Re: [linux-pm] [patch/rft 2.6.17-rc2] swsusp resume must not device_suspend()

[Patrick Mochel <mochel@linux.intel.com>]

On Thu, Apr 27, 2006 at 10:34:16AM -0400, Alan Stern wrote:
> On Thu, 27 Apr 2006, Pavel Machek wrote:
> 
> > Hi!
> > 
> > > The right way to solve this is to make sure that the resuming kernel can
> > > correctly determine whether the power session (way back from the original
> > > sw-suspend) is still intact.  It's expected that in many cases it won't
> > > be, because most systems won't provide suspend current while the machine
> > > is off.  We have to guarantee that the boot kernel's actions won't end up
> > > fooling the resuming kernel into thinking that the power sessions are
> > > intact when in fact they aren't.  (Furthermore, in an ideal world, we
> > > would also make sure that the boot kernel won't destroy any power sessions
> > > that still _are_ intact.  Right now we have no way to do this, because the
> > > drivers in the boot kernel don't know that it _is_ a boot kernel.)
> > 
> > You don't know if it is boot or resume until you read the disk, sorry.
> 
> According to David this shouldn't matter.  During swsusp the system is
> supposed to be completely off, with no suspend power available.  Hence all
> the power sessions are guaranteed to be interrupted, and the boot kernel
> doesn't have to worry about destroying any of them.

Not necessarily. x86 hardware implementations of suspend-to-disk retain some 
power during suspend. Not many (if any) devices will retain context, but the
system is definitely not completely "off". Actually, the same is true for
soft-off on x86 (aka ACPI S5). Some power may be drawn to support various power
on events. The only time a system is truly off is when it is unplugged from
the wall (and/or the battery taken out, if applicable). 

I don't think it matters much in this particular context, but it's important
to keep in mind when making assumptions about "off-ness".

Thanks,


	Patrick


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