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: Christoph Hellwig <hch@lst.de>

1 files changed, 8 insertions, 1 deletions

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 674a8da22844..a5a943836c8e 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 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))