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 <h...@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;
> }
>
