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

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Message-ID: <cc16a489-f711-0c54-8576-ef7974b3cb79@amazon.com>
Date:   Tue, 20 Dec 2022 19:43:06 +0200
From:   "Hawa, Hanna" <hhhawa@...zon.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.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 Khawaly <lareine@...zon.com>
Subject: Re: [PATCH v4 1/1] i2c: designware: use casting of u64 in clock
 multiplication to avoid overflow



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@...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.
> 
> ...
> 
>> -             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
> 
>