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Commit 79ac32a4 authored by Christoph Hellwig's avatar Christoph Hellwig
Browse files

dma-direct: document the zone selection logic 



What we are doing here isn't quite obvious, so add a comment explaining
it.

Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
parent 99c65fa7

kernel/dma/direct.c

+8 −1
Original line number Diff line number Diff line
@@ -84,7 +84,14 @@ static gfp_t __dma_direct_optimal_gfp_mask(struct device *dev, u64 dma_mask, 
	else

		*phys_mask = dma_to_phys(dev, dma_mask);



	/* GFP_DMA32 and GFP_DMA are no ops without the corresponding zones: */

	/*

	 * Optimistically try the zone that the physical address mask falls

	 * into first.  If that returns memory that isn't actually addressable

	 * we will fallback to the next lower zone and try again.

	 *

	 * Note that GFP_DMA32 and GFP_DMA are no ops without the corresponding

	 * zones.

	 */

	if (*phys_mask <= DMA_BIT_MASK(ARCH_ZONE_DMA_BITS))

		return GFP_DMA;

	if (*phys_mask <= DMA_BIT_MASK(32))