The memory barrier dma_mb() is introduced by commit a76a3777
("iommu/arm-smmu-v3: Ensure queue is read after updating prod pointer"),
which is used to ensure that prior (both reads and writes) accesses
to memory by a CPU are ordered w.r.t. a subsequent MMIO write.

Reviewed-by: Arnd Bergmann <arnd@arndb.de> # for asm-generic
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: NMarco Elver <elver@google.com>
Link: https://lore.kernel.org/r/20220523113126.171714-2-wangkefeng.wang@huawei.comSigned-off-by: NWill Deacon <will@kernel.org>