Messages in this thread Patch in this message | | | From | Christoph Hellwig <> | Subject | [PATCH] dma-direct: document the zone selection logic | Date | Mon, 1 Oct 2018 13:10:16 -0700 |
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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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