Thunderbolt3/USB4 targeting: should the wiki materials be more explicit about Thunderbolt 3 vs. Thunderbolt 4 distinctions?

Here is my quick attempt at noting distinctions and some possible
handling points. It is possible that protocols make some of this
more automatic/clear than I presume. (I would not know.)


Overall: Is it a goal to have a Thunderbolt 4 capable PC be able to
operate as a Thunderbolt 3 PC when booted by FreeBSD? That would be
different then just allowing FreeBSD to reject such a PC or to end
up without even Thunderbolt 3 support for such a PC.

I'm not claiming that "converting a Thunderbolt 4 capable PC into
a Thunderbolt 3 PC instead" should be avoided: It should be explicitly
documented if targeted and, if not targeted, the alternative should
instead be explicitly documented. It might be too early to say yet for
some of the example issues. (I view this more issue by issue
contributions to the overall status than just the final overall status.
Different issues might go different directions.)


Ports per accessory:

Thunderbolt 3: Allow configuring up to 2 ports (so: daisy chain only by accessories)
Thunderbolt 4: Allow configuring up to 4 ports (so: star is possible)

So:       No support for star (3 or 4 ports used)?
Possibly: Only support devices with 1 or 2 ports?
But:      What about 2 used of 3 or 4 present?
Note: Mixes with hot plugging!

So:   Lack of SW support could turn a PC capable of Thunderbolt 4
       into a Thunderbolt 3 PC?


Minimum PCIe bandwidth (minimums):

Thunderbolt 3: Must support 16 Gbps or more
Thunderbolt 4: Must support 32 Gbps or more

Not so sure that this aspect makes a difference.


Monitors (minimums):

Thunderbolt 3: No 8K monitor support but 1 5K monitor optionally supported
Thunderbolt 4: Allow configuring for 2 4K monitors or 1 8K monitor
Note: Some Thunderbolt 3 implementations can support 2 4K monitors
Note: USB4 apparently does not specify a minimum nK-ish figure.
Note: Looks like frequency for Dual Full HD is another difference

So: Do not support 8K monitors? (What of 2 4K monitors?)

So: Do not support the higher Dual Full HD frequency?

So: Lack of SW support could turn a PC capable of Thunderbolt 4
     into a Thunderbolt 3 PC?


Power (minimum for most ports):

Thunderbolt 3: At least 15W
Thunderbolt 4: At least 15W
Note: USB4: At least 7.5W

Not so sure that this aspect makes a difference.


At least Power port for PC charging for thin and light notebooks that
require < 100W to charge:

Thunderbolt 3: No requirement
Thunderbolt 4: At least one such port on the notebook

So: Lack of SW support could turn such a notebook capable of Thunderbolt 4
     into a Thunderbolt 3 notebook?


Wake from Sleep Mode:

Thunderbolt 3: Optionally keyboard/mouse input to wake a sleeping computer
Thunderbolt 4: Required keyboard/mouse input to wake a sleeping computer

So: Do not support keyboard and mouse input waking a sleeping computer?
     (Lack of SW support could turn a PC capable of Thunderbolt 4 into
     a Thunderbolt 3 PC?)


Compatibility:

Thunderbolt 3: Need not be compatible with some Thunderbolt 4 specific requirements
Thunderbolt 4: Required to be compatible with Thunderbolt 3 devices/requirements

So: Various examples of SW potentially turning a Thunderbolt 4 capable PC
     into a Thunderbolt 3 PC


Security:

Thunderbolt 3: Allows optional VT-D based DMA or equivalent
Thunderbolt 4: Required to support VT-D based DMA or equivalent

So: Is there to be support for uses "VT-D based DMA or equivalent"?
Or: Is this an example of SW converting a Thunderbolt 4 capable PC
     into a Thunderbolt 3 PC instead?

USB4 Specification Status:

Thunderbolt 3: Compatible
Thunderbolt 4: Compliant
(I've seen references to such a distinction but do not well understand the
specifics.)


===
Mark Millard
marklmi at yahoo.com