Re: [PATCH] dma-direct: document the zone selection logic

[Christoph Hellwig <hch@lst.de>]

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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