On Thursday 13 August 2009, Zhang Rui wrote:> On Thu, 2009-08-13 at 05:43 +0800, Rafael J. Wysocki wrote:> > On Wednesday 12 August 2009, Alan Stern wrote:> > > On Wed, 12 Aug 2009, Rafael J. Wysocki wrote:> > > > > > > Hi,> > > > > > > > The following patches introduce a mechanism allowing us to execute device> > > > drivers' suspend and resume callbacks asynchronously during system sleep> > > > transitions, such as suspend to RAM. The idea is explained in the [1/1] patch> > > > message.> > > > > > > > Comments welcome.> > > > > > I get the idea. Not bad.> > > > Thanks!> > > > > Have you tried it in a serious way? For example, turning on the> > > async_suspend flag for every device?> > > > No, I've only tested it with a few selected drivers. I'm going to try the> > "async everyone" scenario, though.> > > > > In one way it isn't as efficient as it could be. You fire off a bunch> > > of async threads and then make many of them wait for parent or child> > > devices. They could be doing useful work instead.> > > are you talking about this scenario, or I find another problem of this> approach:> there is a part of dpm_list, dev1->dev_aaa->...->dev_bbb->dev2> > dev2 is dev1's first child.> dev1 resume takes 1s> dev_aaa~dev_bbb resume takes 0.1s.> > if we call device_enable_async_suspend(dev1, true) in order to resume> device1 asynchronously, the real asynchronous resume only happens> between dev1 and dev_aaa to dev_bbb because dev2 needs to wait until> dev1 resume finished.
Yes, that's how it works, but I would call it a limitation rather than aproblem. It partially stems from the fact that __async_schedule() executesptr() synchronously in some circumstances (e.g. async_enabled unset), so thesuspend and resume callbacks have to be scheduled in the same order, in whichthey would have been called synchronously.
> so kernel schedules dev1 resume in an async thread first, and then takes> 0.1s to finish the dev_aaa to dev_bbb resume, and then sleep 0.9s> > > I kind of agree, but then the patches would be more complicated.> > > The problem is that we need to invoke device_resume for every device> synchronously.
Yes, we do.
> I wonder if we can make the child devices inherit the> parent's dev->power.async_suspend flag, so that devices that need to> wait are resumed asynchronously, i.e. we never wait/sleep when parsing> the dpm_list.> > this doesn't bring too much benefit in suspend case but it can speed up> the resume process a lot.
We can do that at the core level, because there may be dependencies betweenthe children the core doesn't know about. Subsystems are free to setasync_suspend for the entire branches of device hierarchy if they are knownnot to contain any off-tree dependencies, but the core has no informationabout that.
> Of cause, this is not a problem if we turn on the async_suspend flag for> every device.
Yes, but we cannot do that at this point.
> > > It would be interesting to invent a way of representing explicitly the > > > non-tree dependencies -- assuming there aren't too many of them! (I > > > can just hear the TI guys hollering about power and timer domains...)> > > > I have an idea.> > > > Every such dependency involves two devices, one of which is a "master"> > and the second of which is a "slave", meaning that the "slave" have to be> > suspended before the "master" and cannot be resumed before it. In principle> > we could give each device two lists of "dependency objects", one containing> > "dependency objects" where the device is the "master" and the other containing> > "dependency objects" where the device is the "slave". Then, each "dependency> > object" could be represented as> > > > struct pm_connection {> > struct device *master;> > struct list_head master_hook;> > struct device *slave;> > struct list_head slave_hook;> > };> > > > Add some locking, helpers for adding / removing "dependency objects" etc.> > and it should work. Instead of checking the parent, walk the list of> > "masters", instead of walking the list of children, walk the list of "slaves".> > > > The core could create those objects for parent-child relationships> > automatically, the other ones would have to be added by platforms / bus types /> > drivers etc.> > > this sounds great. :)
It can only be the next step, though, because it will affect the runtime PM aswell, among other things.
Thanks,Rafael_______________________________________________linux-pm mailing listlinux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx://lists.linux-foundation.org/mailman/listinfo/linux-pm
