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

Hanna Hawa
2022-12-20T16:48:27Z

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 v3->v4:
- update line length when possible
- fix change log location in the patch

Change Log v2->v3:
- Avoid changing the ic_clk parameter to u64, and do casting in the
calculation itself instead.
- i2c_dw_clk_rate() returns unsigned long which is confusing because the
function return the value of get_clk_rate_khz() which returns u32.
This is not effect the overflow issue, pushed change in separated
patch.
- 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..2a669da08762 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;
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;
}

int i2c_dw_set_sda_hold(struct dw_i2c_dev *dev)
--
2.38.1