DSC broken after update; 30-bit color HDR 4K @ 144Hz option missing with RX 5700 XT eGPU

Number:rdar://FB8895507 Date Originated:13/11/2020
Status:Open Resolved:
Product:macOS Product Version:macOS 11.0.1 / 11.1
Classification:bug Reproducible:Always
I have recently purchased a new LG monitor specifically the LG 27GN950-B which is capable of displaying 30-bit colors & HDR at 4K resolution @ 144Hz. To achieve this, the monitor uses DSC - Display Stream Compression, which also requires a compatible graphics card, for example an RX 5700 XT that I own and have attached as an eGPU in the Akitio Node Pro enclosure.

This setup worked out of the box on macOS Catalina with support for every feature listed above, but after updating it seems as if DSC has been disabled and the video output to the monitor is limited to 4K @ 60Hz with HDR or 4K @ 95 Hz without HDR, both using 30-bit color.

This is a big regression in support for eGPU and I would love to see this issue fixed so I can use the monitor at it’s full potential.

Steps to reproduce: (taken from rdar://FB8924921)
1. Get a 4K 144hz monitor such as LG 27GN950 and a MBP 16" or eGPU enclosure with AMD 5500XT and newer GPU.
2. Connect the MBP or eGPU to the monitor either via TB to DisplayPort or directly DP to DP in case on eGPU.
3. Go to System Preferences > Displays > Option click "Scaled" > Tick Show low resolution modes.
4. Only 60hz and 90hz options will be available.


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