Dual Monitor Studio Display XDR fails on MacBook Pro M4 Pro

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Tappsters OP

Created 4d

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I have two Macbook Pros:

14" M4 Pro (company)
16" M4 Max (personal)

I work remote full-time and recently purchased 2 of the new Studio Display XDRs. Everything works perfectly however I chose to connect them to the M4 Max. I have a caldigit Element TB5 hub and can daisy chain both monitors through that perfectly.

With that said, no matter how I plug them into the M4 Pro I can only ever get one to light up at a time.

What I have tried to resolve it:

Plug them in individually to the m4 pro
Plug them in one at a time, force them to 60hz and then plug them both in.
Daisy Chaining the displays
Daisy Chaining the displays through the TB5 Hub

Nothing works. Only one display comes on and its whichever is plugged in first. I have even tried lowering the refresh to as low as it goes on both manually then plugging them back in. Still nothing.

From what I am reading it appears to be that the M4 Pro has 3 display lanes and when I plug the first studio display XDR it is using 2 lanes. If I go down to 60hz which is what the original studio display was, then it should theoretically go down to 1 display lane allowing a second to be plugged in. A bunch of people had the older studio display running 2x 5k ASD monitors on the M4 Pro.

Now with the latest Studio Display XDR I am stuck. I was researching possibly editing the EDID of each to mimic the older studio display, but I don't know how to do that easily without BetterDisplay and right now I have no ability to install that. There is a chance I can get approval to run commands / BetterDisplay to get this working if a solution can be found.

What I think the ultimate fix is for the firmware / macOS to realize the limitation, force the studio display XDR to 60hz when a second monitor is plugged in and they both would work. A single Studio Display XDR could run 120hz, but immediately upon plugging a second one it swaps to 60hz. I am completely fine with that scenario.

I have found a few discussions about this topic with the main one being on apple discussions:

https://discussions.apple.com/thread/256262701?sortBy=rank&answerId=261888577022

Someone sort of gave me this idea on Mac because they were trying to use the studio display XDR on windows and it appears to have worked with cloning an older ASD EDID on the new model:

https://www.reddit.com/r/mac/comments/1s3ani5/got_studio_display_xdr_working_on_windows_pc_5k/

I don't really know what else to do. I opened a ticket with support. Case # 102853480566, but it went no where. I got disconnected during the first call after describing everything and when they reached back out they didn't even give me 2 seconds to pick up and they hung up and closed the ticket.

I really don't want to return the displays because they are beautiful and work beautifully on the m4 max. They should work with 60hz on the m4 pro.

Who / How / When can we get this resolved? I would be happy to work with an Apple dev / engineer to help resolve this.

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Answer 1

DTS Engineer OP

Apple

1d

First off, to clarify the purpose of this forum and my role at Apple, this forum is specifically to support engineers building products for our platform, not consumer-level support. I'm responding here because I hope the information will be helpful to you and to other engineers working on our hardware. However, I can't comment on if or when this will be fixed, nor am I the right person to try and pass feedback through.

Secondly, on the feedback side, I'd appreciate you filling a bug on this and then posting the bug number back here so I can ensure it gets to the correct team. That's the best way to get this feedback to the relevant engineering team.

In any case, I can't provide you with a direct solution, but I can confirm that this is basically what's going on:

From what I am reading, it appears to be that the M4 Pro has 3 display lanes, and when I plug the first Studio Display XDR, it is using 2 lanes.

More specifically, the kernel logic that assigns display pipes to monitors is not particularly sophisticated, so it's basically always requiring the number of pipes needed by the display’s maximum configuration. That doesn't matter at lower resolutions/refresh rates because driving the display only requires a single pipe. However, higher resolutions require two pipes, which is where things then break down. This forum thread talks about this in more detail.

What I think the ultimate fix is for the firmware/macOS to realize the limitation, force the Studio Display XDR to 60Hz when a second monitor is plugged in, and they both would work.

Yes, probably alongside a UI that would allow you to manually control pipe assignment between the two screens (so one can be driven at 120 and the other at 60).

Who/How/When can we get this resolved?

The only solution I know to currently work is to add a hardware dongle that artificially restricts one of the displays. I don't have any specific recommendation for that adapter, but there's an example of this working here.

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Kevin Elliott
DTS Engineer, CoreOS/Hardware

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