Re: [PATCH 2/2] ARM: dma-mapping: allow larger DMA mask than supported

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

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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