[PATCH 2/3] i2c: meson: Use 50% duty cycle for I2C clock

[Lucas Tanure]

The duty cycle of 33% is less than the required
by the I2C specs for the LOW period of the SCL
clock.

Move the duty cyle to 50% for 100Khz or lower
clocks, and (40% High SCL / 60% Low SCL) duty
cycle for clocks above 100Khz.

Signed-off-by: Lucas Tanure <tanure at linux.com>
---
 drivers/i2c/busses/i2c-meson.c | 45 +++++++++++++++++++++++++---------
 1 file changed, 33 insertions(+), 12 deletions(-)

diff --git a/drivers/i2c/busses/i2c-meson.c b/drivers/i2c/busses/i2c-meson.c
index 4b4a5b2d77ab..b913ba20f06e 100644
--- a/drivers/i2c/busses/i2c-meson.c
+++ b/drivers/i2c/busses/i2c-meson.c
@@ -140,29 +140,50 @@ static void meson_i2c_add_token(struct meson_i2c *i2c, int token)
 static void meson_i2c_set_clk_div(struct meson_i2c *i2c, unsigned int freq)
 {
 	unsigned long clk_rate = clk_get_rate(i2c->clk);
-	unsigned int div;
+	unsigned int div_h, div_l;
 
-	div = DIV_ROUND_UP(clk_rate, freq);
-	div -= FILTER_DELAY;
-	div = DIV_ROUND_UP(div, i2c->data->div_factor);
+	if (freq <= 100000) {
+		div_h = DIV_ROUND_UP(clk_rate, freq);
+		div_l = DIV_ROUND_UP(div_h, 4);
+		div_h = DIV_ROUND_UP(div_h, 2) - FILTER_DELAY;
+	} else {
+	/* According to I2C-BUS Spec 2.1, in FAST-MODE, the minimum LOW period is 1.3uS, and
+	 * minimum HIGH is least 0.6us.
+	 * For 400000 freq, the period is 2.5us. To keep within the specs, give 40% of period to
+	 * HIGH and 60% to LOW. This means HIGH at 1.0us and LOW 1.5us.
+	 * The same applies for Fast-mode plus, where LOW is 0.5us and HIGH is 0.26us.
+	 * Duty = H/(H + L) = 2/5
+	 */
+		div_h = DIV_ROUND_UP(clk_rate * 2, freq * 5) - FILTER_DELAY;
+		div_l = DIV_ROUND_UP(clk_rate * 3, freq * 5 * 2);
+	}
 
 	/* clock divider has 12 bits */
-	if (div > GENMASK(11, 0)) {
+	if (div_h > GENMASK(11, 0)) {
 		dev_err(i2c->dev, "requested bus frequency too low\n");
-		div = GENMASK(11, 0);
+		div_h = GENMASK(11, 0);
+	}
+	if (div_l > GENMASK(11, 0)) {
+		dev_err(i2c->dev, "requested bus frequency too low\n");
+		div_l = GENMASK(11, 0);
 	}
 
 	meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIV_MASK,
-			   FIELD_PREP(REG_CTRL_CLKDIV_MASK, div & GENMASK(9, 0)));
+			   FIELD_PREP(REG_CTRL_CLKDIV_MASK, div_h & GENMASK(9, 0)));
 
 	meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIVEXT_MASK,
-			   FIELD_PREP(REG_CTRL_CLKDIVEXT_MASK, div >> 10));
+			   FIELD_PREP(REG_CTRL_CLKDIVEXT_MASK, div_h >> 10));
+
+
+	/* set SCL low delay */
+	meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_MASK,
+			   (div_l << REG_SLV_SCL_LOW_SHIFT) & REG_SLV_SCL_LOW_MASK);
 
-	/* Disable HIGH/LOW mode */
-	meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_EN, 0);
+	/* Enable HIGH/LOW mode */
+	meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_EN, REG_SLV_SCL_LOW_EN);
 
-	dev_dbg(i2c->dev, "%s: clk %lu, freq %u, div %u\n", __func__,
-		clk_rate, freq, div);
+	dev_dbg(i2c->dev, "%s: clk %lu, freq %u, divh %u, divl %u\n", __func__,
+		clk_rate, freq, div_h, div_l);
 }
 
 static void meson_i2c_get_data(struct meson_i2c *i2c, char *buf, int len)
-- 
2.35.1