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SubjectRe: [PATCH v12 7/8] usb: Adding SuperSpeed support to dummy_hcd
On Tue, 24 May 2011, Felipe Balbi wrote:

> > It would be more accurate to say the module parameter will be used to
> > force the connection to run at a lower speed than the maximum possible.
> > This is kind of like what happens when you plug in a SuperSpeed device
> > using a USB-2 cable -- the connection runs at a lower speed than it
> > could have.
>
> if it's something like that, for sure we can have the module parameter. But
> plugging a USB2 gadget/function to a USB3-capable UDC/HCD should
> work fine.
>
> With Tatyana's patches, if we load a USB2 g_zero to dummy_hcd, enumeration
> will fail where it shouldn't. This has been my whole point ;-) Maybe I wasn't
> clear enough.

I guess not. I thought Tatyana said she was working to fix that bug...

> >> What about the case where SuperSpeed enumeration
> >> fails and you have to fall back to high speed?
> >
> > If SuperSpeed enumeration fails, say because the device doesn't have
> > any SuperSpeed descriptors, xhci-hcd doesn't fall back to high speed,
> > does it? dummy-hcd should behave the same way.
>
> it should at least. Isn't that what happens between EHCI/OHCI ? HS Chirp
> sequencing fails, then we fall back to FullSpeed.

That's a failure in initialization, not a failure in enumeration.

There are two reasons why the HS chirp might fail: the device doesn't
support high speed operation or hardware errors prevent the chirp from
working. With dummy-hcd there are no hardware errors (because there's
no hardware).

As for whether or not the device supports high-speed or SuperSpeed
operation, that's determined by the usb_gadget_driver->speed field. If
the field doesn't specify SuperSpeed then dummy-hcd should connect the
gadget to the USB-2 root hub rather than the USB-3 root hub. Isn't
that what Tatyana's patch does? It contains a line saying:

dum->gadget.speed = driver->speed;

> >> It seems like you really
> >> need to handle both speeds and the speed fall back parameter in the same
> >> driver. Isn't there some other gadget driver that has a fall back to
> >> full or low speed when high speed enumeration fails?
> >
> > That's a property of the gadget driver, not the UDC driver. dummy-hcd
> > is a UDC driver (and an HCD too).
>
> USB3.0 dummy_hcd should still enumerate USB2.0 gadget drivers.

Yes, certainly it should. If it doesn't, that's a bug, not a design
error.

Alan Stern



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