Bandwidth of Pro Display usb ports.

[romanof]

I saw a blurb about the Pro Display somewhere that indicated that under an xxx configuration, if the panel was run at full res, then the USB-C ports were degraded to 2.0. It probably referenced the Mac Pro and multiple displays, or something like that. Can't find the reference again (or more likely, not seeing it). Have one coming for my M1 Mini and planning the configuration of my far too numerous USB devices.

Thanks, anyone.

 

[joevt]

A Mac that supports DSC (Display Stream Compression), like the M1 Mac, should be able to get full USB 3.0 performance from the USB-C ports of the Apple Pro Display XDR. Other Macs that have DSC are those with AMD Navi or Intel Ice Lake GPUs. DSC is visually lossless compression so that there is plenty of bandwidth remaining for USB 5 Gbps (and even USB 10 Gbps or a Thunderbolt drive). The bandwidth required with DSC is less than half of Thunderbolt 3, so it may even work with a 20 Gbps cable instead of a 40 Gbps cable (but in this case you may get reduced USB speed). For example, a USB-C 10 Gbps cable will probably allow connection of Thunderbolt 3 devices (and the XDR) at 20 Gbps. Connecting the XDR to a Thunderbolt 2 dock may also work (only if the Thunderbolt 2 dock is connected to the Thunderbolt 3 port that is connected to the GPU that supports DSC).

A Mac that doesn't support DSC requires nearly the full bandwidth of Thunderbolt 3 to do 6K60 (there might be a couple Gbps remaining). I'm not sure if this mode causes the USB to connect as USB 2.0 (480 Mb/s) or if the USB device still connects as USB 3.0 but transmits a fraction of the 5 Gbps. I would like to see benchmarks for this mode to confirm, as well as an ioreg dump to see how the USB device is connected.

 

[Lihp8270]

A Mac that supports DSC (Display Stream Compression), like the M1 Mac, should be able to get full USB 3.0 performance from the USB-C ports of the Apple Pro Display XDR. Other Macs that have DSC are those with AMD Navi or Intel Ice Lake GPUs. DSC is visually lossless compression so that there is plenty of bandwidth remaining for USB 5 Gbps (and even USB 10 Gbps or a Thunderbolt drive). The bandwidth required with DSC is less than half of Thunderbolt 3, so it may even work with a 20 Gbps cable instead of a 40 Gbps cable (but in this case you may get reduced USB speed). For example, a USB-C 10 Gbps cable will probably allow connection of Thunderbolt 3 devices (and the XDR) at 20 Gbps. Connecting the XDR to a Thunderbolt 2 dock may also work (only if the Thunderbolt 2 dock is connected to the Thunderbolt 3 port that is connected to the GPU that supports DSC).

A Mac that doesn't support DSC requires nearly the full bandwidth of Thunderbolt 3 to do 6K60 (there might be a couple Gbps remaining). I'm not sure if this mode causes the USB to connect as USB 2.0 (480 Mb/s) or if the USB device still connects as USB 3.0 but transmits a fraction of the 5 Gbps. I would like to see benchmarks for this mode to confirm, as well as an ioreg dump to see how the USB device is connected.

LTT manage to Mac out the bandwidth of the XDR hubbub that video.

 

[romanof]

Ok, Thanks. So I will have three more USB 3 ports of decent speed to add to the pair on the M1. More than I need, with a big 3.0 hub. I looked again for that blog, news item or review that talked about the bandwidth per the Pro Display, but still haven't found it. Now, if the display will just arrive...

 

[joevt]

LTT manage to Mac out the bandwidth of the XDR hubbub that video.

Oh look, they used my script

LTT is in the same town - in one of their videos Linus was in front of my brother in-law's store. It's a small world.

Their 34.12 Gbps number in the video is low for uncompressed 6K60 video - that would leave enough bandwidth for Thunderbolt 3 (40 Gbps) to include USB 3.0 (4 Gbps). How does 6016x3384x60x30 = 34.12 Gbps? It should be at least 36.64 Gbps. It's missing the vertical and horizontal sync data, so they should probably use the pixel clock instead. 6K60 is 1286 MHz, so multiply that by 30 bpp and you get 38.6 Gbps. Actually, the 3008x3384 tiles use 648.91MHz. So that's 38.9 Gbps total.

With DSC compression, they took the 34.12 number and divided by two to get 17.06 Gbps. I think DSC is usually 2:1 (it can be as high as 3:1 or maybe even more) so that would be correct if 34.12 were correct. We know that the XDR only requires HBR2 with DSC, so the number must be less than 17.28 Gbps.