From: hackmiester / Hunter Fuller <hackmiester@hackmiester.com>
Reply-To: hackmiester@hackmiester.com
Organization: hackmiester.com, Ltd.
To: Alan Stern <stern@rowland.harvard.edu>, linux-kernel@vger.kernel.org
Subject: Re: Flames over -- Re: Which is simpler?
Date: Tue, 14 Feb 2006 08:15:41 -0600
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On Sunday 12 February 2006 20:19, Alan Stern wrote:
> On Sun, 12 Feb 2006, Phillip Susi wrote:
> > Alan Stern wrote:
> > > Both of you are missing an important difference between Suspend-to-RAM
> > > and Suspend-to-Disk.
> > >
> > > Suspend-to-RAM is a true suspend operation, in that the hardware's
> > > state is maintained _in the hardware_.  External buses like USB will
> > > retain suspend power, for instance (assuming the motherboard supports
> > > it; some don't).
> > >
> > > Suspend-to-Disk, by contrast, is _not_ a true suspend.  It can more
> > > accurately be described as checkpoint-and-turn-off.  Hardware state is
> > > not maintained.  (Some systems may support a special ACPI state that
> > > does maintain suspend power to external buses during shutdown, I forg=
et
> > > what it's called.  And I down't know whether swsusp uses this state.)
> >
> > I would disagree.  The only difference between the two is WHERE the
> > state is maintained - ram vs. disk.  I won't really argue it though,
> > because it's just semantics -- call it whatever you want.
>
> It's not just semantics.  There's a real difference between maintaining
> state in the hardware and maintaining it somewhere else.  The biggest
> difference is that if the hardware retains suspend power, it is able to
> detect disconnections.  When the system resumes, it _knows_ whether a
> device was attached the entire time, as opposed to being unplugged and
> replugged (or possibly a different device plugged in!) while the system
> was asleep.  If the hardware is down completely, there is no way of
> telling for certain whether a device attached to some port is the same one
> that was there when the system got suspended.
>
> Another difference is the possibility of remote wakeup.  Clearly it can't
> happen when there's no power available.

Well... sort of. I know that my notebook can be brought up over LAN when it=
 is=20
off - suspended to disk or otherwise.
>
> > > So for example, let's say you have a filesystem mounted on a USB flash
> > > or disk drive.  With Suspend-to-RAM, there's a very good chance that
> > > the connection and filesystem will still be intact when you resume.=20
> > > With Suspend-to-Disk, the USB connection will terminate when the
> > > computer shuts down.  When you resume, the device will be gone and yo=
ur
> > > filesystem will be screwed.
> >
> > This is not true.  The USB bus is shut down either way, and provided
> > that you have not unplugged the disk, nothing will be screwed when you
> > resume from disk or ram.
>
> Have you actually tried it?  I have.
>
> In any case, it is undeniably true that if the bus is shut down then all
> the USB connections are lost.  When you resume it will be the same as if
> you had unplugged all the USB devices and then replugged them.  Not a good
> thing to do when they contain mounted filesystems; all the memory mappings
> are invalidated.

It all depends on the machine I guess. Mine keeps even the CD drive running=
=20
when it's off!
>
> (Bear in mind that whether a USB bus gets shut down depends on the
> motherboard; some supply suspend power and some don't.  It depends on the
> USB controller too; some support low-power states other than "completely
> off" and others don't.)
>
> Alan Stern
>
> -
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=2D-=20
=2D-hackmiester
If you can read this, you don't need glasses.

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