The Wii Remote has several extension ports. The first port (EXT) provides
hotplug events whenever an extension is plugged. The second port (MP)
does not provide hotplug events by default. Instead, we have to map MP
into EXT to get events for it.

This patch introduces hotplug support for extensions. It is fairly
complicated to get this right because the Wii Remote sends a lot of
noise-hotplug events while activating extension ports. We need to filter
the events and only handle the events that are real hotplug events.

Mapping MP into EXT is easy. But if we want both, MP _and_ EXT at the same
time, we need to map MP into EXT and enable a passthrough-mode. This will
then send real EXT events through the mapped MP interleaved with real MP
events. But once MP is mapped, we no longer have access to the real EXT
registers so we need to perform setup _before_ mapping MP. Furthermore, we
no longer can read EXT IDs so we cannot verify if EXT is still the same
extension that we expect it to be.
We deal with this by unmapping MP whenever we got into a situation where
EXT might have changed. We then re-read EXT and MP and remap everything.

The real Wii Console takes a fairly easy approach: It simply reconnects to
the device on hotplug events that it didn't expect. So if a program wants
MP events, but MP is disconnected, it fails and reconnects so it can wait
for MP hotplug events again.
This simplifies hotplugging a lot because we just react on PLUG events and
ignore UNPLUG events.
The more sophisticated Wii applications avoid reconnection (well, they
still reconnect during many weird events, but at least not during UNPLUG)
but they start polling the device. This allows them to disable the device,
poll for the extension ports to settle and then initialize them again.
Unfortunately, this approach fails whenever an extension is replugged
while it is initialized. We would loose UNPLUG events and polling the
device later will give unreliable results because the extension port might
be in some weird state, even though it's actually unplugged.

Our approach is a real HOTPLUG approch. We keep track of the EXT and
mapped MP hotplug events whenever they occur. We then re-evaluate the
device state and initialize any possible new extension or deinitialize any
gone extension. Only during initialization, we set an extension port
ACTIVE. However, during an unplug event we mark them as INACTIVE. This
guarantess that a fast UNPLUG -> PLUG event sequence doesn't keep them
marked as PLUGGED+ACTIVE but only PLUGGED.
To deal with annoying noise-hotplug events during extension mapping, we
simply rescan the device before performing any mapping. This allows us to
ignore all the noise events as long as the device is in the correct state.

Long story short: EXT and MP registers are sparsely known and we need to
jump through hoops to get reliable HOTPLUG working. But while Nintendo
needs *FOUR* Bluetooth reconnections for the shortest imaginable
boot->menu->game->menu->shutdown sequence, we now need *ZERO*.

As always, 3rd party devices tend to break whenever we behave differently
than the original Wii. So there are also devices which _expect_ a
disconnect after UNPLUG. Obviously, these devices won't benefit from this
patch. But all official devices were tested extensively and work great
during any hotplug sequence. Yay!
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>