commit 7a18afe8097731b8ffb6cb5b2b3b418ded77c105
author Akshay Bhat <akshay.bhat@timesys.com> Mon Apr 18 15:47:53 2016 -0400
committer Guenter Roeck <linux@roeck-us.net> Tue Apr 19 06:30:28 2016 -0700
tree a6b39b60800292c14a9732f0cd6b0837d5e5c951
parent 9735a22799b9214d17d3c231fe377fc852f042e9

hwmon: (ads7828) Enable internal reference

On ads7828 the internal reference defaults to off upon power up. When
using internal reference, it needs to be turned on and the voltage needs
to settle before normal conversion cycle can be started. Hence perform a
dummy read in the probe to enable the internal reference allowing the
voltage to settle before performing a normal read.

Without this fix, the first read from the ADC when using internal
reference always returns incorrect data.

Signed-off-by: Akshay Bhat <akshay.bhat@timesys.com>
Cc: stable@vger.kernel.org # v4.1+
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/ads7828.c

diff --git a/drivers/hwmon/ads7828.c b/drivers/hwmon/ads7828.c
index 6c99ee7..ee396ff 100644
--- a/drivers/hwmon/ads7828.c
+++ b/drivers/hwmon/ads7828.c
@@ -120,6 +120,7 @@
 	unsigned int vref_mv = ADS7828_INT_VREF_MV;
 	bool diff_input = false;
 	bool ext_vref = false;
+	unsigned int regval;
 
 	data = devm_kzalloc(dev, sizeof(struct ads7828_data), GFP_KERNEL);
 	if (!data)
@@ -154,6 +155,15 @@
 	if (!diff_input)
 		data->cmd_byte |= ADS7828_CMD_SD_SE;
 
+	/*
+	 * Datasheet specifies internal reference voltage is disabled by
+	 * default. The internal reference voltage needs to be enabled and
+	 * voltage needs to settle before getting valid ADC data. So perform a
+	 * dummy read to enable the internal reference voltage.
+	 */
+	if (!ext_vref)
+		regmap_read(data->regmap, data->cmd_byte, &regval);
+
 	hwmon_dev = devm_hwmon_device_register_with_groups(dev, client->name,
 							   data,
 							   ads7828_groups);