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List:       linux-usb-devel
Subject:    Re: [linux-usb-devel] Re: Suspend/resume for HCDs
From:       Alan Stern <stern () rowland ! harvard ! edu>
Date:       2005-04-29 1:45:43
Message-ID: Pine.LNX.4.44L0.0504282140530.28498-100000 () netrider ! rowland ! org
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On Fri, 29 Apr 2005, Oliver Neukum wrote:

> > "Provided that you know when you can safely autosuspend."  If the driver 
> > doesn't know when it can suspend its device, who else would know?
> 
> Those who understand the semantics, which with scsi (sg) might be
> in user space.

Definitely we wouldn't expect sg to do autosuspend.

> > shouldn't make much difference to usb-storage.  It doesn't have to tell
> > the SCSI layer when the USB transport is suspended.  All it has to do is
> > resume the USB connection when the next SCSI command comes along.
> 
> No. It has to guarantee that the device will react the same way to the next
> command whether a suspension has intervened or not. I really doubt you
> can do that for all devices going into power save.

How about self-powered USB mass storage devices?  Suspending the USB 
connection generally won't cause these devices to go into a power save 
mode, although there might be exceptions.

However I take your point.  Eventually we may expect that the upper layer
SCSI drivers sd and sr will autosuspend and will notify their parent when
they do so.  When usb-storage receives such a notification from below it
can then autosuspend safely.

Alan Stern



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