linux-kernel - Re: [PATCH v5 1/1] i2c: designware: use casting of u64 in clock multiplication to avoid overflow

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Date:   Wed, 21 Dec 2022 22:07:44 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Hanna Hawa <hhhawa@...zon.com>
Cc:     wsa@...nel.org, jarkko.nikula@...ux.intel.com,
        mika.westerberg@...ux.intel.com, jsd@...ihalf.com,
        linus.walleij@...aro.org, ben-linux@...ff.org,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        dwmw@...zon.co.uk, benh@...zon.com, ronenk@...zon.com,
        talel@...zon.com, jonnyc@...zon.com, hanochu@...zon.com,
        farbere@...zon.com, itamark@...zon.com, lareine@...zon.com
Subject: Re: [PATCH v5 1/1] i2c: designware: use casting of u64 in clock
 multiplication to avoid overflow

On Wed, Dec 21, 2022 at 07:59:00PM +0000, Hanna Hawa wrote:
> From: Lareine Khawaly <lareine@...zon.com>
> 
> In functions i2c_dw_scl_lcnt() and i2c_dw_scl_hcnt() may have overflow
> by depending on the values of the given parameters including the ic_clk.
> For example in our use case where ic_clk is larger than one million,
> multiplication of ic_clk * 4700 will result in 32 bit overflow.
> 
> Add cast of u64 to the calculation to avoid multiplication overflow, and
> use the corresponding define for divide.

Perfect, thank you!
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> Fixes: 2373f6b9744d ("i2c-designware: split of i2c-designware.c into core and bus specific parts")
> Signed-off-by: Lareine Khawaly <lareine@...zon.com>
> Signed-off-by: Hanna Hawa <hhhawa@...zon.com>
> ---
> Change log v4->v5:
> - Fix indentation
> 
> Change Log v3->v4:
> - update line length when possible
> - fix change log location in the patch
> 
> Change Log v2->v3:
> - Avoid changing the ic_clk parameter to u64, and do casting in the
>   calculation itself instead.
> - i2c_dw_clk_rate() returns unsigned long which is confusing because the
>   function return the value of get_clk_rate_khz() which returns u32.
>   This is not effect the overflow issue, pushed change in separated
>   patch.
> - use DIV_ROUND_CLOSEST_ULL instead of DIV_ROUND_CLOSEST
> 
> Change Log v1->v2:
> - Update commit message and add fix tag.
> 
>  drivers/i2c/busses/i2c-designware-common.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-designware-common.c b/drivers/i2c/busses/i2c-designware-common.c
> index e0a46dfd1c15..6fdb25a5f801 100644
> --- a/drivers/i2c/busses/i2c-designware-common.c
> +++ b/drivers/i2c/busses/i2c-designware-common.c
> @@ -351,7 +351,8 @@ u32 i2c_dw_scl_hcnt(u32 ic_clk, u32 tSYMBOL, u32 tf, int cond, int offset)
>  		 *
>  		 * If your hardware is free from tHD;STA issue, try this one.
>  		 */
> -		return DIV_ROUND_CLOSEST(ic_clk * tSYMBOL, MICRO) - 8 + offset;
> +		return DIV_ROUND_CLOSEST_ULL((u64)ic_clk * tSYMBOL, MICRO) -
> +		       8 + offset;
>  	else
>  		/*
>  		 * Conditional expression:
> @@ -367,7 +368,8 @@ u32 i2c_dw_scl_hcnt(u32 ic_clk, u32 tSYMBOL, u32 tf, int cond, int offset)
>  		 * The reason why we need to take into account "tf" here,
>  		 * is the same as described in i2c_dw_scl_lcnt().
>  		 */
> -		return DIV_ROUND_CLOSEST(ic_clk * (tSYMBOL + tf), MICRO) - 3 + offset;
> +		return DIV_ROUND_CLOSEST_ULL((u64)ic_clk * (tSYMBOL + tf), MICRO) -
> +		       3 + offset;
>  }
>  
>  u32 i2c_dw_scl_lcnt(u32 ic_clk, u32 tLOW, u32 tf, int offset)
> @@ -383,7 +385,8 @@ u32 i2c_dw_scl_lcnt(u32 ic_clk, u32 tLOW, u32 tf, int offset)
>  	 * account the fall time of SCL signal (tf).  Default tf value
>  	 * should be 0.3 us, for safety.
>  	 */
> -	return DIV_ROUND_CLOSEST(ic_clk * (tLOW + tf), MICRO) - 1 + offset;
> +	return DIV_ROUND_CLOSEST_ULL((u64)ic_clk * (tLOW + tf), MICRO) -
> +	       1 + offset;
>  }
>  
>  int i2c_dw_set_sda_hold(struct dw_i2c_dev *dev)
> -- 
> 2.38.1
> 

-- 
With Best Regards,
Andy Shevchenko