Each multipath instance can use a Hardware Handler with hooks for the
particular hardware you're using.

This patch provides the hw_handler infrastructure.

So far 3 hooks are available:

  A status function invoked by device-mapper table and status requests.

  An initialisation function called when a Priority Group is selected for
  use but before any I/O is sent to it.  This function should return straight
  away, and I/O is queued until dm_pg_init_complete() is called indicating
  whether or not the initialisation was successful.  The are three error
  flags, any or all of which may be set: MP_FAIL_PATH, MP_BYPASS_PG,
  MP_ERROR_IO.

  An error handler which gets the opportunity to decode any error that a bio
  generated.  Patches are pending to make scsi error details available for
  dm_scsi_err_handler() to decode.
Signed-Off-By: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>