Force External Display Link Rate

[davewolfs]

Greetings,

I have two external displays. One of which I would prefer to always connect at HBR3. The problem is that the display I would like to connect at HBR3 comes out of sleep slower than my other display.

I can force the other display to connect at HBR2 but is there anyway to have it only connect at HBR? HBR2 isn’t ideal as the display I would like to connect at HBR3 is then limited to HBR2.

Thanks!

 

[joevt]

Take a DisplayPort cable and remove two of the lanes to make it 2 lanes.
HBR2 x2 = HBR x4
I don't think anyone makes such a cable though...

A USB-C dock that supports USB 3.x will limit the DisplayPort connection to two lanes.

Does your display support HDMI? An active DisplayPort to HDMI 1.4 adapter might be sufficiently limited. HDMI 1.4 has slightly less bandwidth than HBR though.

Which GPU / macOS?

A display override file can have a /DisplayPort/MaxBitRate property but this is one of the properties that only works for override files in /System/Library/Displays/Contents/Resources/Overrides and not the override files that are in /Library/Displays/Contents/Resources/Overrides.
I've never seen anyone use this property (not even Apple) so I don't know if it works. Also, it might only work for Intel Macs.

Modern macOS versions don't let you modify files in /System/Library... I have a Lilu/WhateverGreen patch that makes displaypolicyd load override files from /Library/... These are kexts that are loaded by Open Core to run macOS on PCs or to run newer macOS versions on unsupported Macs.

 

[davewolfs]

Mac M2 on Ventura. How do I set the override?

Would a 10gbps max cable USBC cable have any impact?

Windows doesn't have this problem as it can always sort out to connect the DisplayPort on the larger display first.

 

[joevt]

Mac M2 on Ventura. How do I set the override?

The override is loaded by displaypolicyd on Intel Macs. Do Apple Silicon Macs use displaypolicyd?
I would sudo grep the system folder on Apple Silicon Macs for any binary that contains /DisplayPort/MaxBitRate.

You can look in /System/Library/Displays/Contents/Resources/Overrides for examples of override files.
Here's a list of override files that include the DisplayPort dictionary (but none of them use the MaxBitRate property):

grep -l DisplayPort $( find /System/Library/Displays/Contents/Resources/Overrides -type f \( -not -name '*.*' \) )
/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-610/DisplayProductID-ae2e
/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-610/DisplayProductID-ae22
/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-610/DisplayProductID-ae2d
/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-1e6d/DisplayProductID-5b74
/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-1e6d/DisplayProductID-5b11
/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-1e6d/DisplayProductID-5b7b
/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-1e6d/DisplayProductID-5b10
/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-9e6d/DisplayProductID-5b74
/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-9e6d/DisplayProductID-5b11

You can run SwitchResX to create an override file in /Library/Displays/Contents/Resources/Overrides.
Then you can modify the override file.
I won't explain how to move the override file to the /System folder since that would break the system seal and I don't even know if this would work on Apple Silicon Macs.

Would a 10gbps max cable USBC cable have any impact?

Maybe if the display had a USB-C input and an option to switch from USB 2.0 to USB 3.x mode.
A USB-C 10 Gbps cable has 4 lanes that can do 10 Gbps. It can allow HBR3 x4 + USB 2.0 or HBR3 x2 + USB 3.x.

Windows doesn't have this problem as it can always sort out to connect the DisplayPort on the larger display first.

Send feedback to Apple.

 

[davewolfs]

The display has a USB C and a USB B input.

It sounds like this isn't possible without buying a hub and even then it still might not work. Who would have thought that external display support could be this bad.

 

[joevt]

The display has a USB C and a USB B input.

It sounds like this isn't possible without buying a hub and even then it still might not work. Who would have thought that external display support could be this bad.

External displays work fine. It's putting things between the display and the host computer that is the problem.

You're trying to connect two displays to a single Thunderbolt dock? You should connect them to different Thunderbolt ports of the host Mac so that you don't need to worry about issues with HBR3 link rate over Thunderbolt.

 

[davewolfs]

I found a 1.1 cable 😂

Theoretically this should do exactly what I want.

The setup would be easier if I just daisy chained and allowed things to work at 60Hz but I prefer to use 72Hz for the main display.

 

[joevt]

I found a 1.1 cable 😂

Theoretically this should do exactly what I want.

The setup would be easier if I just daisy chained and allowed things to work at 60Hz but I prefer to use 72Hz for the main display.

Is there a difference between a 1.1 and a 1.2 or a 1.4 cable? I don't think so. You have to hope that it doesn't work well enough for the system to attempt 1.2 speed.

A USB-C cable is different - it has a chip that describes the capabilities of the cable.

 

[davewolfs]

@joevt Do you know if the CalDigit element do 2 lanes or does it have to be a soho?

 

[joevt]

CalDigit Element Hub is a Thunderbolt 4 dock, not a USB-C dock.

If you connect it to a Thunderbolt or USB4 host, then it can have 8 lanes (two DisplayPort connections, 4 lanes each). You would still need a method to limit each downstream USB-C port to 2 lanes.

I suppose the CalDigit Element Hub could be connected as a USB-C dock but it would have to be using a special cable that makes it not connect as Thunderbolt such as with a USB + DisplayPort to USB-C cable or adapter.
https://forums.macrumors.com/threads/alternative-to-the-belkin-vr-cable.2378619/
https://www.store.level1techs.com/products/p/dp-repeater-hdmi-splitter-6sha9-yznx5-zm58w
But what would be the point of adding a CalDigit Element Hub on top of that?

I have used a CalDigit Element Hub connected to a Club-3d CAC-1336 to fix the USB-C output of the CAC-1336, but that's 4 lanes of DisplayPort + USB 2.0 from HDMI.
https://insights.club-3d.com/thread/cac-1336-operation/