linux-kernel - Re: [PATCH] [v2] dma-mapping: work around clang bug

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Date:   Thu, 7 Mar 2019 09:17:08 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Arnd Bergmann <arnd@...db.de>, Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Paul Burton <paul.burton@...s.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [v2] dma-mapping: work around clang bug

Hi Arnd,

On 2019-03-07 8:52 am, Arnd Bergmann wrote:
> Clang has a rather annoying behavior of checking for integer
> arithmetic problems in code paths that are discarded by gcc
> before that perfoms the same checks.
> 
> For DMA_BIT_MASK(64), this leads to a warning despite the
> result of the macro being completely sensible:
> 
> arch/arm/plat-iop/adma.c:146:24: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
>                  .coherent_dma_mask = DMA_BIT_MASK(64),
> 
> The best workaround I could come up with is to shift the
> value twice, which makes the macro way less readable but
> always has the same result.
> 
> Link: https://bugs.llvm.org/show_bug.cgi?id=38789
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> v2: fix off-by-one error
> ---
>   include/linux/dma-mapping.h | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 75e60be91e5f..9e438fe6b130 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -138,7 +138,8 @@ struct dma_map_ops {
>   extern const struct dma_map_ops dma_virt_ops;
>   extern const struct dma_map_ops dma_dummy_ops;
>   
> -#define DMA_BIT_MASK(n)	(((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
> +/* double shift to work around https://bugs.llvm.org/show_bug.cgi?id=38789 */
> +#define DMA_BIT_MASK(n)	(((n) == 64) ? ~0ULL : ((1ULL<<((n)-1))<<1)-1)

I think that now makes DMA_BIT_MASK(0) undefined - that shouldn't matter 
in most cases, but it could potentially happen at runtime where callers 
use a non-constant argument. However, it also means we don't need to 
special-case 64 any more (since that's there to avoid the same thing 
anyway), so we could simply flip that to handle 0 instead.

FWIW I'd be very tempted to fold in the second shift as "2ULL<<((n)-1)", 
but that may not be to everyone's taste.

Robin.

>   
>   #define DMA_MASK_NONE	0x0ULL
>   
>