#StackBounty: #displayport #thunderbolt What's the compatibility story for DisplayPort 1.4 and Thunderbolt 3.0?

Bounty: 200

Intel Titan Ridge controllers (and if I gather correctly, Ice Lake CPUs) are DisplayPort 1.4 and TB3 compatible (Intel Ark).

I understand very well how this works in DisplayPort Alternate Mode: the host is told to switch into this alternate mode and from that point on, the cable is basically a DisplayPort cable with a different connector. If we have a laptop with a DisplayPort 1.4 capable GPU and Titan Ridge and a 3440×1440 @ 144 Hz monitor (which requires more DP bandwidth than DP 1.2 has) I would expect every USB C dock / adapter with a DisplayPort output to just work because all the dock needs to do is blindly connect the lanes in the USB C connection carrying DisplayPort signals to the DisplayPort outputs.

I can’t wrap my head around the Thunderbolt story, however. What’s the compatibility expectation here? Let’s take an Alpine Ridge based dock which is the majority of the market currently. Do we expect the monitor to work at full resolution and frequency if plugged into a DisplayPort port on the dock? If monitor has a USB C input but not Thunderbolt input, would plugging that into the downstream port of the dock work? My understanding is the TB3 standard requires the downstream ports to work in DP alt mode (this answer corroborates) but I do not know about the DP version here. How much does the TB3 controller dock “interpret” and how much does it slavishly copy?

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