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SubjectRe: [PATCH] dma-direct: document the zone selection logic
Ben, does this resolve your issues with the confusing zone selection?

On Mon, Oct 01, 2018 at 01:10:16PM -0700, Christoph Hellwig wrote:
> 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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> iommu@lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu
---end quoted text---

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