Traditional IDE interface sucks in that it doesn't have a reliable IRQ
pending bit, so if the controller raises IRQ while the driver is
expecting it not to, the IRQ won't be cleared and eventually the IRQ
line will be killed by interrupt subsystem.  Some controllers have
non-standard mechanism to indicate IRQ pending so that this condition
can be detected and worked around.

This patch adds an optional operation ->sff_irq_check() which will be
called for each port from the ata_sff_interrupt() if an unexpected
interrupt is received.  If the operation returns %true,
->sff_check_status() and ->sff_irq_clear() will be cleared for the
port.  Note that this doesn't mark the interrupt as handled so it
won't prevent IRQ subsystem from killing the IRQ if this mechanism
fails to clear the spurious IRQ.

This patch also implements ->sff_irq_check() for ata_piix.  Note that
this adds slight overhead to shared IRQ operation as IRQs which are
destined for other controllers will trigger extra register accesses to
check whether IDE interrupt is pending but this solves rare screaming
IRQ cases and for some curious reason also helps weird BIOS related
glitch on Samsung n130 as reported in bko#14314.

  http://bugzilla.kernel.org/show_bug.cgi?id=14314

* piix_base_ops dropped as suggested by Sergei.

* Spurious IRQ detection doesn't kick in anymore if polling qc is in
  progress.  This provides less protection but some controllers have
  possible data corruption issues if the wrong register is accessed
  while a command is in progress.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Johannes Stezenbach <js@sig21.net>
Reported-by: Hans Werner <hwerner4@gmx.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>