Re: [PATCH] iommu/dma: Fix condition check in iommu_dma_unmap_sg

Robin Murphy
2019-05-29T10:12:21Z

On 29/05/2019 09:15, Nathan Chancellor wrote:
> Clang warns:
>
> drivers/iommu/dma-iommu.c:897:6: warning: logical not is only applied to
> the left hand side of this comparison [-Wlogical-not-parentheses]
> if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) == 0)
> ^ ~~
> drivers/iommu/dma-iommu.c:897:6: note: add parentheses after the '!' to
> evaluate the comparison first
> if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) == 0)
> ^
> ( )
> drivers/iommu/dma-iommu.c:897:6: note: add parentheses around left hand
> side expression to silence this warning
> if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) == 0)
> ^
> ( )
> 1 warning generated.
>
> Judging from the rest of the commit and the conditional in
> iommu_dma_map_sg, either
>
> if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
>
> or
> if ((attrs & DMA_ATTR_SKIP_CPU_SYNC) == 0)
>
> was intended, not a combination of the two.
>
> I personally think that the former is easier to understand so use that.

Good catch, thanks Nathan. Looks like this was an unintentional mangling
while ratifying the couple of odd "x == 0" instances to "!x" in the
move. The effective double-negation is not what we want at all.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Fixes: 06d60728ff5c ("iommu/dma: move the arm64 wrappers to common code")
> Link: https://github.com/ClangBuiltLinux/linux/issues/497
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
> drivers/iommu/dma-iommu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 0cd49c2d3770..0dee374fc64a 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -894,7 +894,7 @@ static void iommu_dma_unmap_sg(struct device *dev, struct scatterlist *sg,
> struct scatterlist *tmp;
> int i;
>
> - if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) == 0)
> + if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> iommu_dma_sync_sg_for_cpu(dev, sg, nents, dir);
>
> /*
>