On Mon, 20 Feb 2006, Patrick Mochel wrote: > > > > Does swsusp still support these not-completely-powered-off modes? > > > > > > Yes. echo platform > /sys/power/disk. > > > > So if you did that, you could suspend-to-disk while leaving USB up, right? > > Without anything being wrong with the machine. > > No, not likely. S4 is +/- equivalent to a "soft off", except that it > responds to more wake-up events than a soft off will (which BTW is defined > as "S5"). Does it respond to USB wakeup events (like plugging in a new device)? > Even S3 will power down all of the PCI buses, including the USB host > controllers. The chances of having any low-power system state that will > still power the USB ports (by design) is pretty slim. Here is a counterexample from Kyle Moffet (<http://marc.theaimsgroup.com/?l=linux-kernel&m=113980955219914&w=2>): Let me bring up the example of my PowerBook again. It's RAM is fully powered right now, running from battery, and it has another couple days of sleep- charge left before I have to worry about plugging it in again. When I open it, the firmware automatically powers up the CPU and other hardware and returns control to the OS. I can _also_ trigger it to wake by leaving it closed and connecting an external VGA and USB (it wakes every time I connect a USB, but my suspend script puts it to sleep again if it's closed and has no external VGA). There's a difference between powering-down and turning off completely. The USB suspend-power-budget per attached port is 500 uA at 5 V, which amounts to 2.5 mW. While that's more than you need to keep RAM alive, it's not a tremendous amount. And most devices probably use less power when suspended than this maximum. The EHCI specification includes a lot of very careful language about which portions of the controller should be attached to which power well. It's quite clear that most of the controller's circuitry can be turned off while still supplying enough suspend power to keep the bus alive and enable wakeup events. And if the computer can wake up in response to USB events, then obviously the USB controller _is_ receiving some power. Alan Stern
