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Radeon RX 6900 XT + Pro Display XDR = 5K only and no presets

Dae

Dae

macrumors regular
Original poster
Apr 30, 2009
146
13
We're testing the following setup:
  • MacBook Pro or iMac Pro running Big Sur 11.4 (beta)
  • Sonnet eGFX Breakaway Box 650
  • Gigabyte AMD Radeon RX 6900 XT (has USB-C port)
  • Apple Pro Display XDR — connected to the USB-C port of the RX 6900 XT with Apple TB3 Pro cable


All in all it works, but Pro Display XDR only works in 5K, the presets and the brightness controls are missing.



More pics:
https://imgur.com/a/ZSi0WRt

Does anybody know:
  1. Why isn't the Pro Display XDR working in 6K in this setup?
  2. What can we do to be able to control our Pro Display XDR properly (presets, brightness etc)?
Thanks!

Update: output from AGDCDiagnose — https://pastebin.com/y272LLe2
 
Last edited:
Internet sleuthing makes it appear that USB-C doesn't have the bandwidth to support 6k displays.
 
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Technically with DSC enabled it would have enough bandwidth to work for 6k + usb data. Judging by your screenshots it seems that DSC is disabled so max bandwidth for only 5K@60Hz. Wasn't there a thread here with Big Sur not having DSC enabled anymore on third party cards? I guess this is (sadly) the same issue now?
 
Menno87 said:
Technically with DSC enabled it would have enough bandwidth to work for 6k + usb data. Judging by your screenshots it seems that DSC is disabled so max bandwidth for only 5K@60Hz. Wasn't there a thread here with Big Sur not having DSC enabled anymore on third party cards? I guess this is (sadly) the same issue now?
Click to expand...
yeah according to https://www.displayport.org/displayport-over-usb-c-7-reasons/ without DSC DisplayPort over USB-C is supposed to be limited to 4k60. So getting 5K60 out of it is interesting, unless it is 5K30 maybe?
 
Dae said:
Output from:

Code: 
/System/Library/Extensions/AppleGraphicsControl.kext/Contents/MacOS/AGDCDiagnose

pastebin.com

AGDCDiagnose Version: 6.3.2 (AGDC node count: 9)### Start: GPUWrangler ####### - Pastebin.com

Pastebin.com is the number one paste tool since 2002. Pastebin is a website where you can store text online for a set period of time.
pastebin.com pastebin.com
Click to expand...
I don't see a problem. It shows a four lane HBR2 connection with DSC to the 6900 XT so there's no reason 6K shouldn't work.

I'm using 11.3 and noticed there's a new string in the AGDCDiagnose output "Max Swath Width 5120", same as in your output (I'm using a 4K display with a W5700).

My display is DisplayPort 1.4 and can accept timings up to 8K30 (but doesn't support DSC). With my setup, 5K60 is possible with 8bpc (but not 10bpc). I am able to add 6K50 and 8K30 using SwitchResX. They work but they sort of cause the screen to vibrate (maybe just need to adjust the syncs - not much point using timings greater than 4K on a 4K display except to test the GPU)...

When I'm using a 6K50 timing, the "Max Swath Width 5120" still exists in the AGDCDiagnose output so I guess it doesn't mean anything.

Two more things to check:
1) What if you select "Show low resolution modes"? Try holding the Option key and clicking "Scaled" more than once?

2) In SwitchResX, what does the Display Information tab show for Frequency Range, Startup resolution, and Scaled resolution base?
For 6K60, you need a pixel clock of 1287 MHz.
6016x3384@60.000Hz 210.960kHz 1286.01MHz h(8 32 40 +) v(118 8 6 -) 16:9

3) log show with a predicate that filters out all messages except those related to the display and GPU drivers. Starting looking for messages that contain 6016. It may show why it rejects the timing.
 
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Menno87 said:
Hmm nice, interesting. Now I am itching to get a 6900XT again for my Mac Pro to test lol.
Click to expand...
Hm a while ago I was trying to use Tb4 hubs as an extension for my cable to the prodisplay XDR, but none worked for some reason at 6k, only 5k. The XDR is picky!
 
chfilm said:
Hm a while ago I was trying to use Tb4 hubs as an extension for my cable to the prodisplay XDR, but none worked for some reason at 6k, only 5k. The XDR is picky!
Click to expand...
Using Vega II Duo GPUs? Those don't support DSC, which means dual HBR3 connection is required for 6K60 but Apple doesn't allow dual HBR3 with a Thunderbolt device between the host and the display. Does dual HBR3 work at all in Windows? - It might be an Apple hack which doesn't or maybe can't work when there's an intermediate Thunderbolt device.

