On Wed, 26 Sep 2012, Sarah Sharp wrote: > On Wed, Sep 26, 2012 at 05:31:43PM +0200, Oliver Neukum wrote: > > Hi Sarah, > > > > watching your talk and drinking tea I had an idea. > > > > Switching off ports saves power in itself. It also allows > > switching off host controllers. > > The problem with switching off power on external ports is that they > > don't detect hotplug events anymore. > > > > So is there a way to switch off controllers and leave ports on, saving > > at least some power? > > That sounds awfully like host/bus suspend. :) Or if you're talking > about completely powering off the host, a D3 cold mechanism for the PCI > host controller. My thought exactly. > > And it seems to me that in many cases, namely > > EHCI with companion controllers and XHCI controllers that implement > > a vendor specific switching option, we could switch the ports so that > > a minimum number of controllers is connected to ports that are not switched > > off. > > It would be nice to have only one powered host controller, I agree. But > I'm not sure the idea would actually help systems with the EHCI/xHCI > port switching. > > We would want to leave any powered port under the xHCI host controller, > because we want USB 3.0 devices to connect at SuperSpeed. If we can't > switch over the EHCI ports to xHCI, it's either because there isn't a > physical switch, or the OEM is preventing the port switchover because it > has some internal USB device that doesn't work well under xHCI. So > we're forced into having two powered host controllers, and there's no > other way we can switch ports to help. If all the ports are set to xHCI and the EHCI controller doesn't have any others then it could be powered off, assuming there's some way to do that. But this is starting to sound a little peculiar. Intel supports the port-power stuff only on its most recent (i.e., not yet released) systems. Do these same bleeding-edge systems also include the old-style switch-xHCI-ports-back-to-EHCI mechanism? > I think you run into similar issues with EHCI hosts with companion > controllers. You want all the powered ports to be under EHCI by > default. If you have a port under UHCI or OHCI, then it's because you > have a connected LS/FS device. So you're stuck with an two powered host > controllers. Oliver was pointing out that if none of the attached devices are FS/LS then the companion controller and all its ports could be powered off. The EHCI driver knows when a new FS/LS device gets attached, so it could arrange for the companion controller to power back on at that time. 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
