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| author | Christoph Hellwig <hch@lst.de> | 2018-10-01 07:40:53 -0700 |
|---|---|---|
| committer | Christoph Hellwig <hch@lst.de> | 2018-10-09 07:43:25 +0200 |
| commit | 79ac32a427f5d1211fa417021fd04c36f63ab917 (patch) | |
| tree | c203a3573214892eadaad406498bdb0a8a66db35 /kernel/dma/direct.c | |
| parent | 99c65fa7c59ff558e70db8aa61bbdece5d3a9588 (diff) | |
| download | linux-79ac32a427f5d1211fa417021fd04c36f63ab917.tar.bz2 | |
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>
Diffstat (limited to 'kernel/dma/direct.c')
| -rw-r--r-- | kernel/dma/direct.c | 9 |
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)) |