Native Resolution Only Available By Lowering Vertical Backporch

[davefl]

Hello all,

Hoping that someone could explain this to me.

2 Monitors. Resolution is 5120x2160 and 2560x2880. Both are being run through a dock connected by a single Thunderbolt (I am testing two Docks).

On Mac M2 - Both work just fine using a Display Port and Thunderbolt.

On PC (Intel Iris XE on Windows 11) Both work just fine if using Display Port for both Monitors. If using Display Port and Thunderbolt the native resolution will not be available on the 2560x2880 (when both are plugged in, if each is plugged in independently then no issue).

Oddly for the scenario on the PC that a Display Port and Thunderbolt Port is used if I change the Vertical Back Porch from 69 to 49 lines everything works.

What exactly is going on here? Is there a better way of doing this?

Edit:

If I leave the Vertical Back Porch to the Original but Change the Pixel Clock to be 480. Then it works as well. I noticed that when changing the Vertical Back Porch to 49 it put the pixel clock to 479.99. Perhaps a hardware limitation on the PC?

[ToastyX]

480 MHz is the 6 bpc limit for 4-lane DisplayPort HBR mode, so DisplayPort is being limited to HBR mode when Thunderbolt is active. This could be a hardware limitation. Maybe the Thunderbolt port on the PC doesn't have as much bandwidth available as on the Mac, so the dock has to reduce the DisplayPort bandwidth to be able to operate the Thunderbolt port.

[davefl]

But if I have 2 physical display ports (Dell WD22TB4) connected to the Thunderbolt Input then it does work.

So there must be something that is physically setting the Thunderbolt Port on the Dock to 4 Lane HBR Mode ONLY when it is connected to the PC. What seems odd to me is that the underlying Thunderbolt port on the Laptop does have enough bandwidth to handle this (otherwise how would a dock with actual display ports work). Is there anything else that can be done to instruct the port to run in a mode greater than 4 lanes?

Maybe step points are in terms of bandwidth useage are smaller with Display Port which is why there is no issue there? The other Display which is also connected is running at 849.25Mhz.

[ToastyX]

I'm saying when the Thunderbolt port is active on the dock, maybe the dock shares the bandwidth between all DisplayPort and Thunderbolt ports, while with just DisplayPort, it only shares the bandwidth between the DisplayPort displays, so there is more available in that case. The PC's Thunderbolt port might have enough bandwidth for two displays, but the dock might split that bandwidth differently when using both DisplayPort and Thunderbolt. The Mac's Thunderbolt port might have more bandwidth to make that not matter.

[davefl]

This makes sense now. 5120x2160 72Hz requires ~850Mhz. This means that HBR3 must be used. At best that leaves us with HBR. Which is why I am seeing the limits as you mentioned. DisplayLink must be using some sort of MST or Optimization for Bandwidth. And Mac M2 must be doing something unique as well.

It's too bad that you can't select say 60Hz and force the monitor to use HBR2 to free up bandwidth but I guess it doesn't work that way.

Happy that I can get the resolution at least to it's native dimensions with reduced color or refresh rate.

Thank you for your help.