linux-kernel - [PATCH] dma-mapping: Allow use of DMA_BIT

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Message-Id: <20251030-james-fix-dma_bit_mask-v1-1-ad1ce7cfab6e@linaro.org>
Date: Thu, 30 Oct 2025 14:05:27 +0000
From: James Clark <james.clark@...aro.org>
To: Marek Szyprowski <m.szyprowski@...sung.com>, 
 Robin Murphy <robin.murphy@....com>, Nathan Chancellor <nathan@...nel.org>, 
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 Stephen Rothwell <sfr@...b.auug.org.au>, Mark Brown <broonie@...nel.org>, 
 Vinod Koul <vkoul@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
 Guodong Xu <guodong@...cstar.com>
Cc: iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, James Clark <james.clark@...aro.org>
Subject: [PATCH] dma-mapping: Allow use of DMA_BIT_MASK(64) in global scope

Clang doesn't like that (1ULL<<(64)) overflows when initializing a
global scope variable, even if that part of the ternary isn't used when
n = 64. The same initialization can be done without warnings in function
scopes, and GCC doesn't mind either way.

The build failure that highlighted this was already fixed in a different
way [1], which also has detailed links to the Clang issues. However it's
not going to be long before the same thing happens again, so it's better
to fix the root cause.

Fix it by using GENMASK_ULL() which does exactly the same thing, is much
more readable anyway, and doesn't have a shift that overflows.

[1]: https://lore.kernel.org/all/20250918-mmp-pdma-simplify-dma-addressing-v1-1-5c2be2b85696@riscstar.com/

Signed-off-by: James Clark <james.clark@...aro.org>
---
 include/linux/dma-mapping.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 8248ff9363ee..2ceda49c609f 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -90,7 +90,7 @@
  */
 #define DMA_MAPPING_ERROR		(~(dma_addr_t)0)
 
-#define DMA_BIT_MASK(n)	(((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
+#define DMA_BIT_MASK(n)	GENMASK_ULL(n - 1, 0)
 
 struct dma_iova_state {
 	dma_addr_t addr;

---
base-commit: e53642b87a4f4b03a8d7e5f8507fc3cd0c595ea6
change-id: 20251030-james-fix-dma_bit_mask-624dbeb89afa

Best regards,
-- 
James Clark <james.clark@...aro.org>