[PATCH] driver core / PM: Add callbacks for PM domain initialization/cleanup

[Rafael J. Wysocki]

On Thursday, March 19, 2015 03:45:07 PM Ulf Hansson wrote:
> On 19 March 2015 at 15:20, Alan Stern <stern at rowland.harvard.edu> wrote:
> > On Thu, 19 Mar 2015, Rafael J. Wysocki wrote:
> >
> >> > This doesn't make sense to me.  A bus just called bus_add_device, it can
> >> > do whatever it wanted to right before calling this function, no need for
> >> > another callback.
> >>
> >> The only caller of bus_add_device() is device_add().
> >>
> >> What do you mean by "bus just called bus_add_device"?  Do you think that
> >> the pm_domain pointer should be populated before calling device_add()?
> >>
> >> That wouldn't work for the ACPI PM domain at least, because ACPI companions
> >> are generally added during device_add() and we arguably cannot point a
> >> device to the ACPI PM domain before its ACPI companion is set.
> >
> >
> >> There are two alternatives to them.  One is to do PM domain attach/detach in
> >> the bus type's ->probe and ->remove, but that's suboptimal, because it is
> >> then carried out every time a driver is probed/removed for a device.  The
> >> other one would be to have each interested bus type register a bus type
> >> notifier for itself, but that would be rather ugly, wouldn't it?
> >
> > Driver probing and removal is not a hot path.  Successful probes
> > usually occur only once for each device.  I don't know how many
> > unsuccessful probes there are on average, but probably not many.
> >
> > Doing the PM domain attach/detach in ->probe and ->remove makes sense
> > to me.
> 
> Let me elaborate a bit on the current approach, with PM domain
> attach/detach in ->probe() and ->remove(). This comes with the
> following limitations.
> 
> 1) If the device gets probed earlier than the PM domain has been
> initialized, we can't return -EPROBE_DEFER since we just don't know
> anything about the PM domain yet.
> 
> 2) The PM domain won't be able to operate on a device without having a
> driver bound to it, since the device's PM domain pointer is removed at
> ->remove().
> 
> Perhaps Rafael want to add something to these? I don't know whether we
> think it's okay to keep these limitations or not.

There are arguments both ways as you can see.

At this point, the most straightforward approach would be to apply the Russell's
patch adding the ->sync callback alone.

I will then need the ->activate callback for another use case (a PM domain that
may not be ready for the device to be probed), but in that case I actually know
that the pm_domain pointer is populated at the really_probe() time.

Going forward we may want to move the attach/detach operations for genpd and
the ACPI PM domain out of bus type ->probe, but I think that can wait.


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.