Jaimie Vandenbergh <
jai...@usually.sessile.org> wrote:
> Yup, 40Gbps USB4 does the same.
>
> Thanks for the heads up, I wasn't aware of the M1 lanes issue. Can't
> find anything useful about it, do you have any ref?
I'm not saying there *is* a lanes issue, I'm just guessing.
Basically, USB-C provides four differential pairs, normally two in each
direction. Each pair can do 5, 10 or 20Gbps, resulting in up to 40Gbps in
each direction. There is additionally a 480Mbps USB 2 channel on a separate
pair.
There are two ways this is handled. The USB 3.x way is the pairs are
essentially circuit switched. You plug it a docking station and it says
'I'd like some Displayport, some USB superspeed please, and by the way I can
offer charging'. There are four pairs, so you might get two lanes of
Displayport (both going out) and one lane each way of USB 3.x.
The Thunderbolt way is that Thunderbolt is a packet switching network. So
you have the four lanes running the Thunderbolt protocol, and on top of that
you open a tunnel a bit like a VPN for Displayport and a tunnel for USB.
They share the available bandwidth. For example, if you change to a lower
resolution video mode there's more bandwidth for USB. To do this you need
an endpoint that supports Thunderbolt to essentially terminate those VPN
tunnels.
USB 4 brought Thunderbolt into the USB standard. That means a USB 4 device
can implement the packet switching way instead of the circuit switching way.
Thunderbolt 4 is a minor bump of Thunderbolt to align with USB 4, basically
making a bunch of optional things mandatory.
There is talk that the M1 only supported Thunderbolt 3 not Thunderbolt 4 (as
in it failed to tick the TBT4 must-have boxes) because it only supported one
external display - I'm not clear if that's a port limitation or because of a
lack of onboard Displayport controller for a third panel (internal, external
1, external 2). I'm not casting aspersions about M1, just unclear on what
it does and doesn't support (USB-C stuff is always super murky...)
Unless you have a Thunderbolt (or USB 4 running Thunderbolt protocol)
device, it's likely it does the circuit switching method which has the
bandwidth limitation. Which is not an M1 fault per se, it's just that
you're using it in a different, more limited, mode.
Theo