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

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Date:   Thu,  7 Mar 2019 09:52:37 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Robin Murphy <robin.murphy@....com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Paul Burton <paul.burton@...s.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: [PATCH] [v2] dma-mapping: work around clang bug

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>
---
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