The OTG profile could not see USB devices in host mode. On a board with a
UVC webcam on the USB tail, `lsusb` showed only the two root hubs, while the
same board running a plain lite image enumerated the webcam normally.
The controller was in the host role and the host controller was running --
mode file "host", xhci-hcd bound, USBCMD 0x5, PORTSC1 port-power set -- and
the port still never reported a connect (PORTSC1 0x00000280, CCS clear).
dr_mode = "otg" makes dwc3_core_init() program GCTL.PRTCAPDIR = OTG(3), and
dwc3_core_init_mode() registers xhci-hcd on top of that. This 4.9 dwc3 has no
working OTG role-detection block, so xHCI probes, resets the controller and
brings up its root hubs while the core is in a role it cannot drive. The port
is then dead for the life of that probe.
S72usbmode's `usb-mode host` did not repair it and could not: writing the
debugfs mode file reaches dwc3_set_mode(), which flips GCTL.PRTCAPDIR and
nothing else, leaving the already-probed xHCI with the port state machine it
built in OTG. It also skipped the bind, because xhci-hcd was already bound.
Fix it where the ordering is: patch dwc3_core_init() to program
PRTCAPDIR = HOST for the OTG case, so xHCI probes into a role the core can
drive. dwc3_gadget_init() only registers a UDC and does not care, and
`usb-mode device` re-aims the controller itself when it takes the port.
usb-mode's host path also learns to re-probe an xHCI that was bound in some
other role, which is what makes a newer rootfs recover on an older kernel.
