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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
