linux-kernel - Re: Enable CONFIG_USB_SUSPEND by default or some mobile HD can't be unplugged safely

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Date:	Fri, 2 Jan 2009 11:04:40 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Yan Li <elliot.li.tech@...il.com>
cc:	Tino Keitel <tino.keitel@....de>, <gregkh@...e.de>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: Enable CONFIG_USB_SUSPEND by default or some mobile HD can't be
 unplugged safely

On Fri, 2 Jan 2009, Yan Li wrote:

> Now I know 3 methods supposed can be used to shut the device:
> 1. (Alan Stern said Windows use this) cut the USB port's power

I said no such thing!  In fact, I said exactly the opposite: Windows 
does _not_ cut the port's power.  Instead it disables the port.

> 2. send STOP SCSI command to stop it

Note that this is different.  The START-STOP command is used to spin-up 
or spin-down a disk.  It does not affect the state of the data link.

> 3. put it into suspend mode

As far as the device is concerned, there is essentially no difference 
between 1 and 3.  When the device's upstream port is disabled, the 
device must go into suspend mode.

The best approach is to send a SYNCHRONIZE CACHE command followed by 
START-STOP (if the device supports it), and then to disable or suspend 
the port.  In Linux, those two commands will be sent automatically if 
you unbind the device from usb-storage.  The suspend has to be done 
manually unless you have set up a udev rule (or something equivalent) 
to enable autosuspend for the device.  Of course, this requires 
CONFIG_USB_SUSPEND.

Alan Stern

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