joevt

I've received AGDCDiagnose info from someone with a Blackmagic eGPU (contains Radeon Pro 580 which does not support the DSC mode of the XDR display). It shows dual HBR3 connection from the Radeon Pro 580 to the Apple Pro Display XDR (this can be done only through Thunderbolt 3 since the Blackmagic eGPU has no other method to connect to the display). The EDID vendor/product is APPAE2E. This is the first I've seen having the ae2e product ID - it is also the largest because it has both the tile info/timings (tiled 5K and 6K) and the 4K, 5K, and 6K non tiled timings (7 EDID blocks total).

The manual says you need Dual Link DVI for max resolution/refresh rate 1920x1080@144Hz.
Since your RTX card has a USB-C port, you could use a USB-C to Dual Link DVI adapter (if you're not using the USB-C port for a USB-C display or hard drive or dock).
I like Club 3D stuff because they have a user forum and have VESA certified cables for HBR3 speed.
https://www.club-3d.com/en/detail/2471/usb_type_c_to_dvi_i_dual_link_active_adapter/
The adapter doesn't have a separate USB type A power connection because it can get USB power from the USB-C port.
 
If you want to save your USB-C port for something else, then they have a DisplayPort version. Remember, any DisplayPort to Dual Link DVI adapter should have a separate USB type A connection for extra power because the conversion process requires more power than DisplayPort can provide by itself - otherwise the adapter would probably be single link.

I believe so. I'm waiting for someone to post results of the following commands to prove it.

/System/Library/Extensions/AppleGraphicsControl.kext/Contents/MacOS/AGDCDiagnose -a > AGDCDiagnose_a.txt 2>&1 /System/Library/Extensions/AppleGraphicsControl.kext/Contents/MacOS/DisplayDiagnose -a > DisplayDiagnose_a.txt 2>&1
The Apple Pro Display XDR Tech Specs do not mention any reduced visual capability for Macs or MPX modules that do not support DSC (if they at least have AMD graphics and Titan Ridge Thunderbolt controllers).
Google: xdr site:support.apple.com/en-us
The only reduced capability is the bandwidth left remaining for USB.
No-one has made any complaints about reduced capability (except when they do something wrong like try to connect two displays to the same Thunderbolt controller).

I forgot one item missing from the original post:
For DisplayPort or USB-C to VGA adapters, there do exist adapters that can do up to 340 MHz. I have a Plugable usbc-vga adapter which can convert DisplayPort 1.2 (2 lanes of HBR2) to VGA (up to 330 MHz). I don't know why they only advertise 1920×1200@60Hz. For some reason, macOS limits the adapter to 160 MHz. Maybe Apple meant to limit some specific adapter but managed to limit all of them (I made a patch to remove that limit for macOS Intel graphics drivers).
 
Many of the adapters are discussed at
https://hardforum.com/threads/24-widescreen-crt-fw900-from-ebay-arrived-comments.952788/page-426
 
 

The simplest method is to use the DVI input of your monitor:
Use a passive HDMI to DVI cable or adapter. DVI uses the same signals as HDMI.
 
If you want to use the DisplayPort input of your monitor then:
There exist HDMI 1.4 to DisplayPort active adapters (1440p60, 4K30). Your monitor is only 1080p so HDMI 1.4 is good enough. This info is in the original post.
There also exists really expensive HDMI 2.0 to DisplayPort active adapters (4K60). This info is missing from the "HDMI Source to DisplayPort Display" section if the original post (adapters from SIIG and gofanco https://insights.club-3d.com/thread/hdmi-2-0-to-displayport-1-2-2/6/ ) .
 
DisplayPort to HDMI adapters also exist. This is mentioned in the original post. I don't know why you say "displayport adapter in first page one way its possible and another way its not" since the original post doe mention that adapters exist for both directions.
 
