firewire: core: check for 1394a compliant IRM, fix inaccessibility of Sony camcorder

firewire: core: check for 1394a compliant IRM, fix inaccessibility of Sony camcorder

Per IEEE 1394 clause 8.4.2.3, a contender for the IRM role shall check
whether the current IRM complies to 1394a-2000 or later.  If not force a
compliant node (e.g. itself) to become IRM.  This was implemented in the
older ieee1394 driver but not yet in firewire-core.

An older Sony camcorder (Sony DCR-TRV25) which implements 1394-1995 IRM
but neither 1394a-2000 IRM nor BM was now found to cause an
interoperability bug:
  - Camcorder becomes root node when plugged in, hence gets IRM role.
  - firewire-core successfully contends for BM role, proceeds to perform
    gap count optimization and resets the bus.
  - Sony camcorder ignores presence of a BM (against the spec, this is
    a firmware bug), performs its idea of gap count optimization and
    resets the bus.
  - Preceding two steps are repeated endlessly, bus never settles,
    regular I/O is practically impossible.
http://thread.gmane.org/gmane.linux.kernel.firewire.user/3913

This is an interoperability regression from the old to the new drivers.
Fix it indirectly by adding the 1394a IRM check.  The spec suggests
three and a half methods to determine 1394a compliance of a remote IRM;
we choose the method of testing the Config_ROM.Bus_Info.generation
field.  This is data that firewire-core should have readily available at
this point, i.e. does not require extra I/O.

Reported-by: Clemens Ladisch <clemens@ladisch.de> (missing 1394a check)
Reported-by: H. S. <hs.samix@gmail.com> (issue with Sony DCR-TRV25)
Tested-by: H. S. <hs.samix@gmail.com>
Cc: <stable@kernel.org> # .32.x and newer
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>