i2c: mxs: fix broken timing calculation

The timing calculation is rather bogus and gives extremely wrong
results for higher frequencies (on an i.MX28). E.g. instead of 400 kHz
I measured 770 kHz.

Implement a calculation that adheres to the I2C spec and gives exact
results for I2C frequencies from 12.56 kHz to 960 kHz.

Also the bus_free and leadin parameters are programmed according to
the I2C spec for standard and fast mode.

This was tested on a Ka-Ro TX28 module with a DS1339, TSC2007, PCA9554
and SGTL5000 client.
Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Marek Vasut <marex@denx.de>
[wsa: patch fixes whitespace issue, too]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-mxs.c

@@ -114,6 +114,7 @@ struct mxs_i2c_dev {
 
 	uint32_t timing0;
 	uint32_t timing1;
+	uint32_t timing2;
 
 	/* DMA support components */
 	struct dma_chan			*dmach;
@@ -138,7 +139,7 @@ static int mxs_i2c_reset(struct mxs_i2c_dev *i2c)
 	 */
 	writel(i2c->timing0, i2c->regs + MXS_I2C_TIMING0);
 	writel(i2c->timing1, i2c->regs + MXS_I2C_TIMING1);
-	writel(0x00300030, i2c->regs + MXS_I2C_TIMING2);
+	writel(i2c->timing2, i2c->regs + MXS_I2C_TIMING2);
 
 	writel(MXS_I2C_IRQ_MASK << 8, i2c->regs + MXS_I2C_CTRL1_SET);
 
@@ -587,41 +588,79 @@ static const struct i2c_algorithm mxs_i2c_algo = {
 	.functionality = mxs_i2c_func,
 };
 
-static void mxs_i2c_derive_timing(struct mxs_i2c_dev *i2c, int speed)
+static void mxs_i2c_derive_timing(struct mxs_i2c_dev *i2c, uint32_t speed)
 {
-	/* The I2C block clock run at 24MHz */
+	/* The I2C block clock runs at 24MHz */
 	const uint32_t clk = 24000000;
-	uint32_t base;
+	uint32_t divider;
 	uint16_t high_count, low_count, rcv_count, xmit_count;
+	uint32_t bus_free, leadin;
 	struct device *dev = i2c->dev;
 
-	if (speed > 540000) {
-		dev_warn(dev, "Speed too high (%d Hz), using 540 kHz\n", speed);
-		speed = 540000;
-	} else if (speed < 12000) {
-		dev_warn(dev, "Speed too low (%d Hz), using 12 kHz\n", speed);
-		speed = 12000;
+	divider = DIV_ROUND_UP(clk, speed);
+
+	if (divider < 25) {
+		/*
+		 * limit the divider, so that min(low_count, high_count)
+		 * is >= 1
+		 */
+		divider = 25;
+		dev_warn(dev,
+			"Speed too high (%u.%03u kHz), using %u.%03u kHz\n",
+			speed / 1000, speed % 1000,
+			clk / divider / 1000, clk / divider % 1000);
+	} else if (divider > 1897) {
+		/*
+		 * limit the divider, so that max(low_count, high_count)
+		 * cannot exceed 1023
+		 */
+		divider = 1897;
+		dev_warn(dev,
+			"Speed too low (%u.%03u kHz), using %u.%03u kHz\n",
+			speed / 1000, speed % 1000,
+			clk / divider / 1000, clk / divider % 1000);
 	}
 
 	/*
-	 * The timing derivation algorithm. There is no documentation for this
-	 * algorithm available, it was derived by using the scope and fiddling
-	 * with constants until the result observed on the scope was good enough
-	 * for 20kHz, 50kHz, 100kHz, 200kHz, 300kHz and 400kHz. It should be
-	 * possible to assume the algorithm works for other frequencies as well.
+	 * The I2C spec specifies the following timing data:
+	 *                          standard mode  fast mode Bitfield name
+	 * tLOW (SCL LOW period)     4700 ns        1300 ns
+	 * tHIGH (SCL HIGH period)   4000 ns         600 ns
+	 * tSU;DAT (data setup time)  250 ns         100 ns
+	 * tHD;STA (START hold time) 4000 ns         600 ns
+	 * tBUF (bus free time)      4700 ns        1300 ns
 	 *
-	 * Note it was necessary to cap the frequency on both ends as it's not
-	 * possible to configure completely arbitrary frequency for the I2C bus
-	 * clock.
+	 * The hardware (of the i.MX28 at least) seems to add 2 additional
+	 * clock cycles to the low_count and 7 cycles to the high_count.
+	 * This is compensated for by subtracting the respective constants
+	 * from the values written to the timing registers.
 	 */
-	base = ((clk / speed) - 38) / 2;
-	high_count = base + 3;
-	low_count = base - 3;
-	rcv_count = (high_count * 3) / 4;
-	xmit_count = low_count / 4;
+	if (speed > 100000) {
+		/* fast mode */
+		low_count = DIV_ROUND_CLOSEST(divider * 13, (13 + 6));
+		high_count = DIV_ROUND_CLOSEST(divider * 6, (13 + 6));
+		leadin = DIV_ROUND_UP(600 * (clk / 1000000), 1000);
+		bus_free = DIV_ROUND_UP(1300 * (clk / 1000000), 1000);
+	} else {
+		/* normal mode */
+		low_count = DIV_ROUND_CLOSEST(divider * 47, (47 + 40));
+		high_count = DIV_ROUND_CLOSEST(divider * 40, (47 + 40));
+		leadin = DIV_ROUND_UP(4700 * (clk / 1000000), 1000);
+		bus_free = DIV_ROUND_UP(4700 * (clk / 1000000), 1000);
+	}
+	rcv_count = high_count * 3 / 8;
+	xmit_count = low_count * 3 / 8;
+
+	dev_dbg(dev,
+		"speed=%u(actual %u) divider=%u low=%u high=%u xmit=%u rcv=%u leadin=%u bus_free=%u\n",
+		speed, clk / divider, divider, low_count, high_count,
+		xmit_count, rcv_count, leadin, bus_free);
 
+	low_count -= 2;
+	high_count -= 7;
 	i2c->timing0 = (high_count << 16) | rcv_count;
 	i2c->timing1 = (low_count << 16) | xmit_count;
+	i2c->timing2 = (bus_free << 16 | leadin);
 }
 
 static int mxs_i2c_get_ofdata(struct mxs_i2c_dev *i2c)