On Sat, Jan 16, 2016 at 11:40:32AM -0500, Alan Stern wrote: > On Sat, 16 Jan 2016, Arnd Bergmann wrote: > > > > This leaves the question of whether OF will always use the same node to > > > represent the host controller and the root hub. In other words, if a > > > motherboard has a fixed device plugged into a fixed root-hub port, will > > > the DT description make that device a child of the host controller? > > > Or will there be a node in between (to represent the root hub)? > > > > Good question. I'm sure the answer is somewhere in this document > > > > http://www.firmware.org/1275/bindings/usb/usb-1_0.ps > > > > but unfortunately I don't understand USB addressing well enough > > to answer this. I think the key sentence is the definition of the > > "reg" property: > > > > | prop-encoded-array: one integer, encoded as with encode-int. > > | The "reg" property for a device node shall consist of the number of the > > | USB hub port or the USB host controller port to which this USB device > > | is attached. As specified in [2] section 11.11.2.1, port numbers range > > | from 1 to 255. > > > > Does the root hub have a port number relative to the host controller, > > or is it identical to the host controller from the addressing > > perspective? > > The root hub does not have a port number relative to the host > controller. In a sense, the root hub is _part_ of the host controller; > it represents the controller's connection to the USB bus (whereas the > logical device we commonly associate with the host controller > represents the controller's connection to the upstream PCI/whatever > bus). > > You can also ask whether port numbers relative to the root hub are > the same as the port numbers relative to the controller. Under normal > circumstances they are the same. However, it is possible that they > will be different for USB-3 host controllers (!). > > This is because USB-3 uses two buses per controller: the legacy > Low-Speed/Full-Speed/High-Speed bus compatible with USB-1.1 and USB-2 > devices, and the new SuperSpeed bus compatible with USB-3 devices. > Even though there's only the one controller, each bus has its own root > hub and the ports for each root hub are numbered starting from 1. > Thus, for example, an xHCI controller with 6 LS/FS/HS ports and 3 SS > ports would have a USB-2 root hub with ports 1-6 and a USB-3 root hub > with ports 1-3. However, xHCI treats the controller as a single entity > and numbers all the ports sequentially; thus the controller would have > ports 1-9. (Even though the three SS ports would be the same physical > objects as three of the LS/FS/HS ports -- USB-3 ports literally contain > two independent sets of signal wires.) Sorry, I have not USB-3 platform now. I have several questions: - For USB-3, how many times hub_configure() will be called for root hub? One or two? - Which the value of hub->descriptor->bNbrPorts for root hub? > > This isn't merely a convention in the specifications; the hardware > actually uses both numbering schemes (although I don't know about USB-3 > controllers other than xHCI; they might be different). Untangling this > mess might be difficult. I don't know how existing DT files handle it. > -- Best Regards, Peter Chen -- 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
