AR# 70130

DisplayPort RX Subsystem - Why does the video resolution not match between the GPU settings and the RX MSA registers?

Description

When using the DisplayPort application example design (pass-through or RX only), if I change the resolution in my GPU, I cannot see a change on the RX MSA registers.

However, on my monitor (for a pass-through application), I can see that the video resolution is changing.

For example, the video resolution is initially set for 4K. If I change it to 1080p, I can see that there is a zoom in the video stream but the MSA registers still display "Horz Resolution (0x500) : 3840" and "Horz Resolution (0x500) : 2160" (4K resolution).

Why am I seeing this behavior?

Solution

Some GPUs are scaling the video stream to avoid retraining every time the resolution change. So in this case, the stream sent by the GPU is actually a 4K stream but which shows a 1080p video. The values from the RX MSA registers are correct.

There are (at least) two ways to confirm if this is what your source is doing:

  • If you change the resolution from your source and you cannot see that the link retrains
  • If you change the resolution from your source, but this resolution does not match the resolution given by your monitor (usually in the monitor menu)

On some computer, there is an option to avoid the scaling.

AR# 70130
Date 12/07/2017
Status Active
Type General Article
IP