Messages in this thread | | | Subject | Re: [PATCH] [v2] dma-mapping: work around clang bug | From | Robin Murphy <> | Date | Thu, 7 Mar 2019 09:17:08 +0000 |
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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@arndb.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 > >
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