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Subject[PATCH] dma-direct: document the zone selection logic
Date
What we are doing here isn't quite obvious, so add a comment explaining
it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
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
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