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 <a...@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
