Re: [usb-storage] xhci: suspend/resume issues

[Matthew Dharm <mdharm-scsi@one-eyed-alien.net>]

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On Mon, Nov 22, 2010 at 03:15:24PM -0800, Sarah Sharp wrote:
> Ccing the scsi and USB mass storage devices lists on this issue.
> Background: a USB 3.0 mass storage device shows up as a USB 2.0 device
> after resume, and then migrates back to USB 3.0 speeds after a reset.
> 
> On Mon, Nov 22, 2010 at 05:49:47PM -0500, Don Zickus wrote:
> > Looking into my previously mounted /mnt directory yields an i/o error
> > (probably because it is now under /dev/sdc).
> 
> As long as it doesn't produce any kernel oops, that's normal.  The
> kernel has noticed the device went away, but userspace hasn't.  At
> least, that's my general impression.  You'll have to ask the USB mass
> storage guys, or the SCSI folks for the details on how that's supposed
> to work.

That's mostly right.  The USB and SCSI layers know the device has gone
away, but there is no way to signal to the filesystem layers that the
underlying device is gone.  So, all the I/O requests are simply 'failed'
until the device is unmounted, at which point the refcount drops to zero
and the structures are actually deleted.

> > Unmounting everything and remounting 'mount /dev/sdc2 /mnt' (using the new
> > letter 'c' instead of 'b') works fine and no data loss seems to occur.
> > 
> > Just thought it might be annoying and was wondering if you had any ideas
> > on that?
> 
> There's no way to work around it if the device disconnects, AFAIK.

I'm coming late to the discussion, but Sara is right.  If the device
disconnects, there is no way to know that it is the same device when it
reconnects.

Matt

-- 
Matthew Dharm                              Home: mdharm-usb@one-eyed-alien.net 
Maintainer, Linux USB Mass Storage Driver

I could always suspend a few hundred accounts and watch what happens.
					-- Tanya
User Friendly, 7/31/1998

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