Date: Tue, 14 Feb 2006 14:26:31 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
To: "Rafael J. Wysocki" <rjw@sisk.pl>
cc: Kernel development list <linux-kernel@vger.kernel.org>,
       Linux-pm mailing list <linux-pm@lists.osdl.org>
Subject: Re: Flames over -- Re: Which is simpler?
In-Reply-To: <200602132327.10475.rjw@sisk.pl>
Message-ID: <Pine.LNX.4.44L0.0602141417350.1419-100000@iolanthe.rowland.org>
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On Mon, 13 Feb 2006, Rafael J. Wysocki wrote:

> Hi,
> 
> On Monday 13 February 2006 22:24, Alan Stern wrote:
> > On Mon, 13 Feb 2006, Phillip Susi wrote:
> }-- snip --{
> > You are complaining because you don't like the way USB was designed.  
> > That's fine, but it leaves you advocating a non-standardized position.
> > 
> > Can you suggest a _reliable_ way to tell if the USB device present at a 
> > port after resuming is the same device as was there before suspending?
> 
> It seems to follow from your discussion that if I have a mounted filesystem
> on a USB device and I suspend to disk, I can lose data unless the filesystem
> has been mounted with "sync".

That's right.  It depends on your hardware, and it could be true even for
suspend-to-RAM.  In fact, even with "-o sync" you can lose data if your
programs have information in buffers they haven't written out to disk.

If you're lucky, your hardware will support low-power modes for USB
controllers while the system is asleep.  Lots of hardware doesn't,
however.  Shutting off the power to a USB controller is equivalent to 
unplugging all the attached devices.

Remember that it's always a bad idea to unplug a disk drive containing a
mounted filesystem.  With USB that's true even when your system is asleep!
The safest thing is to unmount all USB-based filesystems before suspending 
and remount them after resuming.

> If this is the case, there should be a big fat warning in the swsusp
> documentation, but there's nothing like that in there (at lease I can't find
> it easily).
> 
> [If this is not the case, I've missed something and sorry for the noise.]

I'm not aware of any warnings about this in the documentation.  If you 
would like to add something, please go ahead.

Alan Stern

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