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USB4 spec was really unusual for USB, because it was basically just defining the thunderbolt tunneling protocol, routing system, and connection manager (and its

USB4 spec was really unusual for USB, because it was basically just defining the thunderbolt tunneling protocol, routing system, and connection manager (and its complexities), including the optional PCIe encapsulation, and notably one of the encapulated protocols that can be used is USB3.2. It also specifies the optional faster clock rates that TB3 used. A port featuring USB4 discovers and negotate using USB4 via a slightly modified version of the normal method for discovering TYPE-C alternate modes. If it is determined that the other endpoint does not support USB4, normal alternate mode negotiation will occur, including (if supported) the special analouge audio mode. For displayport alternate mode, the USB4 router becomes involved in supplying the connection. Finally, if neither USB4 nor any alternate mode is negotiated, then the port will use USB 3.2. I'm confused by the claim that USB4 does not support HDMI alternate mode. It is true that HDMI encapsulation is not supported, and that USB4 does not mandate supporting HDMI alternate mode (while it does mandate supporting DisplayPort alternate mode, since not all displayport capable TYPE-C peripherals are designed to handle the encapsulation), but a USB4 port can offer any additional alternate modes that the system designer wants. USB4 is also really weird in that you cannot really add additional ports to the system on via add-on cards or similar, since there needs to be one router for the whole host that is connected to all the USB4 ports, in order to implement the correct routing system, and also implement the guarentees like Display Port being available on USB4 ports, etc.

rektide 4y ago
Awesome details, super informative! Really interesting limitation you've mentioned about not being able to add in USB4. I hope this doesn't end up being true forever!! It's such an amazing set of capabilities; it'd suck to require them all to be built-in to the cpu/chipset. Couldn't a discrete video card (which has a DisplayPort output) implement USB4?
- jsmith45 4y ago
  Theoretically, yes a video card could implement USB4 (by having a USB 3.2 host controller, and a USB 2.0 host controller on board, implementing the routing fabric etc. But in so doing, you have limited the bandwidth available to the GPU (although they actually do still make framebuffer GPUs that don't have any calculation acceleration support). Also don't forget the need to support USB Power delivery (there is no requirement on being able to deliver power, but PD communication is required, and many USB4 devices will expect to be able to negotiate for some level of power delivery), so this card will probably need those extra power cables like normal accelerating GPUs use. But if you had such a card, and your motherboard has one two, then you have one machine that is actually two seperate USB4 hosts. This opens up a bunch of questions like does the host OS actually support this sort of configuration (especially if the Connection Manager is being implemented in software rather than hardware). Then you get weird stuff like being able to connect a Type C cable between the two hosts of the same system to get Host-To-Host Tunneling, but where both hosts are the same system. That is some wacky stuff. Even Host to Host tunneling is esoteric, basically allowing system that implements each host to pass each other arbitrary data over over the link. The example in the spec is exchanging IP packets, but I;m not sure that details of this usage are really standardized. It might also be possible to support add-on cards for USB4 by a routing chip manufacturer that makes use of the device to device communication features of of PCIe, to allow multiple USB4 routers to work together as a single virtual router. That sounds far more plausible, although doing that sort of thing would increase the contention on the PCIe bus.
Schroedingersat 4y ago
> USB4 is also really weird in that you cannot really add additional ports to the system on via add-on cards or similar, since there needs to be one router for the whole host that is connected to all the USB4 ports, in order to implement the correct routing system, and also implement the guarentees like Display Port being available on USB4 ports, etc. That sounds a hell of a lot like 'we're going to add some bullshit DRM in the next version'
- jsmith45 4y ago
  I think it really more reflects that the authors of the spec see the classic desktop form factor as obsolete, with the future being things like laptops, tablets, or small form factor motherboards without PCIe expansion and everything fully integrated by the board designer. On those systems requirements that you can use ANY of the USB4 ports to connect your monitor don't seem as insane. With how fragmented USB already is, having to know that THIS USB4 port will support your external monitor, and none of the rest can, would be really pushing user's patience.
  - zarzavat 4y ago
    That sounds like 2010 thinking, when everybody thought that the iPad was going to take over the world and we would have computing appliances. Actually what happened is that esports became a huge deal, console gaming peaked and desktops are still here undefeated. Sure, gone are the days when everybody had a desktop, but what red blooded man/woman doesn’t desire a juiced out gaming PC with a 3090 and a truckload of RAM?

rektide 4y ago

Awesome details, super informative! Really interesting limitation you've mentioned about not being able to add in USB4. I hope this doesn't end up being true forever!! It's such an amazing set of capabilities; it'd suck to require them all to be built-in to the cpu/chipset. Couldn't a discrete video card (which has a DisplayPort output) implement USB4?

jsmith45 4y ago
Theoretically, yes a video card could implement USB4 (by having a USB 3.2 host controller, and a USB 2.0 host controller on board, implementing the routing fabric etc. But in so doing, you have limited the bandwidth available to the GPU (although they actually do still make framebuffer GPUs that don't have any calculation acceleration support). Also don't forget the need to support USB Power delivery (there is no requirement on being able to deliver power, but PD communication is required, and many USB4 devices will expect to be able to negotiate for some level of power delivery), so this card will probably need those extra power cables like normal accelerating GPUs use. But if you had such a card, and your motherboard has one two, then you have one machine that is actually two seperate USB4 hosts. This opens up a bunch of questions like does the host OS actually support this sort of configuration (especially if the Connection Manager is being implemented in software rather than hardware). Then you get weird stuff like being able to connect a Type C cable between the two hosts of the same system to get Host-To-Host Tunneling, but where both hosts are the same system. That is some wacky stuff. Even Host to Host tunneling is esoteric, basically allowing system that implements each host to pass each other arbitrary data over over the link. The example in the spec is exchanging IP packets, but I;m not sure that details of this usage are really standardized. It might also be possible to support add-on cards for USB4 by a routing chip manufacturer that makes use of the device to device communication features of of PCIe, to allow multiple USB4 routers to work together as a single virtual router. That sounds far more plausible, although doing that sort of thing would increase the contention on the PCIe bus.

Schroedingersat 4y ago

> USB4 is also really weird in that you cannot really add additional ports to the system on via add-on cards or similar, since there needs to be one router for the whole host that is connected to all the USB4 ports, in order to implement the correct routing system, and also implement the guarentees like Display Port being available on USB4 ports, etc. That sounds a hell of a lot like 'we're going to add some bullshit DRM in the next version'

jsmith45 4y ago
I think it really more reflects that the authors of the spec see the classic desktop form factor as obsolete, with the future being things like laptops, tablets, or small form factor motherboards without PCIe expansion and everything fully integrated by the board designer. On those systems requirements that you can use ANY of the USB4 ports to connect your monitor don't seem as insane. With how fragmented USB already is, having to know that THIS USB4 port will support your external monitor, and none of the rest can, would be really pushing user's patience.
- zarzavat 4y ago
  That sounds like 2010 thinking, when everybody thought that the iPad was going to take over the world and we would have computing appliances. Actually what happened is that esports became a huge deal, console gaming peaked and desktops are still here undefeated. Sure, gone are the days when everybody had a desktop, but what red blooded man/woman doesn’t desire a juiced out gaming PC with a 3090 and a truckload of RAM?