Message-Id: <200805160120.09057.rjw@sisk.pl>
Date:	Fri, 16 May 2008 01:20:08 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Huang, Ying" <ying.huang@...el.com>, Pavel Machek <pavel@....cz>,
	nigel@...el.suspend2.net,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vivek Goyal <vgoyal@...hat.com>, linux-kernel@...r.kernel.org,
	linux-pm@...ts.linux-foundation.org,
	Kexec Mailing List <kexec@...ts.infradead.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH -mm] kexec jump -v9

On Friday, 16 of May 2008, Eric W. Biederman wrote:
> "Rafael J. Wysocki" <rjw@...k.pl> writes:
> 
> > Well, it looks like we do similar things concurrently.  Please have a look 
> > here: http://kerneltrap.org/Linux/Separating_Suspend_and_Hibernation
> 
> Yes.  Part of the reason I wanted to separate these two conversations
> I knew something was going on.
> 
> > Similar patches are in the Greg's tree already.
> 
> Taking a look.
> 
> I just can't get past the fact in that the only reason hibernation can
> not use the widely implemented and tested probe/remove is because of
> filesystems on block devices, and that you are proposing to add 4
> methods for each and every driver to handle that case, when they
> don't need ANYTHING!

Why exactly do you think that removing()/probing() devices just for creating
a hibernation image is a good idea?

Also, ->poweroff() is actually similar to the late phase of ->suspend().

> I wonder how hard teaching the upper layers to deal with
> hotplug/remove is?
> 
> The more I look at this the more I get the impression that
> hibernation and suspend should be solved in separate patches.  I'm
> not at all convinced that is what is good for the goose is good for
> the gander for things like your prepare method.

This was discussed a lot with people who had exactly opposite opinions.
With BenH in particular (CCed).
 
> Hibernation seems to be an extreme case of hotplug.

I don't agree with that.

> Suspend seems to be just an extreme case of putting unused
> devices in low power state.

Ditto.

> ....
> 
> 
> I don't like the fact that these methods are power management specific.

Please be more specific.

> How should this impact the greater kernel ecosystem.
> 
> + * The externally visible transitions are handled with the help of the following
> + * callbacks included in this structure:
> + *
> + * @prepare: Prepare the device for the upcoming transition, but do NOT change
> + *	its hardware state.  Prevent new children of the device from being
> + *	registered after @prepare() returns (the driver's subsystem and
> + *	generally the rest of the kernel is supposed to prevent new calls to the
> + *	probe method from being made too once @prepare() has succeeded).  If
> + *	@prepare() detects a situation it cannot handle (e.g. registration of a
> + *	child already in progress), it may return -EAGAIN, so that the PM core
> + *	can execute it once again (e.g. after the new child has been registered)
> + *	to recover from the race condition.  This method is executed for all
> + *	kinds of suspend transitions and is followed by one of the suspend
> + *	callbacks: @suspend(), @freeze(), or @poweroff().
> + *	The PM core executes @prepare() for all devices before starting to
> + *	execute suspend callbacks for any of them, so drivers may assume all of
> + *	the other devices to be present and functional while @prepare() is being
> + *	executed.  In particular, it is safe to make GFP_KERNEL memory
> + *	allocations from within @prepare(), although they are likely to fail in
> + *	case of hibernation, if a substantial amount of memory is requested.
> + *	However, drivers may NOT assume anything about the availability of the
> + *	user space at that time and it is not correct to request firmware from
> + *	within @prepare() (it's too late to do that).
> + *
> + * @complete: Undo the changes made by @prepare().  This method is executed for
> + *	all kinds of resume transitions, following one of the resume callbacks:
> + *	@resume(), @thaw(), @restore().  Also called if the state transition
> + *	fails before the driver's suspend callback (@suspend(), @freeze(),
> + *	@poweroff()) can be executed (e.g. if the suspend callback fails for one
> + *	of the other devices that the PM core has unsucessfully attempted to
> + *	suspend earlier).
> + *	The PM core executes @complete() after it has executed the appropriate
> + *	resume callback for all devices.
> 
> The names above are terrible.  Perhaps: @pause/@...ause.

