mxs: i2c: Implement algorithm to set up arbitrary i2c speed

This algorithm computes the values of TIMING{0,1,2} registers for the
MX28 I2C block. This algorithm was derived by using a scope, but the
result seems correct.

The resulting values programmed into the registers do not correlate
with the contents in datasheet. When using the values from the datasheet,
the I2C clock were completely wrong.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Wolfgang Denk <wd@denx.de>

arch/arm/cpu/arm926ejs/mxs/clock.c

@@ -333,6 +333,8 @@ uint32_t mxc_get_clock(enum mxc_clock clk)
 		return mx28_get_sspclk(MXC_SSPCLK2);
 	case MXC_SSP3_CLK:
 		return mx28_get_sspclk(MXC_SSPCLK3);
+	case MXC_XTAL_CLK:
+		return XTAL_FREQ_KHZ * 1000;
 	}
 
 	return 0;

arch/arm/include/asm/arch-mxs/clock.h

@@ -35,6 +35,7 @@ enum mxc_clock {
 	MXC_SSP1_CLK,
 	MXC_SSP2_CLK,
 	MXC_SSP3_CLK,
+	MXC_XTAL_CLK,
 };
 
 enum mxs_ioclock {

drivers/i2c/mxs_i2c.c

@@ -28,6 +28,7 @@
 
 #include <common.h>
 #include <malloc.h>
+#include <i2c.h>
 #include <asm/errno.h>
 #include <asm/io.h>
 #include <asm/arch/clock.h>
@@ -213,62 +214,39 @@ int i2c_probe(uchar chip)
 	return ret;
 }
 
-static struct mxs_i2c_speed_table {
-	uint32_t	speed;
-	uint32_t	timing0;
-	uint32_t	timing1;
-} mxs_i2c_tbl[] = {
-	{
-		100000,
-		(0x0078 << I2C_TIMING0_HIGH_COUNT_OFFSET) |
-		(0x0030 << I2C_TIMING0_RCV_COUNT_OFFSET),
-		(0x0080 << I2C_TIMING1_LOW_COUNT_OFFSET) |
-		(0x0030 << I2C_TIMING1_XMIT_COUNT_OFFSET)
-	},
-	{
-		400000,
-		(0x000f << I2C_TIMING0_HIGH_COUNT_OFFSET) |
-		(0x0007 << I2C_TIMING0_RCV_COUNT_OFFSET),
-		(0x001f << I2C_TIMING1_LOW_COUNT_OFFSET) |
-		(0x000f << I2C_TIMING1_XMIT_COUNT_OFFSET),
-	}
-};
-
-static struct mxs_i2c_speed_table *mxs_i2c_speed_to_cfg(uint32_t speed)
-{
-	int i;
-	for (i = 0; i < ARRAY_SIZE(mxs_i2c_tbl); i++)
-		if (mxs_i2c_tbl[i].speed == speed)
-			return &mxs_i2c_tbl[i];
-	return NULL;
-}
-
-static uint32_t mxs_i2c_cfg_to_speed(uint32_t timing0, uint32_t timing1)
-{
-	int i;
-	for (i = 0; i < ARRAY_SIZE(mxs_i2c_tbl); i++) {
-		if (mxs_i2c_tbl[i].timing0 != timing0)
-			continue;
-		if (mxs_i2c_tbl[i].timing1 != timing1)
-			continue;
-		return mxs_i2c_tbl[i].speed;
-	}
-
-	return 0;
-}
-
 int i2c_set_bus_speed(unsigned int speed)
 {
 	struct mxs_i2c_regs *i2c_regs = (struct mxs_i2c_regs *)MXS_I2C0_BASE;
-	struct mxs_i2c_speed_table *spd = mxs_i2c_speed_to_cfg(speed);
+	/*
+	 * 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.
+	 */
+	uint32_t clk = mxc_get_clock(MXC_XTAL_CLK);
+	uint32_t base = ((clk / speed) - 38) / 2;
+	uint16_t high_count = base + 3;
+	uint16_t low_count = base - 3;
+	uint16_t rcv_count = (high_count * 3) / 4;
+	uint16_t xmit_count = low_count / 4;
+
+	if (speed > 540000) {
+		printf("MXS I2C: Speed too high (%d Hz)\n", speed);
+		return -EINVAL;
+	}
 
-	if (!spd) {
-		printf("MXS I2C: Invalid speed selected (%d Hz)\n", speed);
+	if (speed < 12000) {
+		printf("MXS I2C: Speed too low (%d Hz)\n", speed);
 		return -EINVAL;
 	}
 
-	writel(spd->timing0, &i2c_regs->hw_i2c_timing0);
-	writel(spd->timing1, &i2c_regs->hw_i2c_timing1);
+	writel((high_count << 16) | rcv_count, &i2c_regs->hw_i2c_timing0);
+	writel((low_count << 16) | xmit_count, &i2c_regs->hw_i2c_timing1);
 
 	writel((0x0030 << I2C_TIMING2_BUS_FREE_OFFSET) |
 		(0x0030 << I2C_TIMING2_LEADIN_COUNT_OFFSET),
@@ -280,12 +258,15 @@ int i2c_set_bus_speed(unsigned int speed)
 unsigned int i2c_get_bus_speed(void)
 {
 	struct mxs_i2c_regs *i2c_regs = (struct mxs_i2c_regs *)MXS_I2C0_BASE;
-	uint32_t timing0, timing1;
+	uint32_t clk = mxc_get_clock(MXC_XTAL_CLK);
+	uint32_t timing0;
 
 	timing0 = readl(&i2c_regs->hw_i2c_timing0);
-	timing1 = readl(&i2c_regs->hw_i2c_timing1);
-
-	return mxs_i2c_cfg_to_speed(timing0, timing1);
+	/*
+	 * This is a reverse version of the algorithm presented in
+	 * i2c_set_bus_speed(). Please refer there for details.
+	 */
+	return clk / ((((timing0 >> 16) - 3) * 2) + 38);
 }
 
 void i2c_init(int speed, int slaveadd)