Peter Chen <hzpeterchen@xxxxxxxxx> writes: > 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. TT is only required for hubs, not for roothubs. Some roothubs will implement it to save silicon area, but regular EHCIs don't ;-) -- balbi
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