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

[Hawa, Hanna]

On 12/20/2022 7:11 PM, Andy Shevchenko wrote:
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>
> On Tue, Dec 20, 2022 at 04:48:06PM +0000, Hanna Hawa wrote:
> > From: Lareine Khawaly <lareine@xxxxxxxxxx>
> >
> > 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