arm: exynos: Wait till ADC stabilizes before checking Odroid HC1 revision

Fix detection of Odroid HC1 (Exynos5422) after reboot if kernel disabled
the LDO4/VDD_ADC regulator.

The LDO4 supplies both ADC block and the ADC input AIN9.  Voltage on
AIN9 will rise slowly, so use delay of 5 milliseconds instead of
timers-based loop to wait for voltage stabilization.

First reads on Odroid HC1 return 305, 1207, 1297 and finally 1308
(reference value is 1309).

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Anand Moon <linux.amoon@gmail.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>

board/samsung/common/exynos5-dt-types.c

@@ -57,12 +57,48 @@ static unsigned int odroid_get_rev(void)
 	return 0;
 }
 
+/*
+ * Read ADC at least twice and check the resuls.  If regulator providing voltage
+ * on to measured point was just turned on, first reads might require time
+ * to stabilize.
+ */
+static int odroid_get_adc_val(unsigned int *adcval)
+{
+	unsigned int adcval_prev = 0;
+	int ret, retries = 20;
+
+	ret = adc_channel_single_shot("adc", CONFIG_ODROID_REV_AIN,
+				      &adcval_prev);
+	if (ret)
+		return ret;
+
+	while (retries--) {
+		mdelay(5);
+
+		ret = adc_channel_single_shot("adc", CONFIG_ODROID_REV_AIN,
+					      adcval);
+		if (ret)
+			return ret;
+
+		/*
+		 * If difference between ADC reads is less than 3%,
+		 * accept the result
+		 */
+		if ((100 * abs(*adcval - adcval_prev) / adcval_prev) < 3)
+			return ret;
+
+		adcval_prev = *adcval;
+	}
+
+	return ret;
+}
+
 static int odroid_get_board_type(void)
 {
 	unsigned int adcval;
 	int ret, i;
 
-	ret = adc_channel_single_shot("adc", CONFIG_ODROID_REV_AIN, &adcval);
+	ret = odroid_get_adc_val(&adcval);
 	if (ret)
 		goto rev_default;