Re: [PATCH v4 1/1] i2c: designware: use casting of u64 in clock multiplication to avoid overflow

Hawa, Hanna
2022-12-20T17:43:24Z

On 12/20/2022 7:11 PM, Andy Shevchenko wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>
>
>
> On Tue, Dec 20, 2022 at 04:48:06PM +0000, Hanna Hawa wrote:
>> 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.
>
> ...
>
>> - return DIV_ROUND_CLOSEST(ic_clk * tSYMBOL, MICRO) - 8 + offset;
>> + return DIV_ROUND_CLOSEST_ULL((u64)ic_clk * tSYMBOL, MICRO) - 8 +
>> + offset;
>
> Broken indentation.
>
> ...
>
>> - return DIV_ROUND_CLOSEST(ic_clk * (tSYMBOL + tf), MICRO) - 3 + offset;
>> + return DIV_ROUND_CLOSEST_ULL((u64)ic_clk * (tSYMBOL + tf),
>> + MICRO) - 3 + offset;
>
> I would still go with 'MICRO) -' part to be on the previous line despite being
> over 80, this is logical split which increases readability.

Okay.. will move the 'MICRO) -' one line before
>
>> - return DIV_ROUND_CLOSEST(ic_clk * (tLOW + tf), MICRO) - 1 + offset;
>> + return DIV_ROUND_CLOSEST_ULL((u64)ic_clk * (tLOW + tf), MICRO) - 1 +
>> + offset;
>
> Broken indentation.

Why it's broken indentation? I'm asking to know for the next time. The
word 'offset' is not part of DIV_ROUND_CLOSEST_ULL parentheses. In wrong
indentation the checkpatch shout about it, but it didn't happen with the
above.

Does the below the correct indentation?

--- 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;
}

>
> --
> With Best Regards,
> Andy Shevchenko
>
>