Re: [PATCH] ARM: dma-api: fix max_pfn off-by-one error in __dma_supported()

[Robin Murphy]

On 11/12/2019 10:41 am, Chen-Yu Tsai wrote:
> From: Chen-Yu Tsai <wens@xxxxxxxx>
>
> max_pfn, as set in arch/arm/mm/init.c:
>
>      static void __init find_limits(unsigned long *min,
> 				   unsigned long *max_low,
> 				   unsigned long *max_high)
>      {
> 	    *max_low = PFN_DOWN(memblock_get_current_limit());
> 	    *min = PFN_UP(memblock_start_of_DRAM());
> 	    *max_high = PFN_DOWN(memblock_end_of_DRAM());
>      }
>
> with memblock_end_of_DRAM() pointing to the next byte after DRAM. As
> such, max_pfn points to the PFN after the end of DRAM.
>
> Thus when using max_pfn to check DMA masks, we should subtract one
> when checking DMA ranges against it.
>
> Commit 8bf1268f48ad ("ARM: dma-api: fix off-by-one error in
> __dma_supported()") fixed the same issue, but missed this spot.
>
> This issue was found while working on the sun4i-csi v4l2 driver on the
> Allwinner R40 SoC. On Allwinner SoCs, DRAM is offset at 0x40000000,
> and we are starting to use of_dma_configure() with the "dma-ranges"
> property in the device tree to have the DMA API handle the offset.
>
> In this particular instance, dma-ranges was set to the same range as
> the actual available (2 GiB) DRAM. The following error appeared when
> the driver attempted to allocate a buffer:
>
>      sun4i-csi 1c09000.csi: Coherent DMA mask 0x7fffffff (pfn 0x40000-0xc0000)
>      covers a smaller range of system memory than the DMA zone pfn 0x0-0xc0001
>      sun4i-csi 1c09000.csi: dma_alloc_coherent of size 307200 failed
>
> Fixing the off-by-one error makes things work.
>
> Fixes: 11a5aa32562e ("ARM: dma-mapping: check DMA mask against available memory")
> Fixes: 9f28cde0bc64 ("ARM: another fix for the DMA mapping checks")
> Fixes: ab746573c405 ("ARM: dma-mapping: allow larger DMA mask than supported")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Chen-Yu Tsai <wens@xxxxxxxx>
> ---
>   arch/arm/mm/dma-mapping.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index e822af0d9219..f4daafdbac56 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -227,12 +227,12 @@ static int __dma_supported(struct device *dev, u64 mask, bool warn)
>   	 * Translate the device's DMA mask to a PFN limit.  This
>   	 * PFN number includes the page which we can DMA to.
>   	 */
> -	if (dma_to_pfn(dev, mask) < max_dma_pfn) {
> +	if (dma_to_pfn(dev, mask) < max_dma_pfn - 1) {

I think this correction actually wants to happen a couple of lines up in 
the definition:

	unsigned long max_dma_pfn = min(max_pfn, arm_dma_pfn_limit);

max_pfn is indeed an exclusive limit, but AFAICS arm_dma_pfn_limit is 
inclusive, so none of these "+1"s and "-1"s can be entirely right for 
both cases.

Robin.

>   		if (warn)
>   			dev_warn(dev, "Coherent DMA mask %#llx (pfn %#lx-%#lx) covers a smaller range of system memory than the DMA zone pfn 0x0-%#lx\n",
>   				 mask,
>   				 dma_to_pfn(dev, 0), dma_to_pfn(dev, mask) + 1,
> -				 max_dma_pfn + 1);
> +				 max_dma_pfn);
>   		return 0;
>   	}
>