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Pixel Format for DP Alt mode 2 lanes configuration

Question asked by bobotu on Mar 26, 2020

I have a ThinkPad USB-C dock (USB 3.1 Gen2), which use 2 lanes for DP Alt mode. My laptop is ThinkPad T495 with Ryzen 3500U. When I connect to an external monitor through this dock, I can only select 6bit RGB 4:4:4 or 8bit YCbCr 4:2:2. I also tried this with my MacBook Pro 2018 with 560X GPU, the supported pixel format and color encoding are the same.

The Radeon Software shows the link speed is 8.1Gbps x 2. As mentioned in VESA's documentation, DP 1.4a Alt mode can support 4k@60fps FULL 4:4:4 color encoding WITHOUT DSC in 2 lanes configuration. So I connect this dock to my RTX2080's USB-C port and connect my monitor through the dock, the result is 8bit FULL RGB 4:4:4.

So is this a hardware limitation on 560X and 3500U or just a driver limitation that can be addressed in a later release?