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

[Robin Murphy]

On 2019-03-07 9:28 am, Arnd Bergmann wrote:
> On Thu, Mar 7, 2019 at 10:17 AM Robin Murphy <robin.murphy@xxxxxxx> 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)

Neat - it was too early in the morning for me to think of a succinct way 
to comment it, but that's great. I suspect there may be a redundant set 
of parentheses around the shift still, but other than that,

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

Cheers,
Robin.