linux-kernel - RE: [PATCH v2] usb: uhci: Work around bogus clang shift overflow warning from DMA_BIT

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Message-ID:
 <TY2PPF5CB9A1BE6DC2C9DD1B9070425284FF2E9A@TY2PPF5CB9A1BE6.apcprd06.prod.outlook.com>
Date: Thu, 16 Oct 2025 03:27:13 +0000
From: Ryan Chen <ryan_chen@...eedtech.com>
To: Nathan Chancellor <nathan@...nel.org>, Greg Kroah-Hartman
	<gregkh@...uxfoundation.org>
CC: Alan Stern <stern@...land.harvard.edu>, Nick Desaulniers
	<nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, Justin
 Stitt <justinstitt@...gle.com>, "linux-usb@...r.kernel.org"
	<linux-usb@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: RE: [PATCH v2] usb: uhci: Work around bogus clang shift overflow
 warning from DMA_BIT_MASK(64)

> Subject: [PATCH v2] 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 only use DMA_BIT_MASK() within
> uhci_hcd_platform_probe() (i.e., function scope) to avoid this global scope
> issue, similar to the approach of commit 274f2232a94f ("usb: ehci:
> Add Aspeed AST2700 support").
> 
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2136
> Link: https://github.com/ClangBuiltLinux/linux/issues/92 [1]
> Reviewed-by: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

Acked-by: Ryan Chen <ryan_chen@...eedtech.com>

> ---
> Changes in v2:
> - Slightly rework solution to follow commit 274f2232a94f ("usb: ehci:
>   Add Aspeed AST2700 support") (Alan)
> - Adjust message to match
> - Apply Alan's Reviewed-by from v1
> - Link to v1:
> https://patch.msgid.link/20251014-usb-uhci-avoid-bogus-clang-shift-warning-v
> 1-1-826585eed055@...nel.org
> ---
>  drivers/usb/host/uhci-platform.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/usb/host/uhci-platform.c b/drivers/usb/host/uhci-platform.c
> index 37607f985cc0..5e02f2ceafb6 100644
> --- a/drivers/usb/host/uhci-platform.c
> +++ b/drivers/usb/host/uhci-platform.c
> @@ -65,13 +65,10 @@ 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 int uhci_hcd_platform_probe(struct platform_device *pdev)  {
>  	struct device_node *np = pdev->dev.of_node;
> -	const u64 *dma_mask_ptr;
> +	bool dma_mask_64 = false;
>  	struct usb_hcd *hcd;
>  	struct uhci_hcd	*uhci;
>  	struct resource *res;
> @@ -85,11 +82,11 @@ 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;
> +	if (of_device_get_match_data(&pdev->dev))
> +		dma_mask_64 = true;
> 
> -	ret = dma_coerce_mask_and_coherent(&pdev->dev, *dma_mask_ptr);
> +	ret = dma_coerce_mask_and_coherent(&pdev->dev,
> +		dma_mask_64 ? DMA_BIT_MASK(64) : DMA_BIT_MASK(32));
>  	if (ret)
>  		return ret;
> 
> @@ -200,7 +197,7 @@ 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 = (void *)1 },
>  	{}
>  };
>  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>