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SubjectRe: [PATCH 2/2] ARM: dma-mapping: allow larger DMA mask than supported
On Tue, May 21, 2019 at 02:47:29PM +0200, Christoph Hellwig wrote:
> Since Linux 5.1 we allow drivers to just set the largest DMA mask they
> support instead of falling back to smaller ones.

This doesn't make sense. "they" is confusing - why would a driver set
a DMA mask larger than the driver supports? Or is "they" not
referring to the drivers (in which case, what is it referring to?)

> When fixing up all the dma ops instances to allow for this behavior
> the arm direct mapping code was missed. Fix it up by removing the
> sanity check, as all the actual mapping code handles this case just
> fine.
>
> Fixes: 9eb9e96e97b3 ("Documentation/DMA-API-HOWTO: update dma_mask sections")
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> arch/arm/mm/dma-mapping.c | 20 +-------------------
> 1 file changed, 1 insertion(+), 19 deletions(-)
>
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index 0a75058c11f3..bdf0d236aaee 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -219,25 +219,7 @@ EXPORT_SYMBOL(arm_coherent_dma_ops);
>
> static int __dma_supported(struct device *dev, u64 mask, bool warn)
> {
> - unsigned long max_dma_pfn;
> -
> - /*
> - * If the mask allows for more memory than we can address,
> - * and we actually have that much memory, then we must
> - * indicate that DMA to this device is not supported.
> - */
> - if (sizeof(mask) != sizeof(dma_addr_t) &&
> - mask > (dma_addr_t)~0 &&
> - dma_to_pfn(dev, ~0) < max_pfn - 1) {
> - if (warn) {
> - dev_warn(dev, "Coherent DMA mask %#llx is larger than dma_addr_t allows\n",
> - mask);
> - dev_warn(dev, "Driver did not use or check the return value from dma_set_coherent_mask()?\n");
> - }
> - return 0;
> - }

The point of this check is to trap the case where we have, for example,
8GB of memory, but dma_addr_t is 32-bit. We can allocate in the high
4GB, but we can't represent the address in a dma_addr_t.

> -
> - max_dma_pfn = min(max_pfn, arm_dma_pfn_limit);
> + unsigned long max_dma_pfn = min(max_pfn, arm_dma_pfn_limit);
>
> /*
> * Translate the device's DMA mask to a PFN limit. This
> --
> 2.20.1
>
>

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