The rest of this post is for other things (unrelated to your problem) that are missing from the original post:
 
If you have a dual link DVI monitor (your monitor is only 1080p so it is only single link):
You can chain a DisplayPort to Dual Link DVI adapter to a HDMI 1.4 to DisplayPort adapter to convert single link HDMI (340 MHz) to dual link DVI (330 MHz). This adapter chaining method may be unreliable - some OSs (Ubuntu) may handle it better than others (macOS) because of some plug detect issue?
I think the "HDMI Source to DVI Display" section of the original post should be updated with this info.
 
If you have a dual link output only (old graphics card) and a DisplayPort monitor:
There exists the Gefen Dual Link DVI to Mini DP Converter. This is missing from the original post.
 
For DisplayPort to DisplayPort:
An MST Hub can convert different DisplayPort lane counts (1,2,4) and link rates (RBR, HBR, HBR2, HBR3) from source to sink (much like a PCIe switch can convert PCIe  3.0 x4 to PCIe 2.0 or 1.0 x16).
There now exists DisplayPort 1.4 MST hubs (Club 3D, Delock, and soon StarTech) that can accept Display Stream Compression input and transmit that to output or decompress it for non DSC supporting displays.  https://insights.club-3d.com/thread/displayport-1-4-mst-hub/
MST hubs can also support multiple displays (but macOS can only use one display from a hub - the rest are mirrored).
The original post doesn't mention MST. Other things not mentioned:
The DisplayPort 1.2 standard can be found on the internet (but isn't supposed to be?)
The DSC spec is freely available from the VESA free standard website. It contains code that you can use to see how visually lossless the algorithm is.
 
Other things to update in the original post:
"Thunderbolt 3 Source to DisplayPort Display": Missing mention of DisplayPort 1.3, DisplayPort 1.4 support since Titan Ridge. "DisplayPort Source to Thunderbolt 3 Display": A Thunderbolt 3 add-in (such as GC-TITAN RIDGE or GC-ALPINE RIDGE) card can be used to convert DisplayPort to Thunderbolt 3 if you want to connect a Thunderbolt display. If the add-in card is not connected by PCIe to your computer, then you won't be able to use the extra functionality of the display that requires PCIe tunnelling (brightness control, camera, USB audio, FireWire, Ethernet, etc.) For some unknown reason, The Apple Thunderbolt Display (Thunderbolt 1) doesn't work from a GC-TITAN RIDGE (but does work from any Titan Ridge of a Mac). "DisplayPort Source to USB Type-C DisplayPort Alternate Mode Display": this can be done with the above Thunderbolt add-in card method. There also exists PCIe cards from Delock and SUNIX. And there are external adapters such as the Wacom Link Plus (but not the Wacom Link). https://hardforum.com/threads/use-usb-c-monitor-without-usb-c.1911817/page-2 Component Video ? Maybe this goes under VGA since they are both analog. Or it doesn't matter since it's old. Also in the VGA category could be the old Apple 15 pin connector which is mostly just VGA but in a wider two row D-SUB connector. Before EDID, Apple used a 3 pin sense line scheme to differentiate different connected displays. The pins were connected together (or not) with wires or diodes and grounded or not for 512 different possible circuits producing 45 different results though maybe only 17 were ever used (one of them being "no-connect"). There were old Mac to VGA display adapters with dip switches to select one of those results to make the old Mac think the VGA display was, for example, an Apple 2 Page Mono display, or RGB 19 inch display, etc. Many of these displays only supported a single refresh rate (they were not multi-sync).  

The video should be updated with more info/testing with different PCs/Macs using different connection modes.
 
