+
+/*
+ * The way speed is changed into duty often results in integer truncation
+ * with 50% duty, so we'll force rounding up to the next duty by adding 1
+ * to the max. In practice this will get us a speed of something like
+ * 385 KHz. The other limit is easy to handle as it is only 8 bits.
+ */
+#define I2C_SPEED_MAX 400000
+#define I2C_SPEED_TO_DUTY(speed) (5000000 / (speed))
+#define I2C_DUTY_MAX (I2C_SPEED_TO_DUTY(I2C_SPEED_MAX) + 1)
+#define I2C_DUTY_MIN 0xff /* 8 bit limited */
+#define SYS_I2C_DUTY I2C_SPEED_TO_DUTY(CONFIG_SYS_I2C_SPEED)
+/* Note: duty is inverse of speed, so the comparisons below are correct */
+#if SYS_I2C_DUTY < I2C_DUTY_MAX || SYS_I2C_DUTY > I2C_DUTY_MIN
+# error "The Blackfin I2C hardware can only operate 20KHz - 400KHz"