linux-kernel - Re: [PATCH] media: staging/ipu7: avoid division by 64-bit value

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Message-ID: <20250711092329.74e52573@pumpkin>
Date: Fri, 11 Jul 2025 09:23:29 +0100
From: David Laight <david.laight.linux@...il.com>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>, Mauro Carvalho Chehab
 <mchehab@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Bingbu Cao <bingbu.cao@...el.com>, Hans Verkuil <hverkuil@...all.nl>, Arnd
 Bergmann <arnd@...db.de>, Stanislaw Gruszka
 <stanislaw.gruszka@...ux.intel.com>, linux-media@...r.kernel.org,
 linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: staging/ipu7: avoid division by 64-bit value

On Wed,  9 Jul 2025 16:56:56 +0200
Arnd Bergmann <arnd@...nel.org> wrote:

> From: Arnd Bergmann <arnd@...db.de>
> 
> On 32-bit targets, this causes a link failure:
> 
> x86_64-linux-ld: drivers/staging/media/ipu7/ipu7-isys-csi-phy.o: in function `ipu7_isys_phy_config':
> ipu7-isys-csi-phy.c:(.text+0x1509): undefined reference to `__udivdi3'
> 
> Note that this does not divide a 64-bit number by a 32-bit one as usual,
> but the other way round, which is something that the compiler should
> really be able to figure out but does not (as of gcc-15).
> 
> A few lines higher, a similar division is done using the incorrect div_u64()
> that truncates the 64-bit divisor to 32 bits.
> 
> Change both to use the safe but slow div64_u64() helper.
> 
> Fixes: a516d36bdc3d ("media: staging/ipu7: add IPU7 input system device driver")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/staging/media/ipu7/ipu7-isys-csi-phy.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/staging/media/ipu7/ipu7-isys-csi-phy.c b/drivers/staging/media/ipu7/ipu7-isys-csi-phy.c
> index 4407750c7f40..b8c5db7ae300 100644
> --- a/drivers/staging/media/ipu7/ipu7-isys-csi-phy.c
> +++ b/drivers/staging/media/ipu7/ipu7-isys-csi-phy.c
> @@ -734,6 +734,7 @@ static void ipu7_isys_cphy_config(struct ipu7_isys *isys, u8 id, u8 lanes,
>  	u16 reg;
>  	u16 val;
>  	u32 i;
> +	u64 r64;
>  	u32 r;
>  
>  	if (is_ipu7p5(isys->adev->isp->hw_ver))
> @@ -806,8 +807,8 @@ static void ipu7_isys_cphy_config(struct ipu7_isys *isys, u8 id, u8 lanes,
>  		dwc_phy_write_mask(isys, id, reg, 2, 0, 2);
>  	}
>  
> -	deass_thresh = (u16)div_u64_rem(7 * 1000 * 6, mbps * 5U, &r) + 1;
> -	if (r != 0)
> +	deass_thresh = (u16)div64_u64_rem(7 * 1000 * 6, mbps * 5U, &r64) + 1;
> +	if (r64 != 0)
>  		deass_thresh++;

While 'mbps' is presumably u64, it can't be big.
So this can just be:
	deass_threas = roundup(7 * 1000 * 6 / 5, (u32)mbps) + 1;


>  
>  	reg = CORE_DIG_RW_TRIO0_2;
> @@ -815,8 +816,7 @@ static void ipu7_isys_cphy_config(struct ipu7_isys *isys, u8 id, u8 lanes,
>  		dwc_phy_write_mask(isys, id, reg + 0x400 * i,
>  				   deass_thresh, 0, 7);
>  
> -	delay_thresh =
> -		((224U - (9U * 7U)) * 1000U) / (5U * mbps) - 7U;
> +	delay_thresh = div64_u64((224U - (9U * 7U)) * 1000U, 5U * mbps) - 7u;

That one just needs a (u32) cast, although the 5 can be moved.

	David

>  
>  	if (delay_thresh < 1)
>  		delay_thresh = 1;