There was a commit last year regarding bandwidth management for Display Port tunnels. Does this allow dual HBR3 connections from a single Titan Ridge Thunderbolt port? From reading the changes, it seems the answer is no? I have not tried the latest Linux kernel yet.
Normally, dual HBR3 would exceed the 40 Gbps bandwidth of Thunderbolt 3, but display timings that don’t require the entire HBR3 bandwidth would just have stuffing symbols added to fill the HBR3 bandwidth, and Thunderbolt doesn’t transmit stuffing symbols. Apple allows dual HBR3 from the ports of its Titan Ridge controllers (and the port of the Blackmagic eGPU after recent firmware update) for its Apple Pro Display XDR for GPUs that have DisplayPort 1.4 but don’t support Display Stream Compression (DSC). This works because 6K does not require the full bandwidth of dual HBR3. The AppleThunderboltDPOutAdapter.kext has a string: “Thunderbolt DP - bandwidth to be derived from local and remote caps instead of common cap”
Currently, if I connect an HBR3 display to a Windows PC with GC-TITAN RIDGE Thunderbolt 3 add-in card, then a second display can only connect at HBR link rate. If an OS sees a display as connected only at HBR link rate, then the OS will not allow the user to choose a higher bandwidth timing, even if the timing used by the HBR3 display is reduced to HBR2 or lower link rate beforehand. It would be preferable for two DisplayPort 1.4 displays to both connect at HBR3 and allow one display to use higher bandwidth (refresh rate, resolution, bit depth) by having the user reduce bandwidth of the other display (I think that’s how displays connected VIA DisplayPort MST work).
A similar issue exists with the Thunderbolt 2 ports of a 2015 MacBook Pro when connecting an HBR2 display. A second display cannot be connected. To work around this, you need to connect an HBR display first, then an HBR2 display, then replace the HBR display with a HBR2 display. Both HBR2 displays will connect as HBR and macOS only shows HBR timings.
Alpine Ridge doesn’t have this problem because it supports only DisplayPort 1.2 and dual HBR2 won’t exceed the Thunderbolt 3 bandwidth (but I suppose the problem will exist like for Thunderbolt 2 if the cable only allows 20 Gbps).
Has the thunderbolt-software(a)lists.01.org mailing list disappeared? The last message is from October. The links at https://01.org/thunderbolt-sw/get-involved don’t work.