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Thunderbolt 3 doesn't use those USB 2.0 wires; that was one of the big changes in USB4/TB4 to use them. Once you're in Thunderbolt 3 mode, all USB ports are pro

Thunderbolt 3 doesn't use those USB 2.0 wires; that was one of the big changes in USB4/TB4 to use them. Once you're in Thunderbolt 3 mode, all USB ports are provided by PCIe xHCI controllers by the TB3 device. In actuality, the mode where the XDR display consumes 38gbps of uncompressed display bandwidth is an Apple-only mode requiring special Titan Ridge firmware that aggregates 6 lanes of HBR3. Contrary to the article, Alpine Ridge does not support 6k, which specifically is why the iMac Pro doesn't support 6k output. It requiring special firmware is also why this mode only works when directly connected to the Mac. But yeah, it's annoying that everyone throws bandwidth numbers around without mentioning or even thinking if it's link rate or data rate.

deleted 3y ago
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dataworm 3y ago
I'm not sure about this mode requiring special Apple firmware, but it definitely does not use 6 lanes of HBR3: https://forums.macrumors.com/threads/caldigit-introduces-usb-c-dock-with-10-ports-and-up-to-94w-charging-for-macs-updated.2280085/page-4?post=29502187#post-29502187:~:text=for%20the%20host.-,No%20such%20thing%20as%206%20lanes%20of%20HBR3.%20A%20DisplayPort%20connection%20uses%201%2C%202%2C%20or%204%20lanes.%20The%20XDR%20uses%20two%20DisplayPort%201.4%20connections%20of%204%20lanes%20each.,-The%20AGDCDiagnose%20output7 https://forums.macrumors.com/threads/caldigit-introduces-usb.... I don't think Thunderbolt ever uses anything other than 4 lanes per DP connection if the GPU doesn't limit the lane count, because Thunderbolt deserializes DP packets, transmits them over the link and serializes them back on the other end, so it can't negotiate the lane count on the host side.
chx 3y ago
Thunderbolt 3 uses the four high speed lanes of the USB C connector. The USB 2.0 separate wires are present and are definitely used. Yes, it's USB4 which added USB packets to the bus, TB3 only had PCIe and DP. That's rather interesting there, what do you mean by aggregating? Thunderbolt could always carry 40gbps worth of DisplayPort packets and afaik it was not lane bound.
- brigade 3y ago
  I mean, if you have access to the actual Thunderbolt spec saying otherwise then so be it. Personally, I sincerely doubt that the PCIe packets containing USB 2.0 data are specially routed over the USB 2.0 wires like you say since that runs counter to literally everything public about how Thunderbolt 3 works. Maybe the spec itself doesn't define a maximum bandwidth allocation for DisplayPort packets, but the DisplayPort stream has to come from somewhere and be output somewhere else, and with one single exception, the actual implementations support two HBR2 streams or one HBR3 stream, with each stream obviously being individually capped to 4 lanes at the physical DP interface. (AFAIK the special mode combines one 4-lane HBR3 stream with a second 2-lane HBR3 stream)
  - chx 3y ago
    What...? No. There are no PCIe packets, it's completely separate. The USB 2.0 wires are not part of the Thunderbolt specification, it's part of the USB C connector specification. USB C is a physical connector with four high speed lanes, a separate pair of wires for USB 2.0, one for negotiating things and one "extra". Here is a TB3 dock with a USB 2.0 port: https://www.bhphotovideo.com/c/product/1512891-REG/belkin_f4u098bt_thunderbolt_3_dock_mini.html https://www.bhphotovideo.com/c/product/1512891-REG/belkin_f4... This is the same reason you see some adapters using DisplayPort alternate mode have a USB 2.0 port only -- in this case, it's DP packets which occupy the four high speed lanes much like Thunderbolt above. Yes, Titan Ridge must connect via DisplayPort lanes and you are right, it definitely looks like it only takes five lanes, wow. https://support.lenovo.com/us/en/accessories/pd029622-display-and-video-output-configurations-docking-stations https://support.lenovo.com/us/en/accessories/pd029622-displa... says two 3840*2160@60hz and a 2560 × 1440@60Hz monitor is supported which is 30.71 Gbit/s -- a third 4K would require 37.62. https://www.dell.com/support/manuals/en-us/dell-wd19tb-dock/wd19_tb_userguide/display-resolution-table?guid=guid-5558fc88-5c31-4e2c-90be-42359855a1ae&lang=en-us https://www.dell.com/support/manuals/en-us/dell-wd19tb-dock/... explicitly says it's five lanes. https://www.dell.com/support/manuals/en-us/wd22tb4-dock/dell_wd22tb4_userguide/display-resolution-table?guid=guid-fafc77a4-c95f-44df-92ad-608231d11cd9&lang=en-us https://www.dell.com/support/manuals/en-us/wd22tb4-dock/dell... even TB4 is?? Wow!

