linux-kernel - Re: [PATCH v3 1/1] i2c: designware: use casting of u64 in clock multiplication to avoid overflow

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Date:   Tue, 20 Dec 2022 12:47:19 +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,
        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 v3 1/1] i2c: designware: use casting of u64 in clock
 multiplication to avoid overflow

On Mon, Dec 19, 2022 at 05:17:13PM +0000, Hanna Hawa wrote:
> From: Lareine Khawaly <lareine@...zon.com>

Thank you for an update, my comments below.

> 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.
> 
> Fixes: 2373f6b9744d ("i2c-designware: split of i2c-designware.c into core and bus specific parts")

It's not clear if the second patch you sent (about unsigned long --> u32) is
required or not, can you clarify this in the changelog?

> Signed-off-by: Lareine Khawaly <lareine@...zon.com>
> Signed-off-by: Hanna Hawa <hhhawa@...zon.com>

This should be last part of the message body. The cutter '---' line makes it
so, currently you finish your message with a changelog and not tags. So, you
need to move the changelog after the cutter line.

> Change Log v2->v3:
> - Avoid changing the ic_clk parameter to u64, and do casting in the
>   calculation itself instead.
> - 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..9cc02d0142df 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;

There is still a room for 'MICRO) -' part on the previous line.
Ditto for the similar cases.

-- 
With Best Regards,
Andy Shevchenko