Re: [RFC][PATCH -mm 1/3] PM: Hibernation and suspend notifiers

Rafael J. Wysocki
2007-05-30T21:11:39Z

On Wednesday, 30 May 2007 22:44, Rafael J. Wysocki wrote:
> Hi,
>
> On Wednesday, 30 May 2007 17:37, Pavel Machek wrote:
> > Hi!
> >
> > > +Suspend notifiers
> > > + (C) 2007 Rafael J. Wysocki <rjw@sisk.pl>, GPL
> > > +
> > > +There are some operations that device drivers may want to carry out in their
> > > +.suspend() routines, but shouldn't, because they can cause the hibernation or
> > > +suspend to fail. For example, a driver may want to allocate a substantial amount
> > > +of memory (like 50 MB) in .suspend(), but that shouldn't be done after the
> > > +swsusp's memory shrinker has run.
> > > +
> > > +Also, there may be some operations, that subsystems want to carry out before a
> > > +hibernation/suspend or after a restore/resume, requiring the system to be fully
> > > +functional, so the drivers' .suspend() and .resume() routines are not suitable
> > > +for this purpose. For example, device drivers may want to upload firmware to
> > > +their devices after a restore from a hibernation image, but they cannot do it by
> > > +calling request_firmware() from their .resume() routines (user land processes
> > > +are frozen at this point). The solution may be to load the firmware into
> > > +memory before processes are frozen and upload it from there in the .resume()
> > > +routine. Of course, a hibernation notifier may be used for this purpose.
> > > +
> > > +The subsystems that have such needs can register suspend notifiers that will be
> > > +called upon the following events by the suspend core:
> > > +
> > > +PM_PRE_FREEZE The system is going to hibernate or suspend, tasks will
> > > + be frozen immediately
> >
> > Hmm, looks like bad idea if we are going to remove freezer from
> > suspend...?
>
> We need PM_PRE_FREEZE anyway and it's a different question whether or not
> it'll be used for suspend (STR) too.
>
> The timing is not the best one, but so far the freezer is in the suspend code
> path and I need to take this into account.
>
> > > +PM_POST_THAW Tasks have just been thawed after a resume or restore
> > > + from a hibernation image
> > > +
> > > +PM_HIBERNATION_PREPARE The system is preparing for hibernation. Tasks have
> > > + been frozen, memory is going to be freed and devices
> > > + are going to be suspended.
> >
> > Is not PRE_FREEZE enough? We can allocate memory for drivers there,
> > too...
>
> Well, there is a reason for not doing this. Namely, if the memory if freed on
> PM_POST_HIBERNATION after the image has been created, we can use it for saving
> the image (and speed up the saving).
>
> Besides, if the freezer is dropped from the suspend code, the notifiers will be
> useful to it anyway IMO, and PRE_FREEZE won't make sense in that case.
>
> I think the rule should be: If you need to do something _before_ tasks are
> frozen, do it in PM_PRE_FREEZE, but if you can do that after the tasks have
> been frozen, do it on PM_HIBERNATION_PREPARE (or PM_SUSPEND_PREPARE in the
> suspend case).

OTOH, having considered it for a while, I think that for now I can add just
PM_PRE_FREEZE and PM_POST_THAW, as I don't have any user for the other ones.
The other events may be added in the future if need be (along with some users).

I'll post revised patches in a new thread.

Greetings,
Rafael