deleted 3y ago

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dataworm 3y ago

I'm not sure about this mode requiring special Apple firmware, but it definitely does not use 6 lanes of HBR3: https://forums.macrumors.com/threads/caldigit-introduces-usb-c-dock-with-10-ports-and-up-to-94w-charging-for-macs-updated.2280085/page-4?post=29502187#post-29502187:~:text=for%20the%20host.-,No%20such%20thing%20as%206%20lanes%20of%20HBR3.%20A%20DisplayPort%20connection%20uses%201%2C%202%2C%20or%204%20lanes.%20The%20XDR%20uses%20two%20DisplayPort%201.4%20connections%20of%204%20lanes%20each.,-The%20AGDCDiagnose%20output7 https://forums.macrumors.com/threads/caldigit-introduces-usb.... I don't think Thunderbolt ever uses anything other than 4 lanes per DP connection if the GPU doesn't limit the lane count, because Thunderbolt deserializes DP packets, transmits them over the link and serializes them back on the other end, so it can't negotiate the lane count on the host side.

chx 3y ago

Thunderbolt 3 uses the four high speed lanes of the USB C connector. The USB 2.0 separate wires are present and are definitely used. Yes, it's USB4 which added USB packets to the bus, TB3 only had PCIe and DP. That's rather interesting there, what do you mean by aggregating? Thunderbolt could always carry 40gbps worth of DisplayPort packets and afaik it was not lane bound.

brigade 3y ago
I mean, if you have access to the actual Thunderbolt spec saying otherwise then so be it. Personally, I sincerely doubt that the PCIe packets containing USB 2.0 data are specially routed over the USB 2.0 wires like you say since that runs counter to literally everything public about how Thunderbolt 3 works. Maybe the spec itself doesn't define a maximum bandwidth allocation for DisplayPort packets, but the DisplayPort stream has to come from somewhere and be output somewhere else, and with one single exception, the actual implementations support two HBR2 streams or one HBR3 stream, with each stream obviously being individually capped to 4 lanes at the physical DP interface. (AFAIK the special mode combines one 4-lane HBR3 stream with a second 2-lane HBR3 stream)
- chx 3y ago
  What...? No. There are no PCIe packets, it's completely separate. The USB 2.0 wires are not part of the Thunderbolt specification, it's part of the USB C connector specification. USB C is a physical connector with four high speed lanes, a separate pair of wires for USB 2.0, one for negotiating things and one "extra". Here is a TB3 dock with a USB 2.0 port: https://www.bhphotovideo.com/c/product/1512891-REG/belkin_f4u098bt_thunderbolt_3_dock_mini.html https://www.bhphotovideo.com/c/product/1512891-REG/belkin_f4... This is the same reason you see some adapters using DisplayPort alternate mode have a USB 2.0 port only -- in this case, it's DP packets which occupy the four high speed lanes much like Thunderbolt above. Yes, Titan Ridge must connect via DisplayPort lanes and you are right, it definitely looks like it only takes five lanes, wow. https://support.lenovo.com/us/en/accessories/pd029622-display-and-video-output-configurations-docking-stations https://support.lenovo.com/us/en/accessories/pd029622-displa... says two 3840*2160@60hz and a 2560 × 1440@60Hz monitor is supported which is 30.71 Gbit/s -- a third 4K would require 37.62. https://www.dell.com/support/manuals/en-us/dell-wd19tb-dock/wd19_tb_userguide/display-resolution-table?guid=guid-5558fc88-5c31-4e2c-90be-42359855a1ae&lang=en-us https://www.dell.com/support/manuals/en-us/dell-wd19tb-dock/... explicitly says it's five lanes. https://www.dell.com/support/manuals/en-us/wd22tb4-dock/dell_wd22tb4_userguide/display-resolution-table?guid=guid-fafc77a4-c95f-44df-92ad-608231d11cd9&lang=en-us https://www.dell.com/support/manuals/en-us/wd22tb4-dock/dell... even TB4 is?? Wow!