When we insert the sync sequence number into the CMD_SYNC.MSIData field,
we do so in CPU-native byte order, before writing out the whole command
as explicitly little-endian dwords. Thus on big-endian systems, the SMMU
will receive and write back a byteswapped version of sync_nr, which would
be perfect if it were targeting a similarly-little-endian ITS, but since
it's actually writing back to memory being polled by the CPUs, they're
going to end up seeing the wrong thing.

Since the SMMU doesn't care what the MSIData actually contains, the
minimal-overhead solution is to simply add an extra byteswap initially,
such that it then writes back the big-endian format directly.

Cc: <stable@vger.kernel.org>
Fixes: 37de98f8 ("iommu/arm-smmu-v3: Use CMD_SYNC completion MSI")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>