The XDR has a Titan Ridge Thunderbolt 3 controller which can accept Thunderbolt or USB-C with DisplayPort alt mode input (Thunderbolt is also an alt mode of USB-C). The XDR supports the following input modes:
1440p HBR (good for booting) 4K 10bpc HBR2 5K 8bpc HBR3 (not sure about this one - needs testing with a DisplayPort 1.4 GPU that doesn't support DSC) 5K 10bpc HBR2x2 Thunderbolt 3 (from Alpine Ridge)  6K 12bpc HBR2 DSC 6K 12bpc? HBR3x2 Thunderbolt 3 (from Titan Ridge) The XDR display is 10bpc but allows 12bpc input. The 4K, 5K, and 6K modes have timings for the following refresh rates (Hz): 47.95, 48.00, 50.00, 59.94, 60.00. I don't know if custom timings are supported. The 1440p mode is only 60Hz. The timings for the lower refresh rates are strange because only the vertical blanking is changed. The pixels are drawn at the same rate for each refresh rate (so the same bandwidth is required for all refresh rates). This means each frame is drawn in the same amount of time but lower refresh rates will show the frame for longer. Bandwidth is reduced for lower frame sizes though (so 5K and 4K don't use the same bandwidth as 6K).
 
For USB functionality, (brightness control, USB ports, presets):
A USB-C with DisplayPort alt mode connection includes USB 2.0 (4 lanes of DisplayPort leaves no lines available for USB 3.x).
A Thunderbolt connection uses PCIe tunnelling to the Titan Ridge USB 3.1 gen 2 controller of the XDR display's Titan Ridge controller. But HBR3x2 reduces bandwidth available to USB 2.0 speeds. The USB ports and devices are connected internally with a USB 3.0 hub. How is the USB speed limited? Is the hub connection reduced to USB 2.0 mode or is it only limited by the reduced available PCIe bandwidth? For example, a USB 3.1 gen 2 (10 Gbps) PCIe card can work fine (but with reduced bandwidth) in a PCIe 1.0 x1 slot (2 Gbps).

Nvidia RTX, AMD 5300M, 5500M, W5700, W5700X, 5700 XT support DSC. Until recently Intel graphics have been limited to DisplayPort 1.2. Now there are Intel CPUs (10th gen CPU, Gen 11 GPU, Ice Lake) that support DisplayPort 1.4 and DSC, so now the Surface Laptop 3 can support 6K.
 
RTX has a USB-C port that supports DisplayPort alt mode so it works with the XDR display. The USB-C port of the RTX also supports VirtualLink alt mode. Do any available VR headsets support VirtualLink? Anyway, the XDR does not use VirtualLink. The W5700 also has a USB-C port but I don't know how it works for macOS/Windows, or if it supports USB with DisplayPort alt mode.
 
For GPUs that don't have USB-C, a bidirectional USB-C to DisplayPort cable (such as the one sold by Moshi) will work but you'll be missing the USB features of the display. Maybe an app can set brightness using DDC/CI?

If the GPU doesn't support DSC, then you'll be limited to 5K or 4K unless the GPU has two DisplayPort HBR3 connections to a Titan Ridge Thunderbolt 3 controller. However, the firmware of the controller might limit one or both connections to HBR2 or HBR which makes 6K impossible (this requires more testing). There was a problem with the Blackmagic eGPU where it would only allow two HBR2 connections to the XDR display (supporting only 5K). They released a firmware update to fix that. With a GC-TITAN RIDGE, when I connect two HBR3 displays, one of them can only connect at HBR speed. In macOS, the AGDCDiagnose command is used to get connection information (DisplayPort lanes, speed, DSC, HDCP, MST, etc.). Even if you get dual HBR3 over Thunderbolt 3 to work, I haven't seen an EDID that includes the 3008x3384 timing that would be required to support the tiled mode but that might be just because I haven't seen the AGDCDiagnose output from this connection mode yet.
 
6K is a lot of pixels (more than dual 4K which is more than 5K). A card that supports DSC and has 6 DisplayPort outputs (such as the W5700) might not support six 6K displays. Apple's Mac Pro tech specs says the AMD Radeon Pro Vega II can only support two XDR displays instead of the expected 3 (only 3 because it doesn't support DSC). The Mac Pro tech specs and the support document for the Radeon Pro Vega II Duo MPX Module have conflicting number of supported XDR displays. Is there a problem with 6K transport from the MPX slot to the I/O card or top Thunderbolt 3 ports? I don't think so, since that is the only way for the MPX 580X to support 6K.

