On Sunday 22 November 2009, Alan Stern wrote: > Rafael: > > Here's the situation. Device A has children B1, ..., Bn (possibly > others too). I need to guarantee that whenever A is active, so are the > children. But it's perfectly valid to have an inactive device under an active parent, so I guess this is not a general case. > One direction seems easy enough. I can make the idle handler for the > B's first check that A and all the B's are ready to be suspended, then > suspend all the B's, and then suspend A. This works because the PM and > driver cores never directly suspend a device; all the do is notify the > idle handler. > > The other direction is a little more complicated. Resumes initiated by > the bus code aren't troublesome; they can always be written so as to > resume the B's along with the A's. The real problem arises when A is > resumed directly by the PM or driver cores. For example, consider what > would happen when one of B1's children sends a remote wakeup signal. > The PM core would automatically resume B1 and A, but there would be no > chance for the bus code to resume the other B's. pm_runtime_resume() schedules an idle request for 'dev' at the end, which theoretically may resume the B's that are still suspended. There still would be a delay, though. > I can't make the runtime_resume method for A call pm_runtime_resume() > for all the B's. It would deadlock inside __pm_runtime_resume() where > the code to resume B1 waits for A's resume to complete. > > Calling pm_request_resume() for all the B's instead isn't a very good > solution. For one thing, it's silly to add latency by using a work > queue instead of just doing the resumes directly. There are also > problems of synchronization, such as the fact that other code expects > all the B's to be resumed when A's resume completes. > > Do you have any ideas on how to approach this? How about allowing A's > runtime_resume method to set A->power.runtime_status to RPM_ACTIVE, > before it tries to resume the B's? That would avoid the deadlock. I don't see a problem with that as long as the A's runtime_resume returns 0 in such a case. Thanks, Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
