ARM: mvebu: do not register custom DMA operations when coherency is disabled

This patch is a partial backport of commit ef01c6c3 ("ARM: mvebu:
remove Armada 375 Z1 workaround for I/O coherency"). This commit was
merged in v3.19, so kernel versions later than v3.19 are not affected
by the problem that this commit fixes.

It does not make a lot of sense to backport this commit entirely,
since it is mainly removing some no longer useful code. However, this
commit is also making sure that the bus_register_notifier that
register the custom DMA operations that should be used for HW I/O
coherency does not get registered when said HW I/O coherency is not
enabled.

This is particularly critical since we have decided to disable HW I/O
coherency completely in all kernels < 4.0, to be on the safe side,
while experimenting a new implementation of the HW I/O coherency in >=
4.0.

Without this commit, kernels earlier than 3.18 have the custom DMA
operations normally used for HW I/O coherency registered (they don't
do cache maintenance operations), while HW I/O coherency is
disabled. It essentially causes every DMA transfer to transfer
garbage.

The issue fixed by this commit was introduced by 5ab5afd8 ("ARM:
mvebu: implement Armada 375 coherency workaround"), but it was not
visible until now since it didn't cause any problem when HW I/O
coherency is enabled.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

arch/arm/mach-mvebu/coherency.c

@@ -448,8 +448,9 @@ static int __init coherency_late_init(void)
 			armada_375_coherency_init_wa();
 	}
 
-	bus_register_notifier(&platform_bus_type,
-			      &mvebu_hwcc_nb);
+	if (coherency_available())
+		bus_register_notifier(&platform_bus_type,
+				      &mvebu_hwcc_nb);
 
 	return 0;
 }