Date:	Thu, 10 Mar 2011 20:04:25 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Kay Sievers <kay.sievers@...e.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, Len Brown <lenb@...nel.org>,
	Greg KH <gregkh@...e.de>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, mingo@...hat.com,
	tglx@...utronix.de
Subject: Re: [RFC][PATCH 0/2] Allow subsystems to avoid using sysdevs for defining "core" PM callbacks

On Thursday, March 10, 2011, Kay Sievers wrote:
> On Thu, 2011-03-10 at 01:31 +0100, Rafael J. Wysocki wrote:
> > 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.
> 
> Do I read that right? We get rid of the entire dance of creating
> sysdevs/sysdev_classes and the pointless and broken stuff in /sys?

That's the plan at least.

> We just dynamically maintain a list of devices/operations, which is
> list-executed when needed?
> 
> These new "core" operations are not included in every device but only
> global per subsystem, just like the sysdev_class did earlier?

Yup.

> Looks all like a nice plan to me.

Good. :-)

Thanks,
Rafael
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