Re: [PATCH v3 1/2] dmaengine: tegra210-adma: Fix build error due to 64-by-32 division

[Thierry Reding]

On Thu, Jan 16, 2025 at 09:50:32PM +0530, Mohan Kumar D wrote:
> Kernel test robot reported the build errors on 32-bit platforms due to
> plain 64-by-32 division. Following build erros were reported.
> 
>    "ERROR: modpost: "__udivdi3" [drivers/dma/tegra210-adma.ko] undefined!
>     ld: drivers/dma/tegra210-adma.o: in function `tegra_adma_probe':
>     tegra210-adma.c:(.text+0x12cf): undefined reference to `__udivdi3'"
> 
> This can be fixed by using lower_32_bits() for the adma address space as
> the offset is constrained to the lower 32 bits
> 
> Fixes: 68811c928f88 ("dmaengine: tegra210-adma: Support channel page")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202412250204.GCQhdKe3-lkp@xxxxxxxxx/
> Signed-off-by: Mohan Kumar D <mkumard@xxxxxxxxxx>
> ---
>  drivers/dma/tegra210-adma.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dma/tegra210-adma.c b/drivers/dma/tegra210-adma.c
> index 6896da8ac7ef..258220c9cb50 100644
> --- a/drivers/dma/tegra210-adma.c
> +++ b/drivers/dma/tegra210-adma.c
> @@ -887,7 +887,8 @@ static int tegra_adma_probe(struct platform_device *pdev)
>  	const struct tegra_adma_chip_data *cdata;
>  	struct tegra_adma *tdma;
>  	struct resource *res_page, *res_base;
> -	int ret, i, page_no;
> +	unsigned int page_no, page_offset;
> +	int ret, i;
>  
>  	cdata = of_device_get_match_data(&pdev->dev);
>  	if (!cdata) {
> @@ -914,9 +915,16 @@ static int tegra_adma_probe(struct platform_device *pdev)
>  
>  		res_base = platform_get_resource_byname(pdev, IORESOURCE_MEM, "global");
>  		if (res_base) {
> -			page_no = (res_page->start - res_base->start) / cdata->ch_base_offset;
> -			if (page_no <= 0)
> +			if (WARN_ON(lower_32_bits(res_page->start) <=
> +						lower_32_bits(res_base->start)))

Don't we technically also want to check that

	res_page->start <= res_base->start

because otherwise people might put in something that's completely out of
range? I guess maybe you could argue that the DT is then just broken,
but since we're checking anyway, might as well check for all corner
cases.

Thierry

> +				return -EINVAL;
> +
> +			page_offset = lower_32_bits(res_page->start) -
> +						lower_32_bits(res_base->start);
> +			page_no = page_offset / cdata->ch_base_offset;
> +			if (page_no == 0)
>  				return -EINVAL;
> +
>  			tdma->ch_page_no = page_no - 1;
>  			tdma->base_addr = devm_ioremap_resource(&pdev->dev, res_base);
>  			if (IS_ERR(tdma->base_addr))
> -- 
> 2.25.1
>

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