arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()

arch_dma_prep_coherent() is called when preparing a non-cacheable region
for a consistent DMA buffer allocation. Since the buffer pages may
previously have been written via a cacheable mapping and consequently
allocated as dirty cachelines, the purpose of this function is to remove
these dirty lines from the cache, writing them back so that the
non-coherent device is able to see them.

On arm64, this operation can be achieved with a clean to the point of
coherency; a subsequent invalidation is not required and serves little
purpose in the presence of a cacheable alias (e.g. the linear map),
since clean lines can be speculatively fetched back into the cache after
the invalidation operation has completed.

Relax the cache maintenance in arch_dma_prep_coherent() so that only a
clean, and not a clean-and-invalidate operation is performed.

Cc: Mark Rutland <[email protected]>
Cc: Robin Murphy <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Signed-off-by: Will Deacon <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

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