i2c: meson: fixup rate calculation with filter delay

Apparently, 15 cycles of the peripheral clock are used by the controller
for sampling and filtering. Because this was not known before, the rate
calculation is slightly off.

Clean up and fix the calculation taking this filtering delay into account.

Fixes: 30021e37 ("i2c: add support for Amlogic Meson I2C controller")
Signed-off-by: Nicolas Belin <nbelin@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-meson.c

@@ -34,10 +34,8 @@
 #define REG_CTRL_ACK_IGNORE	BIT(1)
 #define REG_CTRL_STATUS		BIT(2)
 #define REG_CTRL_ERROR		BIT(3)
-#define REG_CTRL_CLKDIV_SHIFT	12
-#define REG_CTRL_CLKDIV_MASK	GENMASK(21, 12)
-#define REG_CTRL_CLKDIVEXT_SHIFT 28
-#define REG_CTRL_CLKDIVEXT_MASK	GENMASK(29, 28)
+#define REG_CTRL_CLKDIV		GENMASK(21, 12)
+#define REG_CTRL_CLKDIVEXT	GENMASK(29, 28)
 
 #define REG_SLV_ADDR		GENMASK(7, 0)
 #define REG_SLV_SDA_FILTER	GENMASK(10, 8)
@@ -46,6 +44,7 @@
 #define REG_SLV_SCL_LOW_EN	BIT(28)
 
 #define I2C_TIMEOUT_MS		500
+#define FILTER_DELAY		15
 
 enum {
 	TOKEN_END = 0,
@@ -140,19 +139,21 @@ 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;
 
-	div = DIV_ROUND_UP(clk_rate, freq * i2c->data->div_factor);
+	div = DIV_ROUND_UP(clk_rate, freq);
+	div -= FILTER_DELAY;
+	div = DIV_ROUND_UP(div, i2c->data->div_factor);
 
 	/* clock divider has 12 bits */
-	if (div >= (1 << 12)) {
+	if (div > GENMASK(11, 0)) {
 		dev_err(i2c->dev, "requested bus frequency too low\n");
-		div = (1 << 12) - 1;
+		div = GENMASK(11, 0);
 	}
 
-	meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIV_MASK,
-			   (div & GENMASK(9, 0)) << REG_CTRL_CLKDIV_SHIFT);
+	meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIV,
+			   FIELD_PREP(REG_CTRL_CLKDIV, div & GENMASK(9, 0)));
 
-	meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIVEXT_MASK,
-			   (div >> 10) << REG_CTRL_CLKDIVEXT_SHIFT);
+	meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIVEXT,
+			   FIELD_PREP(REG_CTRL_CLKDIVEXT, div >> 10));
 
 	/* Disable HIGH/LOW mode */
 	meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_EN, 0);