dma-direct: warn when coherent allocations aren't supported

Log a warning once when dma_alloc_coherent fails because the platform
does not support coherent allocations at all.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Greg Ungerer <gerg@linux-m68k.org>
Tested-by: Greg Ungerer <gerg@linux-m68k.org>

kernel/dma/direct.c

@@ -240,9 +240,11 @@ void *dma_direct_alloc(struct device *dev, size_t size,
 		 */
 		set_uncached = IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED);
 		remap = IS_ENABLED(CONFIG_DMA_DIRECT_REMAP);
-		if (!set_uncached && !remap)
+		if (!set_uncached && !remap) {
+			pr_warn_once("coherent DMA allocations not supported on this platform.\n");
 			return NULL;
 		}
+	}
 
 	/*
 	 * Remapping or decrypting memory may block, allocate the memory from