On Friday, September 03, 2010, Colin Cross wrote: > On Thu, Sep 2, 2010 at 4:09 PM, Rafael J. Wysocki <rjw@xxxxxxx> wrote: > > On Friday, September 03, 2010, Colin Cross wrote: > >> On Thu, Sep 2, 2010 at 2:34 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > >> > On Thu, 2 Sep 2010, Colin Cross wrote: > >> > > >> >> That would work, but I still don't see why it's better. With either > >> >> of your changes, the power.completion variable is storing state, and > >> >> not just used for notification. However, the exact meaning of that > >> >> state is unclear, especially during the transition from an aborted > >> >> suspend to resume, and the state is duplicating power.status. Setting > >> >> it to complete in dpm_prepare is especially confusing, because at that > >> >> point nothing is completed, it hasn't even been started. > >> > > >> > The state being waited for varies from time to time and is only > >> > partially related to power.status. Instead of using a completion I > >> > suppose we could have used a new "transition_complete" variable > >> > together with a waitqueue. Would you prefer that? It's effectively > >> > the same thing as a completion, but without the nice packaging already > >> > provided by the kernel. > >> No, that doesn't change anything. What I'd prefer to see is a > >> wait_for_condition on the desired state of the parent. As is, > >> power.completion means one thing during suspend (the device has > >> started, but not finished, suspending), and a different thing during > >> resume (the device has not finished resuming, and may not have started > >> resuming). That difference is exactly what caused the bug - the > >> completion has to be set on init so that it is set before the device > >> starts suspend. > > > > Not really. The bug is there, because my analysis of the suspend error code > > path was wrong. Sorry about that, but it has nothing to do with the "different > > meaning" of the completions during suspend and resume. > > > > The completions here are simply used to enforce a specific ordering of > > operations, nothing more. They have no meaning beyond that. > > The completion variable maintains state. So what? Locks also maintain state. > It has meaning whether or not you want it to. Leaving it as a completion > variable requires that you manage that state, which is difficult considering > there is no documentation and no clear idea in the code of exactly when that > state is set or clear. Please run "git show 5af84b82701a96be4b033aaa51d86c72e2ded061" and read the changelog. It's described in there quite clearly (I think). > It would be much cleaner to use a wait queue, and use > wait_on_condition to wait for the device to be in the desired state. Well, in fact that was used in one version of the patchset that introduced asynchronous suspend-resume, but it was rejected by Linus, because it was based on non-standard synchronization. The Linus' argument, that I agreed with, was that standard snychronization constructs, such as locks or completions, were guaranteed to work accross different architectures and thus were simply _safer_ to use than open-coded synchronization that you seem to be preferring. Completions simply allowed us to get the desired behavior with the least effort and that's why we used them. Thanks, Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
