On Tue, Apr 19, 2016 at 10:20:33AM -0400, Alan Stern wrote: > 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. > Thanks, Alan, Felipe, Oliver. After reading the beginning content of EHCI spec, I understand what is companion controller. At chipidea USB2 design, it handles FS/LS by internal, so I have no knowledge about it. Integrated Transaction Translator (Multi Port implementations) supports directly connected USB legacy (USB 1.1) Full and Low speed devices without a companion USB 1.1 host controller or host controller driver software. Embedded Transaction Translator - Allows direct attachment of FS and LS devices in host mode without the need for a companion controller. -- 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
