[RFC][PATCH 0/2] Allow subsystems to avoid using sysdevs for defining "core" PM callbacks

["Rafael J. Wysocki" <rjw@sisk.pl>]

Hi,

There are multiple problems with sysdevs, or struct sys_device objects to
be precise, that are so annoying that some people have started to think
of removind them entirely from the kernel.  To me, personally, the most
obvious issue is the way sysdevs are used for defining suspend/resume
callbacks to be executed with one CPU on-line and interrupts disabled.
Greg and Kay may tell you more about the other problems with sysdevs. :-)

Some subsystems need to carry out certain operations during suspend after
we've disabled non-boot CPUs and interrupts have been switched off on the
only on-line one.  Currently, the only way to achieve that is to define
sysdev suspend/resume callbacks, but this is cumbersome and inefficient.
Namely, to do that, one has to define a sysdev class providing the callbacks
and a sysdev actually using them, which is excessively complicated.  Moreover,
the sysdev suspend/resume callbacks take arguments that are not really used
by the majority of subsystems defining sysdev suspend/resume callbacks
(or even if they are used, they don't really _need_ to be used, so they
are simply unnecessary).  Of course, if a sysdev is only defined to provide
suspend/resume (and maybe shutdown) callbacks, there's no real reason why
it should show up in sysfs.

For this reason, I thought it would be a good idea to provide a simpler
interface for subsystems to define "very late" suspend callbacks and
"very early" resume callbacks (and "very late" shutdown callbacks as well)
without the entire bloat related to sysdevs.  The interface is introduced
by the first of the following patches, while the second patch converts some
sysdev users related to the x86 architecture to using the new interface.

I believe that call sysdev users who need to define suspend/resume/shutdown
callbacks may be converted to using the interface provided by the first patch,
which in turn should allow us to convert the remaining sysdev functionality
into "normal" struct device interfaces.  Still, even if that turns out to be
too complicated, the bloat reduction resulting from the second patch kind of
shows that moving at least some sysdev users to a simpler interface (like in
the first patch) is a good idea anyway.

This is a proof of concept, so the patches have not been tested.  Please be
extrememly careful, because they touch sensitive code, so to speak.  In the
majority of cases the changes are rather straightforward, but there are some
more interesting cases as well (io_apic.c most importantly).

Please have a look and tell me what you think.

Thanks,
Rafael