On Thu, 27 Jan 2005, Mohan wrote:
> Hello All,
>
> I had a question abt the "suspend" state in USB. It is known from the
> USB specifications that when there is inactivity on the USB bus for 3ms
> the device is put in Suspended State.
> Can I from the application put the USB device in the suspended state on
> inactivity. (say there is no activity for 3mins). This i want to do to
> conserve power.
If CONFIG_USB_SUSPEND is set in your kernel configuration then
applications can suspend and resume USB devices. Detecting the absence of
activity is up to you though; right now the kernel does not have any way
to say "Suspend this device after 3 minutes of inactivity".
Writing a '3' (with no newline) to <device's sysfs directory>/power/state
will suspend the device, and writing a '0' will resume it. For example,
you can do
echo -n 3 >/sys/bus/usb/devices/1-2/power/state
to suspend the device plugged in to port 2 of bus 1. (This requires root
privileges, by the way.)
Note that this method is likely to change in the future. There ought to
be ioctls in usbfs for suspending and resuming devices, but there aren't.
Alan Stern
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