On Tue, 19 Apr 2016, Felipe Balbi wrote: > Hi, > > Peter Chen <hzpeterchen@xxxxxxxxx> writes: > > Hi all, > > > > When I review the patch [1] for adding companion controller support for > > OTG framework, I am puzzled several concepts, like shared hcd and > > companion controller, companion hcd, companion port, would someone > > please explain them? And why EHCI/OHCI do not use shared hcd, but > > xHCI uses it? Thanks. > > xHCI is modeled as two separate buses (High-speed and Super-speed) which > *share* the same IP block. In the case of EHCI/OHCI, there is a port > being handed over to the a completely separate IP. OHCI is EHCI's > companion for non-HS signalling. That's right. In more detail: Companion controller and companion hcd are the same thing. They exist because EHCI can only handle high speed connections; it can't handle full speed or low speed. When a USB-1.1 device is plugged into an EHCI controller, the EHCI controller can't handle it and so the connection is handed over to the companion controller, which is either OHCI or UHCI. xHCI doesn't have that problem; it can handle all speeds. However the hardware design (not just the software design!) uses two separate buses: one for SuperSpeed and one for high/full/low speed. There actually are two separate sets of wires in a USB-3 cable. If the USB core were designed better, we would have used a single usb_hcd structure for xHCI, with two usb_bus structures inside it. But the structures were designed long before USB-3 was announced, and it would have taken a lot of changes to make that approach work. Instead, xhci-hcd uses two usb_hcd structures, each containing a single usb_bus. The structure for the high/full/low-speed bus is called the primary_hcd and the structure for the SuperSpeed bus is called the shared_hcd. As a result of xHCI's design, the same physical port on the computer will show up as two different ports in the kernel: one attached to the SuperSpeed hcd and one attached to the high/full/low-speed hcd. The peer pointer (not companion!) in struct usb_port connects these two logical ports. 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
