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From: Arnd Bergmann <arnd@arndb.de>
To: Christoph Hellwig <hch@lst.de>,
	Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Robin Murphy <robin.murphy@arm.com>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Paul Burton <paul.burton@mips.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org
Subject: [PATCH] [v2] dma-mapping: work around clang bug
Date: Thu,  7 Mar 2019 09:52:37 +0100	[thread overview]
Message-ID: <20190307085246.1477426-1-arnd@arndb.de> (raw)

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@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)
 
 #define DMA_MASK_NONE	0x0ULL
 
-- 
2.20.0


WARNING: multiple messages have this Message-ID (diff)
From: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>
To: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>,
	Marek Szyprowski
	<m.szyprowski-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>
Cc: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>,
	Nick Desaulniers
	<ndesaulniers-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
	iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org,
	linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Paul Burton <paul.burton-8NJIiSa5LzA@public.gmane.org>,
	Geert Uytterhoeven
	<geert-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.org>,
	Jesper Dangaard Brouer
	<brouer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
Subject: [PATCH] [v2] dma-mapping: work around clang bug
Date: Thu,  7 Mar 2019 09:52:37 +0100	[thread overview]
Message-ID: <20190307085246.1477426-1-arnd@arndb.de> (raw)

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-r2nGTMty4D4@public.gmane.org>
---
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

             reply	other threads:[~2019-03-07  8:53 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-07  8:52 Arnd Bergmann [this message]
2019-03-07  8:52 ` [PATCH] [v2] dma-mapping: work around clang bug 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
2019-03-07  9:43     ` Robin Murphy
2019-03-07  9:43       ` Robin Murphy

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