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If a Thunderbolt device has two display outputs, it might take both DisplayPort signals off the bus or it might be using an MST hub. Is there any way to tell which one it is? Aside from, of course, trying to connect two 4k @ 60Hz monitors.

Linux has DP branch device info on debugfs. What about Windows?

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You could find the number of monitors and used lanes by using the free TechPowerUp GPU-Z.

Under the Advanced tab, you will find the monitors and the number of lanes taken by each one. From this data you may find the number of DisplayPort signals taken off the bus:

enter image description here

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  • Hrm, I was hoping for a utility to show the MST hub itself but this is very interesting information. So an MST hub would have Lanes (max) 2? Google has nothing on this. Note: there's a GPU chooser on the bottom which is cut off on this screenshot.
    – chx
    Sep 20 at 20:40
  • The DisplayPort port on the hub will be using lanes. It probably won't be using anything if nothing is connected to the port.
    – harrymc
    Sep 21 at 7:36
  • No, I mean, if a monitor is connected via DisplayPort, then it'll display Lanes (max) 2?
    – chx
    Sep 21 at 11:01
  • Wikipedia says: "A standard DisplayPort connection has 4 lanes, though some applications of DisplayPort implement more, such as the Thunderbolt 3 interface which implements up to 8 lanes of DisplayPort". The above screenshot for my monitor uses 4.
    – harrymc
    Sep 21 at 11:09

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