Messages in this thread | | | Date | Fri, 2 Jan 2009 11:04:40 -0500 (EST) | From | Alan Stern <> | Subject | Re: Enable CONFIG_USB_SUSPEND by default or some mobile HD can't be unplugged safely |
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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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