On Fri, 25 Aug 2006, Woodruff, Richard wrote: > > ? Wouldn't suspending the entire bus completely stop the throughput > of > > any attached device? > > Not necessarily (right?). If I shut off VBUS then yes then I need to > re-enumerate for sure. One might figure out a cleaver way of shutting > it off and turning it back on in between host requests. No, you can't do that. Without VBUS power there's no reliable way to detect disconnects or media changes. > If I have a USB > drive attached it might well be the protocol layers are smart enough > keep using the device once it is restarted at the low layers. They just > have to wait longer for the data to arrive. > > If I just do USB bus suspend the device at the other end can signal > remote wake up to me. I tell him suspend and he agrees to drop to a low > power state, then I lower current capacity. When he has some data he > can let me know and I'll up the VBUS current capacity and then go talk > to him. If I have data I want from him I directly wake him up. Assuming the device has remote wake-up capability. And assuming the latency of a remote wakeup isn't too high. I tried doing some tests using a USB keyboard; when the device was suspended and I woke it up by typing on it, nearly every time the first few keystrokes were lost. > > But this doesn't require any over-reaching global coordination. All > it > > needs is for each driver to know when it's not being used. > > Applying an activity time of sorts to each driver 'might' end up in > situations where you get good power savings. Assuming everything lines > up. However, given hardware and software bugs and errata, it seems > forcing the situation is much more likely to succeed and make sure you > hit your targets. Activity timers might be appropriate for some devices but not for others. For instance, a USB mass-storage device will always have a lower-level driver below the USB driver (for instance, a disk or CD driver that uses the USB driver for its transport). Suspending the USB driver can't be done unless the lower-level driver is suspended first, because it might have unexpected side effects such as spinning down a drive. My point being that an inactivity timer might be appropriate at the level of the disk or CD driver, but not at the level of a USB mass-storage driver. > Even with a per-driver activity timer how does one set the time out > levels for the whole system? You need some kind of policy pieces to set > all the knobs. Letting the self adjust won't likely work for QOS > (quality of service type things) unless they are set very > conservatively. I would imagine it depends very heavily on the type of system you're talking about. On desktops and laptops, for example, Windows seems to get along okay with a small handful of system-level inactivity timers. Alan Stern
