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

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Date:   Tue, 20 Dec 2022 16:48:06 +0000
From:   Hanna Hawa <hhhawa@...zon.com>
To:     <wsa@...nel.org>, <jarkko.nikula@...ux.intel.com>,
        <andriy.shevchenko@...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>
CC:     <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>,
        Hanna Hawa <hhhawa@...zon.com>
Subject: [PATCH v4 1/1] i2c: designware: use casting of u64 in clock multiplication to avoid overflow

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.

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