Hi WayneWWW:
Sorry for the late response. We are using Synaptics vmm5320.
We are trying to use DP → DP + HDMI or DP → HDMI + HDMI. DP → 2 HDMI works fine. However, when the HDMI is connected (FHD or 800x480), DP output is not able to display 4K @ 60.
After confirming with the vendor of the MST hub, we found that someone also encounter an issue about this. Hence, we gave it a shot about disabling DSC feature in the driver:
Now, the configuration of DP → DP (4K) + HDMI can work properly.
Huge sorry for the lack of investigation.