The video should be updated with more info/testing with different PCs/Macs using different connection modes.
 
The XDR has a Titan Ridge Thunderbolt 3 controller which can accept Thunderbolt or USB-C with DisplayPort alt mode input (Thunderbolt is also an alt mode of USB-C). The XDR supports the following input modes:
1440p HBR (good for booting) 4K 10bpc HBR2 5K 8bpc HBR3 (not sure about this one - needs testing with a DisplayPort 1.4 GPU that doesn't support DSC) 5K 10bpc HBR2x2 Thunderbolt 3 (from Alpine Ridge)  6K 12bpc HBR2 DSC 6K 12bpc? HBR3x2 Thunderbolt 3 (from Titan Ridge) The XDR display is 10bpc but allows 12bpc input. The 4K, 5K, and 6K modes have timings for the following refresh rates (Hz): 47.95, 48.00, 50.00, 59.94, 60.00. I don't know if custom timings are supported. The 1440p mode is only 60Hz. The timings for the lower refresh rates are strange because only the vertical blanking is changed. The pixels are drawn at the same rate for each refresh rate (so the same bandwidth is required for all refresh rates). This means each frame is drawn in the same amount of time but lower refresh rates will show the frame for longer. Bandwidth is reduced for lower frame sizes though (so 5K and 4K don't use the same bandwidth as 6K).
 
For USB functionality, (brightness control, USB ports, presets):
A USB-C with DisplayPort alt mode connection includes USB 2.0 (4 lanes of DisplayPort leaves no lines available for USB 3.x).
A Thunderbolt connection uses PCIe tunnelling to the Titan Ridge USB 3.1 gen 2 controller of the XDR display's Titan Ridge controller. But HBR3x2 reduces bandwidth available to USB 2.0 speeds. The USB ports and devices are connected internally with a USB 3.0 hub. How is the USB speed limited? Is the hub connection reduced to USB 2.0 mode or is it only limited by the reduced available PCIe bandwidth? For example, a USB 3.1 gen 2 (10 Gbps) PCIe card can work fine (but with reduced bandwidth) in a PCIe 1.0 x1 slot (2 Gbps).

Nvidia RTX, AMD 5300M, 5500M, W5700, W5700X, 5700 XT support DSC. Until recently Intel graphics have been limited to DisplayPort 1.2. Now there are Intel CPUs (10th gen CPU, Gen 11 GPU, Ice Lake) that support DisplayPort 1.4 and DSC, so now the Surface Laptop 3 can support 6K.
 
RTX has a USB-C port that supports DisplayPort alt mode so it works with the XDR display. The USB-C port of the RTX also supports VirtualLink alt mode. Do any available VR headsets support VirtualLink? Anyway, the XDR does not use VirtualLink. The W5700 also has a USB-C port but I don't know how it works for macOS/Windows, or if it supports USB with DisplayPort alt mode.
 
For GPUs that don't have USB-C, a bidirectional USB-C to DisplayPort cable (such as the one sold by Moshi) will work but you'll be missing the USB features of the display. Maybe an app can set brightness using DDC/CI?

If the GPU doesn't support DSC, then you'll be limited to 5K or 4K unless the GPU has two DisplayPort HBR3 connections to a Titan Ridge Thunderbolt 3 controller. However, the firmware of the controller might limit one or both connections to HBR2 or HBR which makes 6K impossible (this requires more testing). There was a problem with the Blackmagic eGPU where it would only allow two HBR2 connections to the XDR display (supporting only 5K). They released a firmware update to fix that. With a GC-TITAN RIDGE, when I connect two HBR3 displays, one of them can only connect at HBR speed. In macOS, the AGDCDiagnose command is used to get connection information (DisplayPort lanes, speed, DSC, HDCP, MST, etc.). Even if you get dual HBR3 over Thunderbolt 3 to work, I haven't seen an EDID that includes the 3008x3384 timing that would be required to support the tiled mode but that might be just because I haven't seen the AGDCDiagnose output from this connection mode yet.
 
