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

[Ulf Hansson]

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.

Kind regards
Uffe