From 79ac32a427f5d1211fa417021fd04c36f63ab917 Mon Sep 17 00:00:00 2001 From: Christoph Hellwig Date: Mon, 1 Oct 2018 07:40:53 -0700 Subject: [PATCH] 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 --- 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 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)) -- 2.20.1