fixup! i2c: designware: Support non-standard bus speeds

[1] calculates timings for arbitrary clock speeds, but it does so aiming
for a 50% SCL duty cycle. This is the wrong goal, particularly for high
clock speeds, because it doesn't allow the device sufficient time to
pull the bus low to issue an ACK.

Change the algorithm to aim for the minimum SCL high time (tHIGH) for
the requested speed according to the I2C Specification, using linear
interpolation between the values for the standard speeds.

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

[1] commit cea76e5 ("i2c: designware: Support non-standard bus speeds")

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

@@ -38,20 +38,31 @@ 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)
 {
-	struct i2c_timings *t = &dev->timings;
-	u32 wanted_speed = dev->wanted_bus_speed ?: t->bus_freq_hz;
-	u32 clk_khz = i2c_dw_clk_rate(dev);
-	u32 extra_ns = want_high ? t->scl_fall_ns : t->scl_rise_ns;
-	u32 extra_cycles = (u32)((u64)clk_khz * extra_ns / 1000000);
-	u32 period = ((u64)clk_khz * 1000 + wanted_speed - 1) / wanted_speed;
-	u32 cycles = (period + want_high)/2 - extra_cycles;
+	return ((x - x1) * y2 + (x2 - x) * y1) / (x2 - x1);
+}
 
-	if (cycles > 0xffff)
-		cycles = 0xffff;
+static u16 u16_clamp(u32 v)
+{
+	return (u16)min(v, 0xffff);
+}
 
-	return (u16)cycles;
+static void clock_calc(struct dw_i2c_dev *dev, u32 *hcnt, u32 *lcnt)
+{
+	struct i2c_timings *t = &dev->timings;
+	u32 wanted_khz = (dev->wanted_bus_speed ?: t->bus_freq_hz)/1000;
+	u32 clk_khz = i2c_dw_clk_rate(dev);
+	u32 min_high_ns = (wanted_khz <= 100) ? 4000 :
+			  (wanted_khz <= 400) ? linear_interpolate(wanted_khz, 100, 400, 4000, 600) :
+			  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;
+
+	*hcnt = u16_clamp(high_cycles - extra_high_cycles);
+	*lcnt = u16_clamp(period - high_cycles - extra_low_cycles);
 }
 
 static int i2c_dw_set_timings_master(struct dw_i2c_dev *dev)
@@ -77,8 +88,7 @@ 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);
-	lcnt = clock_calc(dev, false);
+	clock_calc(dev, &hcnt, &lcnt);
 
 	/* Calculate SCL timing parameters for standard mode if not set */
 	if (!dev->ss_hcnt || !dev->ss_lcnt) {