[U-Boot] [PATCH v3 5/9] arm: exynos: Wait till ADC stabilizes before checking Odroid HC1 revision

[Krzysztof Kozlowski <krzk@kernel.org>]

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 be patient and wait for it to stabilize.

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

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
---
 board/samsung/common/exynos5-dt-types.c | 38 ++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/board/samsung/common/exynos5-dt-types.c b/board/samsung/common/exynos5-dt-types.c
index 7c1271d6547a..516c32923e44 100644
--- a/board/samsung/common/exynos5-dt-types.c
+++ b/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;
 
-- 
2.17.1