On Thursday 22 March 2007 7:45 am, Alan Stern wrote: > On Thu, 22 Mar 2007, Scott E. Preece wrote: > > > I would normally call designs that expect important functions (like > > power on/power off) to happen as side effects of other operations (like > > opening and closing files) broken to begin with. It's still a bad idea > > to hide policy inside the driver. > > Even though other people have already answered this, I'd like to add my > own comments. > > Firstly, doing power on/power off as side effects of other operations is > _not_ a policy choice. It is a design principle: Yes ... and one that's widely used in other contexts: deallocate resources when they're not in active use. > When device D has been idle for more than N ms, it should be > put in a low-power state (unless such state changes have been > disabled for D by userspace). THAT is however something I would call a heuristic. It's a widely used one -- disk spindown uses one value for N, displays enter their lowpower states using other values for N, etc -- but it's still not as definitive as for example "if the device isn't opened, it can't possibly be in use". Not that I see a way around having such a heuristic for things like mice, or anything wrong with that one ... I just want to call a spade a spade here, and not confuse (a) the design principle, with (b) a heuristic that's used to implement that principle in certain cases. > Of course N will vary for different D's, and the exact choice of N _is_ > policy. Thus N should be exposed and configurable by userspace. So > should the ability to disable the state changes. But the principle > above isn't a policy, it is part of the design. > > Secondly, this principle _requires_ that power on/power off occur as side > effects of other operations, since those other operations affect whether > or not the device is idle. > > If anybody wants to argue against the principle itself, then go ahead and > say so. I, for one, don't see anything objectionable about it. Me either -- for either principle or, in general, that heuristic. Of course, choosing to apply that heuristic to a given device is a different kettle of fish. That's why it's got an "off" switch. :) - Dave _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
