dma-mapping: provide CONFIG_ARCH_DMA_DEFAULT_COHERENT

Provide a kconfig option to allow arches to manipulate default
value of dma_default_coherent in Kconfig.
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/Kconfig

@@ -76,6 +76,13 @@ config ARCH_HAS_DMA_PREP_COHERENT
 config ARCH_HAS_FORCE_DMA_UNENCRYPTED
 	bool
 
+#
+# Select this option if the architecture assumes DMA devices are coherent
+# by default.
+#
+config ARCH_DMA_DEFAULT_COHERENT
+	bool
+
 config SWIOTLB
 	bool
 	select NEED_DMA_MAP_STATE

kernel/dma/mapping.c

@@ -20,7 +20,7 @@
 #if defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE) || \
 	defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) || \
 	defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL)
-bool dma_default_coherent;
+bool dma_default_coherent = IS_ENABLED(CONFIG_ARCH_DMA_DEFAULT_COHERENT);
 #endif
 
 /*