linux-kernel - Re: [PATCH 9/10] sh: Use struct syscore_ops instead of sysdev class and sysdev

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Date:	Wed, 23 Mar 2011 00:50:20 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Kay Sievers <kay.sievers@...e.de>
Cc:	Paul Mundt <lethal@...ux-sh.org>,
	LKML <linux-kernel@...r.kernel.org>, Greg KH <gregkh@...e.de>,
	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Magnus Damm <magnus.damm@...il.com>, linux-sh@...r.kernel.org
Subject: Re: [PATCH 9/10] sh: Use struct syscore_ops instead of sysdev class and sysdev

On Wednesday, March 23, 2011, Kay Sievers wrote:
> On Wed, 2011-03-23 at 00:32 +0100, Rafael J. Wysocki wrote:
> > On Tuesday, March 22, 2011, Kay Sievers wrote:
> > > On Tue, 2011-03-22 at 23:23 +0100, Rafael J. Wysocki wrote:
> > > > On Tuesday, March 22, 2011, Kay Sievers wrote:
> > ...
> > > > 
> > > > Now, I can easily understand arguments about representing everything under
> > > > /sys/devices/ by struct device objects, no question about that.  However,
> > > > I also think there should be a place for things like those mentioned in the
> > > > comment in sys.c, presumably outside of /sys/devices/.
> > > 
> > > No, please. We have all we need. Let's do one example, which you might
> > > apply to any other thing, because you never know what's the next big
> > > thing in hardware. We need to be a future-proof-as-possible, and that's
> > > not some second-class out-of-scope sysfs directory.
> > > 
> > > Lets' take CPUs:
> > >   - they send events when registered
> > >   - they want to export device specific properties
> > >   - userspace wants to take actions when such devices are available
> > > 
> > > That all fits properly into the driver model in theory. Unless you do
> > > coldplug and bootup a box.
> > > 
> > > These devices are already there before userspace even starts, hence we
> > > find all these devices and "trigger" an fake uevent for all of them at
> > > bootup. That will match execute all the rules specified for that device,
> > > just as it would be hotplugges in that moment, hence we call it
> > > coldplug, which works for all devices with the hotplug code, even when
> > > they are never hot-pluggable.
> > > 
> > > What we do for coldplug is that we iterate over all flat lists of
> > > subsystems and find the devices lists and trigger the event by poking in
> > > the "uevent" sysfs file. Now all the sysdevs do not have a subsystem to
> > > find, and do not have a standard "uevent" file.
> > > 
> > > Back to the CPUs, we have all the nice device directories which could
> > > have all the CPU features in properties we need to make autoloading of
> > > cpufreq, governer, kvm possible (patch exists from Andi Kleen already) 
> > > 
> > > But these dumb CPU sysfs device directories are completely invisible for
> > > the *usual* logic, and should just join the model and all will just work
> > > out-of-the-box.
> > 
> > That all is cool, but I'm not sure how it is related to things like
> > available_clocksource and current_clocksource (which happen to be located
> > under /sys/devices/system/clocksource/clocksource0/ being simply a path
> > in sysfs).
> 
> Sure, it isn't related to clocksource at all. I didn't really get the
> idea that there are users that just fake core devices only to get a
> place to put a couple of attributes. I was still in the context of the
> $SUBJECT of this thread.
> 
> This stuff should just stay away from devices, not sysdev, not "struct
> device".
> 
> For other things like CPUs, which are fine to be represented as driver
> core devices, all the above is still valid, and they should be real
> devices and have their own subsystem, which exposes them to coldplug and
> usual event handling.
> 
> > Well, Greg apparently thinks that available_clocksource and current_clocksource
> > could be located under /sys/bus/clock/.  Perhaps other attributes now exported
> > through sysdevs could be moved to places like this?
> 
> Sure, we could do that. All such subsystems have a directory to put
> subsystem-global stuff. In this case it would be a subsystem without any
> registered device. But it leaves us open to add real devices to it
> later, which might be the case for some similar subsystems.
> 
> The other option would be /sys/kernel/clocksource/ with the few
> attributes to create.
> 
> We should decide if "clocksource" is kind of "device-related" or not. Do
> you have any list of subsystems besides "clocksource", which would help
> to get a bigger picture what we should expect?

Not at the moment.  I'll prepare one while working on syscore_ops patches
for the non-x86 architectures.

Thanks,
Rafael
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