i2c: exynos5: Calculate t_scl_l, t_scl_h according to i2c spec

Previously the duty cycle was divided equally into h_scl_l, t_scl_h.
This makes the low period of the SCL clock in Fast Mode is only 1.25us
which is way lower than the minimal value (1.3) specified in i2c
specification. In order to make sure t_scl_l, t_scl_h always fullfill
i2c specification, this commit calculates t_scl_l using this formula:

t_scl_l = clk_cycle *
    ((t_low_min + (scl_clock - t_low_min - t_high_min) / 2) / scl_clock)

where:
t_low_min is the minimal value of low period of the SCL clock in us;
t_high_min is the minimal value of high period of the SCL clock in us;
scl_clock is converted from SCL clock frequency into us.
Signed-off-by: Camel Guo <camel.guo@axis.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-exynos5.c

@@ -270,7 +270,7 @@ static void exynos5_i2c_clr_pend_irq(struct exynos5_i2c *i2c)
  * exynos5_i2c_set_timing: updates the registers with appropriate
  * timing values calculated
  *
- * Timing values for operation are calculated against either 100kHz
+ * Timing values for operation are calculated against 100kHz, 400kHz
  * or 1MHz controller operating frequency.
  *
  * Returns 0 on success, -EINVAL if the cycle length cannot
@@ -333,6 +333,23 @@ static int exynos5_i2c_set_timing(struct exynos5_i2c *i2c, bool hs_timings)
 	 *
 	 * Constraints: 4 <= temp, 0 <= CLK_DIV < 256, 2 <= clk_cycle <= 510
 	 *
+	 * To split SCL clock into low, high periods appropriately, one
+	 * proportion factor for each I2C mode is used, which is calculated
+	 * using this formula.
+	 * ```
+	 * ((t_low_min + (scl_clock - t_low_min - t_high_min) / 2) / scl_clock)
+	 * ```
+	 * where:
+	 * t_low_min is the minimal value of low period of the SCL clock in us;
+	 * t_high_min is the minimal value of high period of the SCL clock in us;
+	 * scl_clock is converted from SCL clock frequency into us.
+	 *
+	 * Below are the proportion factors for these I2C modes:
+	 *                t_low_min, t_high_min, scl_clock, proportion
+	 * Standard Mode:     4.7us,      4.0us,      10us,      0.535
+	 * Fast Mode:         1.3us,      0.6us,     2.5us,       0.64
+	 * Fast-Plus Mode:    0.5us,     0.26us,       1us,       0.62
+	 *
 	 */
 	t_ftl_cycle = (readl(i2c->regs + HSI2C_CONF) >> 16) & 0x7;
 	temp = clkin / op_clk - 8 - t_ftl_cycle;
@@ -346,8 +363,19 @@ static int exynos5_i2c_set_timing(struct exynos5_i2c *i2c, bool hs_timings)
 		return -EINVAL;
 	}
 
-	t_scl_l = clk_cycle / 2;
-	t_scl_h = clk_cycle / 2;
+	/*
+	 * Scale clk_cycle to get t_scl_l using the proption factors for individual I2C modes.
+	 */
+	if (op_clk <= I2C_MAX_STANDARD_MODE_FREQ)
+		t_scl_l = clk_cycle * 535 / 1000;
+	else if (op_clk <= I2C_MAX_FAST_MODE_FREQ)
+		t_scl_l = clk_cycle * 64 / 100;
+	else
+		t_scl_l = clk_cycle * 62 / 100;
+
+	if (t_scl_l > 0xFF)
+		t_scl_l = 0xFF;
+	t_scl_h = clk_cycle - t_scl_l;
 	t_start_su = t_scl_l;
 	t_start_hd = t_scl_l;
 	t_stop_su = t_scl_l;