usbfs: Add ioctls for runtime power management

usbfs: Add ioctls for runtime power management

It has been requested that usbfs should implement runtime power
management, instead of forcing the device to remain at full power as
long as the device file is open.  This patch introduces that new
feature.

It does so by adding three new usbfs ioctls:

	USBDEVFS_FORBID_SUSPEND: Prevents the device from going into
	runtime suspend (and causes a resume if the device is already
	suspended).

	USBDEVFS_ALLOW_SUSPEND: Allows the device to go into runtime
	suspend.  Some time may elapse before the device actually is
	suspended, depending on things like the autosuspend delay.

	USBDEVFS_WAIT_FOR_RESUME: Blocks until the call is interrupted
	by a signal or at least one runtime resume has occurred since
	the most recent ALLOW_SUSPEND ioctl call (which may mean
	immediately, even if the device is currently suspended).  In
	the latter case, the device is prevented from suspending again
	just as if FORBID_SUSPEND was called before the ioctl returns.

For backward compatibility, when the device file is first opened
runtime suspends are forbidden.  The userspace program can then allow
suspends whenever it wants, and either resume the device directly (by
forbidding suspends again) or wait for a resume from some other source
(such as a remote wakeup).  URBs submitted to a suspended device will
fail or will complete with an appropriate error code.

This combination of ioctls is sufficient for user programs to have
nearly the same degree of control over a device's runtime power
behavior as kernel drivers do.

Still lacking is documentation for the new ioctls.  I intend to add it
later, after the existing documentation for the usbfs userspace API is
straightened out into a reasonable form.

Suggested-by: Mayuresh Kulkarni <mkulkarni@opensource.cirrus.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/Pine.LNX.4.44L0.1908071013220.1514-100000@iolanthe.rowland.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>