[v3,1/1] i2c: designware: use casting of u64 in clock multiplication to avoid overflow

From: Lareine Khawaly <lareine@amazon.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.

Fixes: 2373f6b9744d ("i2c-designware: split of i2c-designware.c into core and bus specific parts")
Signed-off-by: Lareine Khawaly <lareine@amazon.com>
Signed-off-by: Hanna Hawa <hhhawa@amazon.com>

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(-)

On Mon, Dec 19, 2022 at 05:17:13PM +0000, Hanna Hawa wrote:
> From: Lareine Khawaly <lareine@amazon.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@amazon.com>
> Signed-off-by: Hanna Hawa <hhhawa@amazon.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.

On 12/20/2022 12:47 PM, Andy Shevchenko wrote:
>> 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?
> 

Sure will add to the changelog, it's not required to have the second 
change to avoid the overflow.

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

Thanks for the info. will update.

> 
>> Change Log v2->v3:
.
.
>> -             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.

Ack will be fixed