i2c: ti: Update method to calculate psc, sscl and ssch I2C parameters

This patch updates the way in which psc, sscl and ssch I2C parameters are
calculated to be in sync with v4.9 Linux kernel
SHA1: 69973b830859bc6529a7a0468ba0d80ee5117826
in the ./drivers/i2c/busses/i2c-omap.c

The previous method was causing several issues:
- The internal I2C frequency (after prescaler) was far above recommended
one (7 - 12 MHz [*]) - the current approach brings better noise suppression
(as stated in Linux commit: SHA1: 84bf2c868f3ca996e5bb)

- The values calculated (psc, sscl and ssch) were far from optimal, which
caused on the test platform (AM57xx) the I2C0 SCL signal low time (Fast
Mode) of ~1.0us (the standard requires > 1.3 us).

[*] for AM57xx TRM SPRUHZ6G, Table 24,7
"HS I2C Register Values for Maximum I2C Bit Rates in I2C F/S, I2C HS Modes"

Signed-off-by: Lukasz Majewski <lukma@denx.de>

drivers/i2c/omap24xx_i2c.c

@@ -64,36 +64,52 @@ struct omap_i2c {
 
 static int omap24_i2c_findpsc(u32 *pscl, u32 *psch, uint speed)
 {
-	unsigned int sampleclk, prescaler;
-	int fsscll, fssclh;
+	unsigned long internal_clk = 0, fclk;
+	unsigned int prescaler;
 
-	speed <<= 1;
-	prescaler = 0;
 	/*
-	 * some divisors may cause a precission loss, but shouldn't
-	 * be a big thing, because i2c_clk is then allready very slow.
+	 * This method is only called for Standard and Fast Mode speeds
+	 *
+	 * For some TI SoCs it is explicitly written in TRM (e,g, SPRUHZ6G,
+	 * page 5685, Table 24-7)
+	 * that the internal I2C clock (after prescaler) should be between
+	 * 7-12 MHz (at least for Fast Mode (FS)).
+	 *
+	 * Such approach is used in v4.9 Linux kernel in:
+	 * ./drivers/i2c/busses/i2c-omap.c (omap_i2c_init function).
 	 */
-	while (prescaler <= 0xFF) {
-		sampleclk = I2C_IP_CLK / (prescaler+1);
 
-		fsscll = sampleclk / speed;
-		fssclh = fsscll;
-		fsscll -= I2C_FASTSPEED_SCLL_TRIM;
-		fssclh -= I2C_FASTSPEED_SCLH_TRIM;
-
-		if (((fsscll > 0) && (fssclh > 0)) &&
-		    ((fsscll <= (255-I2C_FASTSPEED_SCLL_TRIM)) &&
-		    (fssclh <= (255-I2C_FASTSPEED_SCLH_TRIM)))) {
-			if (pscl)
-				*pscl = fsscll;
-			if (psch)
-				*psch = fssclh;
-
-			return prescaler;
-		}
-		prescaler++;
+	speed /= 1000; /* convert speed to kHz */
+
+	if (speed > 100)
+		internal_clk = 9600;
+	else
+		internal_clk = 4000;
+
+	fclk = I2C_IP_CLK / 1000;
+	prescaler = fclk / internal_clk;
+	prescaler = prescaler - 1;
+
+	if (speed > 100) {
+		unsigned long scl;
+
+		/* Fast mode */
+		scl = internal_clk / speed;
+		*pscl = scl - (scl / 3) - I2C_FASTSPEED_SCLL_TRIM;
+		*psch = (scl / 3) - I2C_FASTSPEED_SCLH_TRIM;
+	} else {
+		/* Standard mode */
+		*pscl = internal_clk / (speed * 2) - I2C_FASTSPEED_SCLL_TRIM;
+		*psch = internal_clk / (speed * 2) - I2C_FASTSPEED_SCLH_TRIM;
 	}
-	return -1;
+
+	debug("%s: speed [kHz]: %d psc: 0x%x sscl: 0x%x ssch: 0x%x\n",
+	      __func__, speed, prescaler, *pscl, *psch);
+
+	if (*pscl <= 0 || *psch <= 0 || prescaler <= 0)
+		return -EINVAL;
+
+	return prescaler;
 }
 
 /*