On Fri, 11 Mar 2005, Pavel Machek wrote: > > This is why I'm in favor of power domains (power parents and children > > that can be independent of physical device relations). Power domains > > are common for internal devices, not just external devices. Still, if > > you disable a physical parent, then the device is no longer operational. > > Disable the usb host controller, and the external disk drive cannot be > > used (in fact data may be lost). So it's necessary to stop every child > > device first, and generally also power it off if we can control the > > power source. > > I just don't see it like that. > > Remember old scsi disks where formatting took half an hour? You may do > something like > > invoke scsi format operation > power down usb controller > <half an hour later> > power up usb controller > get the results > > Now, that was probably bad example, but I do think some devices might > do usefull work even with their parents disabled. There's another aspect to it as well. You can suspend the USB host controller and then _still_ use the external disk, if the host controller is automatically resumed when the next disk I/O request arrives. Whereas if you would stop the disk and its driver, the next I/O request might simply languish in a plugged queue. I suppose this would depend on whether the disk driver was smart enough to leave its queue unplugged under the right circumstances, so perhaps this isn't a great example. (And you're right about the possibility of data being lost -- although it's pretty unlikely -- which makes this an even worse example.) Alan Stern
