Patchwork [v3] dma-mapping: work around clang bug

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Submitter Arnd Bergmann
Date March 7, 2019, 10:48 a.m.
Message ID <20190307104900.2506657-1-arnd@arndb.de>
Download mbox | patch
Permalink /patch/743251/
State New
Headers show

Comments

Arnd Bergmann - March 7, 2019, 10:48 a.m.
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
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
v3: use (2ull << n-1) instead of ((1ull << n-1) << 1)
    special-case 0 instead of 64
v2: fix off-by-one error
---
 include/linux/dma-mapping.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
Christoph Hellwig - March 8, 2019, 3:27 p.m.
NAK on this whole patch.  Please fix the compiler instead of making
a complete mess of a common macro.
Benjamin Serebrin via iommu - March 8, 2019, 6 p.m.
On Fri, Mar 8, 2019 at 7:27 AM Christoph Hellwig <hch@lst.de> wrote:
>
> NAK on this whole patch.  Please fix the compiler instead of making
> a complete mess of a common macro.

Definitely on the TODO list; higher priority, but not highest right
now.  I'll see if I can get some help with it.

Patch

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 75e60be91e5f..5788d60c2223 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -138,7 +138,11 @@  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))
+/*
+ * 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)
 
 #define DMA_MASK_NONE	0x0ULL