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I am a little bit confused here. Here's what I know:

USB C has four high speed lanes. USB 3.2 Gen 2 uses one lane for transmitting USB and one for receiving USB both at 10gbps speeds. This leaves two lanes empty. The so called "DP Alt Mode with Multi-function support", however, turns these two over to DisplayPort.

What I do not understand is why would the USB lanes drop down to 5gbps when the other two are used for DisplayPort. The reason I think they would is because the the cable specifications mentions this mode as follows:

DP Alt Mode with Multi-function support (DP_BR 1 channel signaling combined with USB 3.2 Gen 1x1 support)

emphasis mine. Gen 1x1 is 5gbps.

The question: is MFDP 10gbps or 5gbps USB? I see no reason for it to be 5gbps. For example, Gen 2x2 uses 2-2 lanes for Tx and Rx reaching 20gbps effective speed showing 10gbps per lane is achievable even with four lanes in use. There are docks that advertise 10gbps and "4k@30Hz" support which is clearly this mode but advertisment and reality can be different things.

The DisplayPort alternate mode standard, however, is not available to the general public only to VESA members. Could someone who have access to that check whether it includes anything about this? Is this confusion only because I latched on a piece of a related standard or is there a real signal integrity reason to drop down to 5gbps?

chx
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The answer below is kept for historical reasons, it is now moot. USB Type-C specification Revision 2.1 edited section 5.2.3.1 USB4 Alternate Mode Support on Hosts and removed the confusing bit:

enter image description here

Historical answer:

I have managed to get a copy of one of the DisplayPort Alternate Mode standard drafts. It uses the term "Enhanced SuperSpeed" throughout the documentation and the Terms and Abbreviations table very clearly states this can mean both 5gbps and 10gbps ("It is used in place of phrases like SuperSpeed/SuperSpeedPlus."):

enter image description here

A Synopsys document also explains this term:

an Enhanced SuperSpeed device is operating either in SuperSpeed or SuperSpeedPlus mode.

Further research finds "An adjective referring to any valid collection of USB defined features defined for the bus that runs over the SSRx and SSTx differential pairs in a USB 3.0 or USB 3.1 system. It is used in place of phrases like SuperSpeed/SuperSpeedPlus." appears word-for-word in the USB 3.1 standard revision 1.0 published July 26, 2013 so this is not some new devilry VESA cooked up.

I have heard rumors about some USB working group reading this post and a later revision or an ECN might appear to clarify this as well but the DP Alt Mode standard is crystal clear: it can be either 5gpbs or 10gbps.

chx
  • 3,903