Dave already answered some of these questions. Here are my views:
On Sun, 1 Jul 2007, jidong xiao wrote:
> about device descriptor,according to usb spec 2.0,
> 1. A hub returns different descriptors based on whether it is
> operating at high speed or full/low speed.A hub can report three
> different sets of the descriptors.
> 2. A hub must operate at high-speed when its upstream facing port is
> connected at high-speed.The hub must operate at full speed when its
> upstream facing port is connected at full-speed.
>
> Take root hub for example, how does the hub itself know whether its
> upstream facing port is connected at high-speed or
> full-speed?
Like Dave said, this notion makes no sense for root hubs since they
don't have upstream ports.
> Futhermore, if a host send a GET_DESCRIPTOR request to a
> hub,how does it know which set of descriptor should be reported?
In Linux, the root hub returns a descriptor suitable for high speed if
the controller supports high-speed operation (i.e., if it is an EHCI
controller). Otherwise it returns a descriptor suitable for full
speed.
> Let's see following section of code,sampled from hub_configure(),we
> need to judge bDeviceProtocol of the device descriptor, it can be
> 0,1,or 2, but it's the value reported by hub,how does the hub know
> which one should be reported when it received GET_DESCRIPTOR from the
> host.
>
> 570 switch (hdev->descriptor.bDeviceProtocol) {
> 571 case 0:
> 572 break;
> 573 case 1:
> 574 dev_dbg(hub_dev, "Single TT\n");
> 575 hub->tt.hub = hdev;
> 576 break;
> 577 case 2:
> 578 ret = usb_set_interface(hdev, 0, 1);
> 579 if (ret == 0) {
> 580 dev_dbg(hub_dev, "TT per port\n");
> 581 hub->tt.multi = 1;
> 582 } else
> 583 dev_err(hub_dev, "Using single
> TT (err %d)\n",
> 584 ret);
> ...
If the root hub doesn't operate at high speed then it returns 0. If
the root hub supports high speed then it returns 1. See
rh_call_control() in drivers/usb/core/hcd.c.
As far as I know, there are no root hubs with more than one TT.
Alan Stern
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