linux-kernel - [PATCH] usb: uhci: Work around bogus clang shift overflow warning from DMA_BIT

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Message-Id: <20251014-usb-uhci-avoid-bogus-clang-shift-warning-v1-1-826585eed055@kernel.org>
Date: Tue, 14 Oct 2025 16:38:19 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Alan Stern <stern@...land.harvard.edu>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 Ryan Chen <ryan_chen@...eedtech.com>
Cc: Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] usb: uhci: Work around bogus clang shift overflow warning
 from DMA_BIT_MASK(64)

After commit 18a9ec886d32 ("usb: uhci: Add Aspeed AST2700 support"),
clang incorrectly warns:

  In file included from drivers/usb/host/uhci-hcd.c:855:
  drivers/usb/host/uhci-platform.c:69:32: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
     69 | static const u64 dma_mask_64 = DMA_BIT_MASK(64);
        |                                ^~~~~~~~~~~~~~~~
  include/linux/dma-mapping.h:93:54: note: expanded from macro 'DMA_BIT_MASK'
     93 | #define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
        |                                                      ^ ~~~

clang has a long outstanding and complicated problem [1] with generating
a proper control flow graph at global scope, resulting in it being
unable to understand that this shift can never happen due to the
'n == 64' check.

Restructure the code to do the DMA_BIT_MASK() assignments within
uhci_hcd_platform_probe() (i.e., function scope) to avoid this global
scope issue.

Closes: https://github.com/ClangBuiltLinux/linux/issues/2136
Link: https://github.com/ClangBuiltLinux/linux/issues/92 [1]
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 drivers/usb/host/uhci-platform.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/usb/host/uhci-platform.c b/drivers/usb/host/uhci-platform.c
index 37607f985cc0..f532d3a0acbf 100644
--- a/drivers/usb/host/uhci-platform.c
+++ b/drivers/usb/host/uhci-platform.c
@@ -65,16 +65,16 @@ static const struct hc_driver uhci_platform_hc_driver = {
 	.hub_control =		uhci_hub_control,
 };
 
-static const u64 dma_mask_32 = DMA_BIT_MASK(32);
-static const u64 dma_mask_64 = DMA_BIT_MASK(64);
+static const bool use_dma_mask_64 = true;
 
 static int uhci_hcd_platform_probe(struct platform_device *pdev)
 {
 	struct device_node *np = pdev->dev.of_node;
-	const u64 *dma_mask_ptr;
+	u64 dma_mask = DMA_BIT_MASK(32);
 	struct usb_hcd *hcd;
 	struct uhci_hcd	*uhci;
 	struct resource *res;
+	const bool *of_data;
 	int ret;
 
 	if (usb_disabled())
@@ -85,11 +85,11 @@ static int uhci_hcd_platform_probe(struct platform_device *pdev)
 	 * Since shared usb code relies on it, set it here for now.
 	 * Once we have dma capability bindings this can go away.
 	 */
-	dma_mask_ptr = (u64 *)of_device_get_match_data(&pdev->dev);
-	if (!dma_mask_ptr)
-		dma_mask_ptr = &dma_mask_32;
+	of_data = of_device_get_match_data(&pdev->dev);
+	if (of_data && *of_data == use_dma_mask_64)
+		dma_mask = DMA_BIT_MASK(64);
 
-	ret = dma_coerce_mask_and_coherent(&pdev->dev, *dma_mask_ptr);
+	ret = dma_coerce_mask_and_coherent(&pdev->dev, dma_mask);
 	if (ret)
 		return ret;
 
@@ -200,7 +200,7 @@ static void uhci_hcd_platform_shutdown(struct platform_device *op)
 static const struct of_device_id platform_uhci_ids[] = {
 	{ .compatible = "generic-uhci", },
 	{ .compatible = "platform-uhci", },
-	{ .compatible = "aspeed,ast2700-uhci", .data = &dma_mask_64},
+	{ .compatible = "aspeed,ast2700-uhci", .data = &use_dma_mask_64 },
 	{}
 };
 MODULE_DEVICE_TABLE(of, platform_uhci_ids);

---
base-commit: 877c80dfbf788e57a3338627899033b7007037ee
change-id: 20251014-usb-uhci-avoid-bogus-clang-shift-warning-a80166a24472

Best regards,
--  
Nathan Chancellor <nathan@...nel.org>