dma-direct: add support for allocation from ZONE_DMA and ZONE_DMA32

This allows to dip into zones for lower memory if they are available.
If one of the zones is not available the corresponding GFP_* flag
will evaluate to 0 so they won't change anything.  We provide an
arch tunable for those architectures that do not use GFP_DMA for
the lowest 24-bits, given that there are a few.

Roughly based on the x86 code.
Signed-off-by: NChristoph Hellwig <hch@lst.de>

lib/dma-direct.c

@@ -12,6 +12,14 @@
 
 #define DIRECT_MAPPING_ERROR		0
 
+/*
+ * Most architectures use ZONE_DMA for the first 16 Megabytes, but
+ * some use it for entirely different regions:
+ */
+#ifndef ARCH_ZONE_DMA_BITS
+#define ARCH_ZONE_DMA_BITS 24
+#endif
+
 static bool
 check_addr(struct device *dev, dma_addr_t dma_addr, size_t size,
 		const char *caller)
@@ -34,6 +42,12 @@ static void *dma_direct_alloc(struct device *dev, size_t size,
 	int page_order = get_order(size);
 	struct page *page = NULL;
 
+	/* GFP_DMA32 and GFP_DMA are no ops without the corresponding zones: */
+	if (dev->coherent_dma_mask <= DMA_BIT_MASK(ARCH_ZONE_DMA_BITS))
+		gfp |= GFP_DMA;
+	if (dev->coherent_dma_mask <= DMA_BIT_MASK(32) && !(gfp & GFP_DMA))
+		gfp |= GFP_DMA32;
+
 	/* CMA can be used only in the context which permits sleeping */
 	if (gfpflags_allow_blocking(gfp))
 		page = dma_alloc_from_contiguous(dev, count, page_order, gfp);