Re: [PATCH 5/5] dma-direct: always allow dma mask <= physiscal memory size

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2018-09-20 at 20:52 +0200, Christoph Hellwig wrote:
> This way an architecture with less than 4G of RAM can support dma_mask
> smaller than 32-bit without a ZONE_DMA.  Apparently that is a common
> case on powerpc.

Anything that uses a b43 wifi adapter which has a 31-bit limitation
actually :-)
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  kernel/dma/direct.c | 28 ++++++++++++++++------------
>  1 file changed, 16 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 64466b7ef67b..d1e103c6b107 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -4,6 +4,7 @@
>   *
>   * DMA operations that map physical memory directly without using an IOMMU.
>   */
> +#include <linux/bootmem.h> /* for max_pfn */
>  #include <linux/export.h>
>  #include <linux/mm.h>
>  #include <linux/dma-direct.h>
> @@ -283,21 +284,24 @@ int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
>  	return nents;
>  }
>  
> +/*
> + * Because 32-bit DMA masks are so common we expect every architecture to be
> + * able to satisfy them - either by not supporting more physical memory, or by
> + * providing a ZONE_DMA32.  If neither is the case, the architecture needs to
> + * use an IOMMU instead of the direct mapping.
> + */
>  int dma_direct_supported(struct device *dev, u64 mask)
>  {
> -#ifdef CONFIG_ZONE_DMA
> -	if (mask < phys_to_dma(dev, DMA_BIT_MASK(ARCH_ZONE_DMA_BITS)))
> -		return 0;
> -#else
> -	/*
> -	 * Because 32-bit DMA masks are so common we expect every architecture
> -	 * to be able to satisfy them - either by not supporting more physical
> -	 * memory, or by providing a ZONE_DMA32.  If neither is the case, the
> -	 * architecture needs to use an IOMMU instead of the direct mapping.
> -	 */
> -	if (mask < phys_to_dma(dev, DMA_BIT_MASK(32)))
> +	u64 min_mask;
> +
> +	if (IS_ENABLED(CONFIG_ZONE_DMA))
> +		min_mask = DMA_BIT_MASK(ARCH_ZONE_DMA_BITS);
> +	else
> +		min_mask = min_t(u64, DMA_BIT_MASK(32),
> +				 (max_pfn - 1) << PAGE_SHIFT);
> +
> +	if (mask >= phys_to_dma(dev, min_mask))
>  		return 0;

nitpick ... to be completely "correct", I would have written

	if (IS_ENABLED(CONFIG_ZONE_DMA))
		min_mask = DMA_BIT_MASK(ARCH_ZONE_DMA_BITS);
	else
		min_mask = DMA_BIT_MASK(32);

	min_mask = min_t(u64, min_mask,	(max_pfn - 1) << PAGE_SHIFT);

In "theory" it's also ok to have a mask < ZONE_DMA_BITS as long as it's
big enough to fit all memory :-)
	
> -#endif
>  	return 1;
>  }
>