The names have been discussed either and I don't intend to change them now.
Sorry.

> @pause Stop all device driver user space facing activities, and prepare
>        for a possible power state transition.
> 
> Essentially these should be very much like bringing an ethernet
> interface down.  The device is still there but we can't do anything
> with it.  The only difference is that this may not be user visible.
> 
> + * @suspend: Executed before putting the system into a sleep state in which the
> + *	contents of main memory are preserved.  Quiesce the device, put it into
> + *	a low power state appropriate for the upcoming system state (such as
> + *	PCI_D3hot), and enable wakeup events as appropriate.
> + *
> + * @resume: Executed after waking the system up from a sleep state in which the
> + *	contents of main memory were preserved.  Put the device into the
> + *	appropriate state, according to the information saved in memory by the
> + *	preceding @suspend().  The driver starts working again, responding to
> + *	hardware events and software requests.  The hardware may have gone
> + *	through a power-off reset, or it may have maintained state from the
> + *	previous suspend() which the driver may rely on while resuming.  On most
> + *	platforms, there are no restrictions on availability of resources like
> + *	clocks during @resume().
> 
> Unless I have misread something.

Yes, you have.

> These are exactly the same as @poweroff and @restore.

For many drivers @suspend will be equivalent to @freeze + @poweroff probably.

Also, @restore is not the same as @resume, because @restore cannot assume
anything about the state of devices, whereas @resume can.

> @suspend place the device in a low power state.
>          Enable wakeup events.
> 
>          Can we use this for cases when we need low power but haven't
>          stopped the cpu?  I think so.

And you are wrong.

We tried that, it didn't work.

> + * @freeze: Hibernation-specific, executed before creating a hibernation image.
> + *	Quiesce operations so that a consistent image can be created, but do NOT
> + *	otherwise put the device into a low power device state and do NOT emit
> + *	system wakeup events.  Save in main memory the device settings to be
> + *	used by @restore() during the subsequent resume from hibernation or by
> + *	the subsequent @thaw(), if the creation of the image or the restoration
> + *	of main memory contents from it fails.
> + *
> + * @thaw: Hibernation-specific, executed after creating a hibernation image OR
> + *	if the creation of the image fails.  Also executed after a failing
> + *	attempt to restore the contents of main memory from such an image.
> + *	Undo the changes made by the preceding @freeze(), so the device can be
> + *	operated in the same way as immediately before the call to @freeze().
> 
> Just @detach/@...ttach.
> 
> @detach Detach the driver from the hardware, while keeping the driver
>         instance for the hardware alive.
> 
>         Essentially this is what the shutdown method is today.
>         Except for being ready for a reattach.
> 
> @reattach
>         See if the hardware for the driver is present and reclaim
>         it and bring it up to speed for processing requests.

No, I don't think so.  I don't want the driver to detach, but to quiesce the
hardware.

> + * @poweroff: Hibernation-specific, executed after saving a hibernation image.
> + *	Quiesce the device, put it into a low power state appropriate for the
> + *	upcoming system state (such as PCI_D3hot), and enable wakeup events as
> + *	appropriate.
> + *
> + * @restore: Hibernation-specific, executed after restoring the contents of main
> + *	memory from a hibernation image.  Driver starts working again,
> + *	responding to hardware events and software requests.  Drivers may NOT
> + *	make ANY assumptions about the hardware state right prior to @restore().
> + *	On most platforms, there are no restrictions on availability of
> + *	resources like clocks during @restore().
> + *
> 
> If we have events we care about we just need to do:
> reattach(); suspend();  It is all the same from the point of view of
> the device.  Not the system but the device.

That I can agree with, if I understood you correctly. :-)

Still, having more specialized callbacks is not generally bad IMO, they
can reuse the code just fine.

Thanks,
Rafael
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