+static inline void arch_dma_cache_wback(phys_addr_t paddr, size_t size)
 {
+	dma_cache_wback(paddr, size);
+}
 
+static inline void arch_dma_cache_inv(phys_addr_t paddr, size_t size)
+{
+	dma_cache_inv(paddr, size);
 }

+static inline void arch_dma_cache_wback_inv(phys_addr_t paddr, size_t size)
 {
+	dma_cache_wback_inv(paddr, size);
+}

There are the only calls for the three functions for each of the
involved functions.  So I'd rather rename the low-level symbols
(and drop the pointless exports for two of them) rather than adding
these wrapppers.

The same is probably true for many other architectures.

+static inline bool arch_sync_dma_clean_before_fromdevice(void)
+{
+	return false;
+}
 
+static inline bool arch_sync_dma_cpu_needs_post_dma_flush(void)
+{
+	return true;
 }

Is there a way to cut down on this boilerplate code by just having
sane default, and Kconfig options to override them if they are not
runtime decisions?

+#include <linux/dma-sync.h>

I can't really say I like the #include version here despite your
rationale in the commit log.  I can probably live with it if you
think it is absolutely worth it, but I'm really not in favor of it.

+config ARCH_DMA_MARK_DCACHE_CLEAN
+	def_bool y

What do we need this symbol for?  Unless I'm missing something it is
always enable for arm32, and only used in arm32 code.