[PATCH] i2c: aspeed: fixed invalid clock parameters for very large divisors

[Brendan Higgins]

The function that computes clock parameters from divisors did not
respect the maximum size of the bitfields that the parameters were
written to. This fixes the bug.

This bug can be reproduced with (and this fix verified with) the test
at: https://kunit-review.googlesource.com/c/linux/+/1035/

Discovered-by-KUnit: https://kunit-review.googlesource.com/c/linux/+/1035/
Signed-off-by: Brendan Higgins <brendanhiggins at google.com>
---
 drivers/i2c/busses/i2c-aspeed.c | 38 +++++++++++++++++++++++----------
 1 file changed, 27 insertions(+), 11 deletions(-)

diff --git a/drivers/i2c/busses/i2c-aspeed.c b/drivers/i2c/busses/i2c-aspeed.c
index c258c4d9a4c0..c1c3f0a4d805 100644
--- a/drivers/i2c/busses/i2c-aspeed.c
+++ b/drivers/i2c/busses/i2c-aspeed.c
@@ -705,9 +705,18 @@ static const struct i2c_algorithm aspeed_i2c_algo = {
 #endif /* CONFIG_I2C_SLAVE */
 };
 
-static u32 aspeed_i2c_get_clk_reg_val(u32 clk_high_low_max, u32 divisor)
+static u32 aspeed_i2c_get_clk_reg_val(u32 clk_high_low_mask, u32 divisor)
 {
-	u32 base_clk, clk_high, clk_low, tmp;
+	u32 base_clk, clk_high_low_max, clk_high, clk_low, tmp;
+
+	/*
+	 * SCL_high and SCL_low represent a value 1 greater than what is stored
+	 * since a zero divider is meaningless. Thus, the max value each can
+	 * store is every bit set + 1. Since SCL_high and SCL_low are added
+	 * together (see below), the max value of both is the max value of one
+	 * them times two.
+	 */
+	clk_high_low_max = (clk_high_low_mask + 1) * 2;
 
 	/*
 	 * The actual clock frequency of SCL is:
@@ -731,15 +740,22 @@ static u32 aspeed_i2c_get_clk_reg_val(u32 clk_high_low_max, u32 divisor)
 	 */
 	base_clk = divisor > clk_high_low_max ?
 			ilog2((divisor - 1) / clk_high_low_max) + 1 : 0;
-	tmp = (divisor + (1 << base_clk) - 1) >> base_clk;
-	clk_low = tmp / 2;
-	clk_high = tmp - clk_low;
 
-	if (clk_high)
-		clk_high--;
+	if (base_clk > ASPEED_I2CD_TIME_BASE_DIVISOR_MASK) {
+		base_clk = ASPEED_I2CD_TIME_BASE_DIVISOR_MASK;
+		clk_low = clk_high_low_mask;
+		clk_high = clk_high_low_mask;
+	} else {
+		tmp = (divisor + (1 << base_clk) - 1) >> base_clk;
+		clk_low = tmp / 2;
+		clk_high = tmp - clk_low;
+
+		if (clk_high)
+			clk_high--;
 
-	if (clk_low)
-		clk_low--;
+		if (clk_low)
+			clk_low--;
+	}
 
 
 	return ((clk_high << ASPEED_I2CD_TIME_SCL_HIGH_SHIFT)
@@ -755,7 +771,7 @@ static u32 aspeed_i2c_24xx_get_clk_reg_val(u32 divisor)
 	 * clk_high and clk_low are each 3 bits wide, so each can hold a max
 	 * value of 8 giving a clk_high_low_max of 16.
 	 */
-	return aspeed_i2c_get_clk_reg_val(16, divisor);
+	return aspeed_i2c_get_clk_reg_val(GENMASK(2, 0), divisor);
 }
 
 static u32 aspeed_i2c_25xx_get_clk_reg_val(u32 divisor)
@@ -764,7 +780,7 @@ static u32 aspeed_i2c_25xx_get_clk_reg_val(u32 divisor)
 	 * clk_high and clk_low are each 4 bits wide, so each can hold a max
 	 * value of 16 giving a clk_high_low_max of 32.
 	 */
-	return aspeed_i2c_get_clk_reg_val(32, divisor);
+	return aspeed_i2c_get_clk_reg_val(GENMASK(3, 0), divisor);
 }
 
 /* precondition: bus.lock has been acquired. */
-- 
2.19.0.444.g18242da7ef-goog