[PATCH] dma-direct: document the zone selection logic

[Christoph Hellwig]

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

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
 kernel/dma/direct.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index ba6f5956a291..14b966e2349a 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -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 physicall 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))
-- 
2.19.0