regmap: mmio: add config option to allow relaxed MMIO accesses

On some platforms (eg armv7 due to the CONFIG_ARM_DMA_MEM_BUFFERABLE)
MMIO R/W operations always add memory barriers which can increase load,
decrease battery life or in general reduce performance unnecessarily
on devices which access a lot of configuration registers and where
ordering does not matter (eg. media accelerators like the Verisilicon /
Hantro video decoders).

Drivers used to call the relaxed MMIO variants directly but since they
are now accessing the MMIO registers via regmaps (to compensate for
different VPU HW reg layouts via regmap fields), there is a need for a
relaxed API / config to preserve existing behaviour.

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Link: https://lore.kernel.org/r/20201014203024.954369-1-adrian.ratiu@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/regmap.h

@@ -315,6 +315,10 @@ typedef void (*regmap_unlock)(void *);
  *                   masks are used.
  * @zero_flag_mask: If set, read_flag_mask and write_flag_mask are used even
  *                   if they are both empty.
+ * @use_relaxed_mmio: If set, MMIO R/W operations will not use memory barriers.
+ *                    This can avoid load on devices which don't require strict
+ *                    orderings, but drivers should carefully add any explicit
+ *                    memory barriers when they may require them.
  * @use_single_read: If set, converts the bulk read operation into a series of
  *                   single read operations. This is useful for a device that
  *                   does not support  bulk read.
@@ -388,6 +392,7 @@ struct regmap_config {
 
 	bool use_single_read;
 	bool use_single_write;
+	bool use_relaxed_mmio;
 	bool can_multi_write;
 
 	enum regmap_endian reg_format_endian;