-0/amdgpu_current_colorspace
sudo cat /sys/kernel/debug/dri/0/crtc-0/amdgpu_current_bpc

Colorspace should be BT2020_RGB, bpc should be 10 but is instead 8