i2c: mxs: Implement arbitrary clock speed derivation algorithm

This patch drops the i2c timing tables from this driver and instead
derives the timing based from the requested clock sleep. The timing
tables were completely wrong anyway when observed on a scope.

This new algorithm is also only derived by using a scope, but it seems
to produce much more accurate result.

Signed-off-by: Marek Vasut <marex@denx.de>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
[wsa: changed messages from dev_err to dev_warn]
Signed-off-by: Wolfram Sang <wolfram@the-dreams.de>

drivers/i2c/busses/i2c-mxs.c

@@ -93,35 +93,6 @@
 #define MXS_CMD_I2C_READ	(MXS_I2C_CTRL0_SEND_NAK_ON_LAST | \
 				 MXS_I2C_CTRL0_MASTER_MODE)
 
-struct mxs_i2c_speed_config {
-	uint32_t	timing0;
-	uint32_t	timing1;
-	uint32_t	timing2;
-};
-
-/*
- * Timing values for the default 24MHz clock supplied into the i2c block.
- *
- * The bus can operate at 95kHz or at 400kHz with the following timing
- * register configurations. The 100kHz mode isn't present because it's
- * values are not stated in the i.MX233/i.MX28 datasheet. The 95kHz mode
- * shall be close enough replacement. Therefore when the bus is configured
- * for 100kHz operation, 95kHz timing settings are actually loaded.
- *
- * For details, see i.MX233 [25.4.2 - 25.4.4] and i.MX28 [27.5.2 - 27.5.4].
- */
-static const struct mxs_i2c_speed_config mxs_i2c_95kHz_config = {
-	.timing0	= 0x00780030,
-	.timing1	= 0x00800030,
-	.timing2	= 0x00300030,
-};
-
-static const struct mxs_i2c_speed_config mxs_i2c_400kHz_config = {
-	.timing0	= 0x000f0007,
-	.timing1	= 0x001f000f,
-	.timing2	= 0x00300030,
-};
-
 /**
  * struct mxs_i2c_dev - per device, private MXS-I2C data
  *
@@ -137,7 +108,9 @@ struct mxs_i2c_dev {
 	struct completion cmd_complete;
 	int cmd_err;
 	struct i2c_adapter adapter;
-	const struct mxs_i2c_speed_config *speed;
+
+	uint32_t timing0;
+	uint32_t timing1;
 
 	/* DMA support components */
 	int				dma_channel;
@@ -153,9 +126,16 @@ static void mxs_i2c_reset(struct mxs_i2c_dev *i2c)
 {
 	stmp_reset_block(i2c->regs);
 
-	writel(i2c->speed->timing0, i2c->regs + MXS_I2C_TIMING0);
-	writel(i2c->speed->timing1, i2c->regs + MXS_I2C_TIMING1);
-	writel(i2c->speed->timing2, i2c->regs + MXS_I2C_TIMING2);
+	/*
+	 * Configure timing for the I2C block. The I2C TIMING2 register has to
+	 * be programmed with this particular magic number. The rest is derived
+	 * from the XTAL speed and requested I2C speed.
+	 *
+	 * For details, see i.MX233 [25.4.2 - 25.4.4] and i.MX28 [27.5.2 - 27.5.4].
+	 */
+	writel(i2c->timing0, i2c->regs + MXS_I2C_TIMING0);
+	writel(i2c->timing1, i2c->regs + MXS_I2C_TIMING1);
+	writel(0x00300030, i2c->regs + MXS_I2C_TIMING2);
 
 	writel(MXS_I2C_IRQ_MASK << 8, i2c->regs + MXS_I2C_CTRL1_SET);
 }
@@ -553,6 +533,43 @@ static bool mxs_i2c_dma_filter(struct dma_chan *chan, void *param)
 	return true;
 }
 
+static void mxs_i2c_derive_timing(struct mxs_i2c_dev *i2c, int speed)
+{
+	/* The I2C block clock run at 24MHz */
+	const uint32_t clk = 24000000;
+	uint32_t base;
+	uint16_t high_count, low_count, rcv_count, xmit_count;
+	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;
+	}
+
+	/*
+	 * 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.
+	 *
+	 * 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.
+	 */
+	base = ((clk / speed) - 38) / 2;
+	high_count = base + 3;
+	low_count = base - 3;
+	rcv_count = (high_count * 3) / 4;
+	xmit_count = low_count / 4;
+
+	i2c->timing0 = (high_count << 16) | rcv_count;
+	i2c->timing1 = (low_count << 16) | xmit_count;
+}
+
 static int mxs_i2c_get_ofdata(struct mxs_i2c_dev *i2c)
 {
 	uint32_t speed;
@@ -572,12 +589,12 @@ static int mxs_i2c_get_ofdata(struct mxs_i2c_dev *i2c)
 	}
 
 	ret = of_property_read_u32(node, "clock-frequency", &speed);
-	if (ret)
+	if (ret) {
 		dev_warn(dev, "No I2C speed selected, using 100kHz\n");
-	else if (speed == 400000)
-		i2c->speed = &mxs_i2c_400kHz_config;
-	else if (speed != 100000)
-		dev_warn(dev, "Unsupported I2C speed selected, using 100kHz\n");
+		speed = 100000;
+	}
+
+	mxs_i2c_derive_timing(i2c, speed);
 
 	return 0;
 }
@@ -621,7 +638,6 @@ static int mxs_i2c_probe(struct platform_device *pdev)
 		return err;
 
 	i2c->dev = dev;
-	i2c->speed = &mxs_i2c_95kHz_config;
 
 	init_completion(&i2c->cmd_complete);