6K is a lot of pixels (more than dual 4K which is more than 5K). A card that supports DSC and has 6 DisplayPort outputs (such as the W5700) might not support six 6K displays. Apple's Mac Pro tech specs says the AMD Radeon Pro Vega II can only support two XDR displays instead of the expected 3 (only 3 because it doesn't support DSC). The Mac Pro tech specs and the support document for the Radeon Pro Vega II Duo MPX Module have conflicting number of supported XDR displays. Is there a problem with 6K transport from the MPX slot to the I/O card or top Thunderbolt 3 ports? I don't think so, since that is the only way for the MPX 580X to support 6K.

Us mere mortals don't get access to the real Thunderbolt documentation. The USB4 spec is publicly available and may describe most of how Thunderbolt works. Section 10 is all about DisplayPort tunnelling. Section 10.4.2.1 is about Path Configuration: Setup. The next page (pg 362) describes how the Connection Manager calculates bandwidth available to limit the maximal link rate and lane count of the DP IN adapter's DP_REMOTE_CAP register. I guess Apple changed this calculation to allow two HBR3 four lane connections. I need an AGDCDiagnose output to prove that (but I believe there's no other way to get 6K without DSC). I wonder if two HBR3 connections is allowed only for the XDR display, or if it will also work with my two displays (It's actually one Acer XV273K with two DisplayPort 1.4 inputs). If it does work with the Acer to allow two HBR3 connections, I wonder what happens if I increase the refresh rates so that the bandwidth exceeds the 40 Gbps limit (does it gracefully disconnect, stop, or show garbage). I don't have a Titan Ridge Mac with AMD dGPU to test.
 
A correction to what I said previously about connecting two 4K displays to Thunderbolt 2 (method: connect HBR display first, then first 4K, then replace HBR display with second 4K). The AGDCDiagnose outputs show that they both connect with DisplayPort 1.1 HBR four lanes. To get one to be connected with DisplayPort 1.2 HBR2 four lanes, they both need to be disconnected first. This behavior matches what is described by the USB4 spec (I'm not sure why the DisplayPort version needs to change - just the link width and lane count should be sufficient).
 
The limit calculation described by USB4 has a problem. If I have 4 lanes of HBR available, will it allow 2 lanes of HBR2? I did this test with a two lane DisplayPort 1.2 adapter after setting up a single HBR x4 connection to Thunderbolt 2. The answer is that it connects with 2 lanes of HBR instead of HBR2. This is not optimal. Apple is right to allow max link rate and lanes (exceeding 40 Gbps) instead of that silly calculation.
 
The problem (that Apple bypasses for the XDR?) is that a limit is placed on both lanes and link rate, but bandwidth is the product of both, where HBR x4 is the same as HBR2 x2. Another but: for Thunderbolt, bandwidth is about pixels per second and not related to DisplayPort lanes and link rate (stuffing symbols are removed - see Figure 10-22 in the USB4 spec). 6K doesn't require full dual HBR3 but does require more than dual HBR2 so Thunderbolt 3 Titan Ridge should be able to handle 6K as Apple has made it do. Ideally, a user should be able to lower the bandwidth used by a display (decrease refresh rate or resolution or pixel depth) to increase the bandwidth allowed for another display connected to the same Thunderbolt 3 port without disconnecting/reconnecting the displays.
 
Another situation where the limit calculation has a problem: If I have a Thunderbolt 3 dock with an internal DisplayPort 1.4 MST hub (like the HP Thunderbolt Dock G2) connected to a Thunderbolt 2 port, then any display (even if it is HBR only) connected to the hub will use the full DisplayPort 1.2 HBR2 four lane connection of the hub so a second display cannot be connected to the Thunderbolt port of the HP Thunderbolt Dock G2. Of course, an MST hub allows multiple displays to be connected, but that doesn't work in macOS which doesn't support MST for multiple displays (but does support MST for old 4K displays that use a separate stream for each side of the display).