firewire: fix use of multiple AV/C devices, allow multiple FCP listeners

Control of more than one AV/C device at once --- e.g. camcorders, tape
decks, audio devices, TV tuners --- failed or worked only unreliably,
depending on driver implementation.  This affected kernelspace and
userspace drivers alike and was caused by firewire-core's inability to
accept multiple registrations of FCP listeners.

The fix allows multiple address handlers to be registered for the FCP
command and response registers.  When a request for these registers is
received, all handlers are invoked, and the Firewire response is
generated by the core and not by any handler.

The cdev API does not change, i.e., userspace is still expected to send
a response for FCP requests; this response is silently ignored.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de> (changelog, rebased, whitespace)

include/linux/firewire-cdev.h

@@ -340,6 +340,9 @@ struct fw_cdev_send_response {
  * The @closure field is passed back to userspace in the response event.
  * The @handle field is an out parameter, returning a handle to the allocated
  * range to be used for later deallocation of the range.
+ *
+ * The address range is allocated on all local nodes.  The address allocation
+ * is exclusive except for the FCP command and response registers.
  */
 struct fw_cdev_allocate {
 	__u64 offset;