i2c: designware: Support non-standard bus speeds

Add support for non-standard bus speeds by treating them as detuned
versions of the slowest standard speed not less than the requested
speed.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

drivers/i2c/busses/i2c-designware-common.c

@@ -362,6 +362,9 @@ static void i2c_dw_adjust_bus_speed(struct dw_i2c_dev *dev)
 {
 	u32 acpi_speed = i2c_dw_acpi_round_bus_speed(dev->dev);
 	struct i2c_timings *t = &dev->timings;
+	u32 wanted_speed;
+	u32 legal_speed = 0;
+	int i;
 
 	/*
 	 * Find bus speed from the "clock-frequency" device property, ACPI
@@ -373,6 +376,30 @@ static void i2c_dw_adjust_bus_speed(struct dw_i2c_dev *dev)
 		t->bus_freq_hz = max(t->bus_freq_hz, acpi_speed);
 	else
 		t->bus_freq_hz = I2C_MAX_FAST_MODE_FREQ;
+
+	wanted_speed = t->bus_freq_hz;
+
+	/* For unsupported speeds, scale down the lowest speed which is faster. */
+	for (i = 0; i < ARRAY_SIZE(supported_speeds); i++) {
+		/* supported speeds is in decreasing order */
+		if (wanted_speed == supported_speeds[i]) {
+			legal_speed = 0;
+			break;
+		}
+		if (wanted_speed > supported_speeds[i])
+			break;
+
+		legal_speed = supported_speeds[i];
+	}
+
+	if (legal_speed) {
+		/*
+		 * Pretend this was the requested speed, but preserve the preferred
+		 * speed so the clock counts can be scaled.
+		 */
+		t->bus_freq_hz = legal_speed;
+		dev->wanted_bus_speed = wanted_speed;
+	}
 }
 
 int i2c_dw_fw_parse_and_configure(struct dw_i2c_dev *dev)

drivers/i2c/busses/i2c-designware-core.h

@@ -297,6 +297,7 @@ struct dw_i2c_dev {
 	u16			fp_lcnt;
 	u16			hs_hcnt;
 	u16			hs_lcnt;
+	u32			wanted_bus_speed;
 	int			(*acquire_lock)(void);
 	void			(*release_lock)(void);
 	int			semaphore_idx;

drivers/i2c/busses/i2c-designware-master.c

@@ -41,20 +41,32 @@ static void i2c_dw_configure_fifo_master(struct dw_i2c_dev *dev)
 	regmap_write(dev->map, DW_IC_CON, dev->master_cfg);
 }
 
-static u16 clock_calc(struct dw_i2c_dev *dev, bool want_high)
+static u32 linear_interpolate(u32 x, u32 x1, u32 x2, u32 y1, u32 y2)
+{
+	return ((x - x1) * y2 + (x2 - x) * y1) / (x2 - x1);
+}
+
+static u16 u16_clamp(u32 v)
+{
+	return (u16)min(v, 0xffff);
+}
+
+static void clock_calc(struct dw_i2c_dev *dev, u32 *hcnt, u32 *lcnt)
 {
 	struct i2c_timings *t = &dev->timings;
-	u32 wanted_speed = t->bus_freq_hz;
+	u32 wanted_khz = (dev->wanted_bus_speed ?: t->bus_freq_hz)/1000;
 	u32 clk_khz = i2c_dw_clk_rate(dev);
-	u32 extra_ns = want_high ? t->scl_fall_ns : t->scl_rise_ns;
+	u32 min_high_ns = (wanted_khz <= 100) ? 4000 :
-	u32 extra_cycles = (u32)((u64)clk_khz * extra_ns / 1000000);
+			  (wanted_khz <= 400) ?
-	u32 period = ((u64)clk_khz * 1000 + wanted_speed - 1) / wanted_speed;
+				linear_interpolate(wanted_khz, 100, 400, 4000, 600) :
-	u32 cycles = (period + want_high)/2 - extra_cycles;
+				linear_interpolate(wanted_khz, 400, 1000, 600, 260);
+	u32 high_cycles = (u32)(((u64)clk_khz * min_high_ns + 999999) / 1000000) + 1;
+	u32 extra_high_cycles = (u32)((u64)clk_khz * t->scl_fall_ns / 1000000);
+	u32 extra_low_cycles = (u32)((u64)clk_khz * t->scl_rise_ns / 1000000);
+	u32 period = ((u64)clk_khz + wanted_khz - 1) / wanted_khz;
 
-	if (cycles > 0xffff)
+	*hcnt = u16_clamp(high_cycles - extra_high_cycles);
-		cycles = 0xffff;
+	*lcnt = u16_clamp(period - high_cycles - extra_low_cycles);
-
-	return (u16)cycles;
 }
 
 static int i2c_dw_set_timings_master(struct dw_i2c_dev *dev)
@@ -80,8 +92,7 @@ static int i2c_dw_set_timings_master(struct dw_i2c_dev *dev)
 	sda_falling_time = t->sda_fall_ns ?: 300; /* ns */
 	scl_falling_time = t->scl_fall_ns ?: 300; /* ns */
 
-	hcnt = clock_calc(dev, true);
+	clock_calc(dev, &hcnt, &lcnt);
-	lcnt = clock_calc(dev, false);
 
 	/* Calculate SCL timing parameters for standard mode if not set */
 	if (!dev->ss_hcnt || !dev->ss_lcnt) {