On Thu, 24 Jan 2013, Sarah Sharp wrote:
> On Thu, Jan 24, 2013 at 10:51:04AM -0500, Alan Stern wrote:
> > On Wed, 23 Jan 2013, Sarah Sharp wrote:
> >
> > > > So I guess my next question is: What are you plans, if any, to add the
> > > > "new" enumeration scheme to xHCI? Is this a huge amount of work?
> > >
> > > Not a huge amount of work, but it does require an API change to the USB
> > > host controller driver functions, so the patch itself is not likely to
> > > get backported to stable kernels. And I have no idea how the current
> > > batch of USB 3.0 devices will react to an enumeration sequence change.
> >
> > I thought this wasn't possible at all, because the xHCI hardware
> > automatically assigns an address to the device when a reset completes.
>
> No, the xHCI hardware doesn't assign an address on reset completion.
> The xHCI host assigns an address and sends the Set Address control
> transfer when the Set Address command issued (when hcd->address_device
> is called in hub_set_address).
>
> The Set Address command does two things: it tells the xHC that the
> default control endpoint ring and associated device context information
> is set up, and it tells the xHC to issue the Set Address control
> transfer.
>
> There's a flag you can pass to the Set Address command to make it not
> actually send the control transfer. It's called 'Block Set Address
> Request' (BSR). After you issue the Set Address command with the BSR
> flag set, you can get the device descriptor through the default control
> endpoint. Then you send the Set Address command again, with BSR
> cleared, when you want to actually set the device address.
>
> I asked the xHCI spec author to put the flag in, because I saw the two
> enumeration schemes. I just haven't had time or motivation to implement
> it.
I see. Thanks for the explanation.
> > "Old" scheme:
> >
> > Reset
> > Get device descriptor (8 bytes)
> > Set address
> > Get device descriptor (18 bytes)
>
> Does the core really get the 8 byte device descriptor before set
> address? I'm pretty sure that would be skipped in the if block when we
> try the old scheme:
>
> for (i = 0; i < GET_DESCRIPTOR_TRIES; (++i, msleep(100))) {
> ...
> if (udev->wusb == 0) {
> for (j = 0; j < SET_ADDRESS_TRIES; ++j) {
> retval = hub_set_address(udev, devnum);
> if (retval >= 0)
> break;
> msleep(200);
> }
> ...
> }
>
> I thought the old scheme was:
>
> Reset
> Set address
> Get device descriptor (8 bytes)
> Get device descriptor (18 bytes)
You're right. I haven't looked at this stuff in years; my memory
cells are decaying...
> I think it's still possible to do the new enumeration scheme with the
> flag to the Set Address command. The new scheme would look like:
>
> "New" scheme:
> Reset
> Set Address command (BSR flag set)
> Get device descriptor (64 bytes)
> Reset
> Reset Device command
> Set address
> Set Address command (BSR flag clear)
> Get device descriptor (18 bytes)
>
> Does that make sense?
Yes.
Alan Stern
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