linux-kernel - Re: [PATCH] iommu/dma: Fix condition check in iommu_dma_unmap

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Message-ID: <e8c21972-cb70-34ff-949d-5e2555c3a1fb@arm.com>
Date:   Wed, 29 May 2019 11:12:16 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Joerg Roedel <joro@...tes.org>, Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] iommu/dma: Fix condition check in iommu_dma_unmap_sg

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@....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@...il.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);
>   
>   	/*
>