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

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Message-ID: <CAK8P3a3=GcE1bN1HNwoMVG5heKfAipFP7+a4pRgnF=QbfiBBGA@mail.gmail.com>
Date:   Thu, 7 Mar 2019 09:50:27 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Robin Murphy <robin.murphy@....com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Paul Burton <paul.burton@...s.com>,
        Linux IOMMU <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dma-mapping: work around clang bug

On Thu, Mar 7, 2019 at 9:28 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> On Thu, Mar 7, 2019 at 9:01 AM Arnd Bergmann <arnd@...db.de> 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>
> > ---
> >  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..380d3a95d02e 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))
>
> The second "-1" should be done on the final result, not on the
> intermediate value.

Ah, of course. I'll send an update patch in a bit, sorry about this.

      Arnd