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

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Message-ID: <c0d1dc65-6f55-40b9-bbfa-09e8639a28e0@rowland.harvard.edu>
Date: Tue, 14 Oct 2025 23:07:27 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ryan Chen <ryan_chen@...eedtech.com>,
	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
Subject: Re: [PATCH] usb: uhci: Work around bogus clang shift overflow
 warning from DMA_BIT_MASK(64)

On Tue, Oct 14, 2025 at 04:38:19PM -0700, Nathan Chancellor wrote:
> 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>
> ---

Do you think you could instead copy the approach used in:

https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git/commit/?id=274f2232a94f6ca626d60288044e13d9a58c7612

IMO it is cleaner, and it also moves the DMA_BIT_MASK() computations 
into a function scope.

Alan Stern

>  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>
>