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 a
problem. It partially stems from the fact that __async_schedule() executes
ptr() synchronously in some circumstances (e.g. async_enabled unset), so the
suspend and resume callbacks have to be scheduled in the same order, in which
they 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 between
the children the core doesn't know about. Subsystems are free to set
async_suspend for the entire branches of device hierarchy if they are known
not to contain any off-tree dependencies, but the core has no information
about 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 as
well, among other things.
Thanks,
Rafael
--
To unsubscribe from this list: send the line "unsubscribe linux-acpi" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
