From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Robin Murphy <robin.murphy@arm.com>,
Christoph Hellwig <hch@lst.de>,
Marek Szyprowski <m.szyprowski@samsung.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Jesper Dangaard Brouer <brouer@redhat.com>,
Paul Burton <paul.burton@mips.com>,
"open list:IOMMU DRIVERS" <iommu@lists.linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] [v2] dma-mapping: work around clang bug
Date: Thu, 7 Mar 2019 10:34:36 +0100 [thread overview]
Message-ID: <CAMuHMdXmd9NURXvjVQeUe06QQP=degkY+VNUAXwO5eHDm3Lh0w@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a19dyTxE2o9KAw3G1qH8p4qXO-dhhc885Z2_w9BDSoFHQ@mail.gmail.com>
On Thu, Mar 7, 2019 at 10:28 AM Arnd Bergmann <arnd@arndb.de> wrote:
> On Thu, Mar 7, 2019 at 10:17 AM Robin Murphy <robin.murphy@arm.com> wrote:
> > On 2019-03-07 8:52 am, Arnd Bergmann wrote:
> > >
> > > -#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.
>
> Yes, good idea.
>
> > 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.
>
> I like that. So shall we do this?
>
> /*
> * Shifting '2' instead of '1' because of
> * https://bugs.llvm.org/show_bug.cgi?id=38789
> */
> #define DMA_BIT_MASK(n) (((n) == 0) ? 0ULL : ((2ULL<<((n)-1)))-1)
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
next prev parent reply other threads:[~2019-03-07 9:34 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-07 8:52 [PATCH] [v2] dma-mapping: work around clang bug Arnd Bergmann
2019-03-07 8:52 ` Arnd Bergmann
2019-03-07 9:17 ` Robin Murphy
2019-03-07 9:17 ` Robin Murphy
2019-03-07 9:28 ` Arnd Bergmann
2019-03-07 9:34 ` Geert Uytterhoeven [this message]
2019-03-07 9:43 ` Robin Murphy
2019-03-07 9:43 ` Robin Murphy
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