Power down USB when suspending

[John Watlington]

On Sep 27, 2010, at 6:46 PM, Matías Poloni wrote:

> We are engineer students from Uruguay and we are working on our grade thesis with a XO-1 olpc laptop. We need to take a photo after resume with a usb webcam as quick as possible. We already know that the usb is powered down when suspending and this leads to excesive delay taking a picture after resume. We can`t find any documentation explaining why it is not possible not to power down the usb. We`ll be very grateful if you help us with information related with this issue because we need it for the documentation of our thesis.

Matías,
  It is true that the XO does not maintain power to the USB device while the laptop is suspended.
But this is just one of the problems you face.   You could maintain power to your USB camera
during suspend by placing a powered hub between the camera and the laptop, but on resume
you won't be able to use the camera until after the USB driver in the Linux kernel has reset the
USB bus and "re-enumerated" the devices on it (which takes seconds.)   Furthermore, most
if not all device drivers will treat the device as if it was just powered back up.

Eventually, given well enough designed hardware (capable of both powering the USB port in
suspend, and waking up the system on insertion or removal of a device while suspended), the
drivers could be extended to support this mode of operation.   But it is not simply a matter of
keeping the port powered.

Regards,
John