Dual external monitors (4k@120hz) & DSC works until wakeup from sleep

[kk123]

I wasn't sure whether DSC would work on MacOS, but my thunderbolt dock is able to output 4k144hz and 5120x1440@120hz simultaneously (via a DisplayPort and a thunderbolt port on the dock), from a single thunderbolt port on M1 pro Macbook. I think those would exceed 40gbps without DSC, so it must be in effect right?
But it won't last after waking up from sleep, or reboot. The 4k monitor would only get 60hz. In order to get the desired refresh rate, I have to unplug both the outputs on the dock, and first connect 4k144hz monitor, then the 5120x1440@120hz monitor.

What I think happened was:
when first connecting 4k144hz, MacOS knows DSC is required (otherwise dp1.4 only supports 120hz without DSC), and enables DSC upfront so it's able to handle both.
But when connecting 5120x1440@120hz first, which does not require DSC, MacOS just won't bother re-enabling DSC even if needed later.
i wonder if it's possible to arrange the wakeup orders, like "do not connect to the second monitor until 4k144hz is connected".

Anyone worked around this problem? well it works fine if I just use a second thunderbolt port to connect the second monitor, but i'll sleep better at night knowing everything works perfectly

 

[joevt]

4K144 (1306 MHz) by itself exceeds the limit of HBR3 x4 (1080 MHz assuming 8bpc RGB) unless it's using chroma sub sampling or DSC.
5120x1440@120hz (951 MHz) is doable by HBR3 x4 but Thunderbolt can't do two HBR3 x4 connections.

So it's either using DSC or chroma sub sampling.

HBR x4 can't do either of those timings even with DSC, therefore they both must be using HBR2 x4.

The 5120x1440@120Hz display probably defaults to HBR3 x4 leaving only HBR x4 remaining for a second display connected to the same Thunderbolt dock.

The 4K144Hz display requires DSC, and DSC can do that timing with HBR2 x4 leaving enough Thunderbolt bandwidth for a second HBR2 x4 connection.

If switching the ports that the displays are connected to doesn't change the order that they are connected at after sleep, then you need to find a way to limit the 5K display to < HBR3. Some displays have a DisplayPort version option - HBR2 is DisplayPort 1.2 but it might not include HDR? Some displays connected as USB-C have an option to switch from USB 2.0 to USB 3.x speed which switches the number of DisplayPort lanes from 4 down to 2. A USB-C dock that supports USB 3.x will only use 2 lanes of DisplayPort.

 

[kk123]

4K144 (1306 MHz) by itself exceeds the limit of HBR3 x4 (1080 MHz assuming 8bpc RGB) unless it's using chroma sub sampling or DSC.
5120x1440@120hz (951 MHz) is doable by HBR3 x4 but Thunderbolt can't do two HBR3 x4 connections.

So it's either using DSC or chroma sub sampling.

HBR x4 can't do either of those timings even with DSC, therefore they both must be using HBR2 x4.

The 5120x1440@120Hz display probably defaults to HBR3 x4 leaving only HBR x4 remaining for a second display connected to the same Thunderbolt dock.

The 4K144Hz display requires DSC, and DSC can do that timing with HBR2 x4 leaving enough Thunderbolt bandwidth for a second HBR2 x4 connection.

If switching the ports that the displays are connected to doesn't change the order that they are connected at after sleep, then you need to find a way to limit the 5K display to < HBR3. Some displays have a DisplayPort version option - HBR2 is DisplayPort 1.2 but it might not include HDR? Some displays connected as USB-C have an option to switch from USB 2.0 to USB 3.x speed which switches the number of DisplayPort lanes from 4 down to 2. A USB-C dock that supports USB 3.x will only use 2 lanes of DisplayPort.

Thanks! I don't see any fringing around text so they should be full RGB, is there a way to tell for sure?

Switching the ports did not help probably because the 5120x1440 always wakes up faster than the 4K. Neither monitor has usb-c port (one using usb-c to dp1.4 cable) so there is no settings to limit the bandwidth.

Yes, my older dock only got DP 1.2 and was able to do 5120x1440@120hz (with chroma subsampling). But I want to run both monitors at their full specs, so the solution I'm looking for is probably "force DSC no matter which monitor connceted", but I can't find the way to do it

 

[joevt]

Thanks! I don't see any fringing around text so they should be full RGB, is there a way to tell for sure?

Only on an Intel Mac. I don't know how to get the info for a Apple Silicon Mac.

 

[kk123]

Only on an Intel Mac. I don't know how to get the info for a Apple Silicon Mac.

I've come across a few adapters that do not run full RGB and I could tell almost immediately...so yes I think DSC was in the play when connected in the right order (UHD@120hz first, DQHD@120hz second).

So it's been a while and I've been trying different adapters / thunderbolt docks but none worked for my case
I tried to look for HBR2 adapters like CDP2DP (www.startech.com/en-us/display-video-adapters/cdp2dp) but it did not change the behavior.

FWIW, thanks to DSC, thunderbolt dock can run a higher refresh rate than specs, e.g. some dual 4k@60hz docks now may support 4k@120hz + 4k@60hz.