On Mon, 11 Jan 2010, Oliver Neukum wrote:
> Hi,
>
> this made me wonder:
>
> void usb_autopm_put_interface(struct usb_interface *intf)
> {
> struct usb_device *udev = interface_to_usbdev(intf);
> int status;
>
> udev->last_busy = jiffies;
>
> Why do we set last_busy here?
For one thing, the current implementation does so.
More to the point, usb_autopm_put_interface() can cause the usage count
to drop to 0. If the last_busy value was too far in the past, the
device would be autosuspended immediately. That's not what we want;
the autosuspend shouldn't occur until after the appropriate delay. And
the delay should begin at the time the device is no longer being used,
which is the time when usb_autopm_put_interface() is called.
Conversely, if the usage count still remains positive or if other
interfaces are still in use then setting last_busy doesn't cause any
harm.
Alan Stern
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