You need a Navi GPU to get DSC which should allow intermediate Thunderbolt devices.
 
Curious as to whether or not you have been experiencing the kernel panics that plague other AMD cards when used with Macs and eGPUs? Fingers crossed this is resolved with 11.4 and and the 6900!
 
We've bought Belkin Charge and Sync Cable [for Huawei VR glass]. Using this cable we get the presets and brightness controls, but only 5K resolution, so still no 6K. Beware!

cba20k said:
Curious as to whether or not you have been experiencing the kernel panics that plague other AMD cards when used with Macs and eGPUs? Fingers crossed this is resolved with 11.4 and and the 6900!
Click to expand...
No kernel panics so far.

I will post more information as advised by @joevt.
 
apple officially claimed that only MPX support 6K . With regard to W5700X with DSC, it is to offer the full speed of thunderbolt on Pro Display XDR otherwise the port will decrease to lower speed like USB 2.0. all of non-MPX can offer 5K max.
 
fritzzzzzz said:
apple officially claimed that only MPX support 6K
Click to expand...

Interesting, I didn't know that.

(On a mostly unrelated note, we've got 6K and USB features of XDR working through Sonnet eGPU Breakaway Puck RX 5700).
 
fritzzzzzz said:
apple officially claimed that only MPX support 6K . With regard to W5700X with DSC, it is to offer the full speed of thunderbolt on Pro Display XDR otherwise the port will decrease to lower speed like USB 2.0. all of non-MPX can offer 5K max.
Click to expand...
Macs can do a lot of things Apple doesn't claim.
DSC is to get full speed of USB 3.0 on Pro Display XDR when connected with Thunderbolt. Non-DSC or non-Thunderbolt limits you to USB 2.0. DSC is possible without Thunderbolt.
All RDNA (MPX or not) can do 6K with DSC if the OS isn't broken. Apple can probably fix it by changing one line of code.
If DSC isn't working in Big Sur then switch to Catalina, but that doesn't work with RDNA2 cards...
 
joevt said:
Macs can do a lot of things Apple doesn't claim.
DSC is to get full speed of USB 3.0 on Pro Display XDR when connected with Thunderbolt. Non-DSC or non-Thunderbolt limits you to USB 2.0. DSC is possible without Thunderbolt.
All RDNA (MPX or not) can do 6K with DSC if the OS isn't broken. Apple can probably fix it by changing one line of code.
If DSC isn't working in Big Sur then switch to Catalina, but that doesn't work with RDNA2 cards...
Click to expand...
what are the point of saying all these statements ? back to ur first point: if you want to leverage the maximum speed of USB 3 (or whatever you want to call it) on Pro Display XDR then you have to use MPX with DSC , which means it has to W5700X at this very moment. this is fact. cause of MPX without DSC cards dont have enough bandwidth

Apple can do this or that. The thing is: Apple sets the limitation on 6K functionality to its own MPX unless you keep at least one APPLE GPU so it is no way that you can bypass it - you imagine Apple can fix it by changing one line of code - yes you might be right but things are easy to do - it doesn't mean apple will do it. This is called Apple Ecosystem - buy or keep the MPX and Pro Display XDR in order to fully enjoy the benefits. And, not attract people to buy third-party GPU and enjoy the same benefits from Apple's technology.
 
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fritzzzzzz said:
what are the point of saying all these statements ? back to ur first point: if you want to leverage the maximum speed of USB 3 (or whatever you want to call it) on Pro Display XDR then you have to use MPX with DSC , which means it has to W5700X at this very moment. this is fact. cause of MPX without DSC cards dont have enough bandwidth
Click to expand...
USB 3 on XDR requires DSC and Thunderbolt. There are other ways to get DSC and Thunderbolt besides MPX. MacBook Pro with 5500M or 5700M? Sonnet eGPU Breakaway Puck RX 5500 XT or 5700? Hacintosh or Mac with 5700 XT and Thunderbolt AIC?
 
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