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

[Arnd Bergmann]

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@xxxxxxxx>
---
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)
 
 #define DMA_MASK_NONE	0x0ULL
 
-- 
2.20.0