ARM: 7161/1: errata: no automatic store buffer drain

This patch implements a workaround for PL310 erratum 769419. On
revisions of the PL310 prior to r3p2, the Store Buffer does not
automatically drain. This can cause normal, non-cacheable writes to be
retained when the memory system is idle, leading to suboptimal I/O
performance for drivers using coherent DMA.

This patch adds an optional wmb() call to the cpu_idle loop. On systems
with an outer cache, this causes an explicit flush of the store buffer.

Cc: [email protected]
Acked-by: Catalin Marinas <[email protected]>
Tested-by: Marc Zyngier <[email protected]>
Signed-off-by: Will Deacon <[email protected]>
Signed-off-by: Russell King <[email protected]>

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