include/linux/dma-mapping.h: add dma_zalloc_coherent()

Lots of driver code does a dma_alloc_coherent() and then zeroes out the
memory with a memset.  Make it easy for them.

Cc: Alexandre Bounine <alexandre.bounine@idt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DMA-API.txt

@@ -50,6 +50,13 @@ specify the GFP_ flags (see kmalloc) for the allocation (the
 implementation may choose to ignore flags that affect the location of
 the returned memory, like GFP_DMA).
 
+void *
+dma_zalloc_coherent(struct device *dev, size_t size,
+			     dma_addr_t *dma_handle, gfp_t flag)
+
+Wraps dma_alloc_coherent() and also zeroes the returned memory if the
+allocation attempt succeeded.
+
 void
 dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
 			   dma_addr_t dma_handle)

include/linux/dma-mapping.h

@@ -1,6 +1,7 @@
 #ifndef _LINUX_DMA_MAPPING_H
 #define _LINUX_DMA_MAPPING_H
 
+#include <linux/string.h>
 #include <linux/device.h>
 #include <linux/err.h>
 #include <linux/dma-attrs.h>
@@ -117,6 +118,15 @@ static inline int dma_set_seg_boundary(struct device *dev, unsigned long mask)
 		return -EIO;
 }
 
+static inline void *dma_zalloc_coherent(struct device *dev, size_t size,
+					dma_addr_t *dma_handle, gfp_t flag)
+{
+	void *ret = dma_alloc_coherent(dev, size, dma_handle, flag);
+	if (ret)
+		memset(ret, 0, size);
+	return ret;
+}
+
 #ifdef CONFIG_HAS_DMA
 static inline int dma_get_cache_alignment(void)
 {