[PATCH] hwmon: (ntc_thermistor): try reading processed

[Linus Walleij <linus.walleij@linaro.org>]

Before trying the custom method of reading the sensor
as raw and then converting assuming 1000 scaling, just
use iio_read_channel_processed() which first tries to
see if the ADC can provide a processed value directly,
else reads raw and applies scaling inside of IIO
using the scale attributes of the ADC.

The code that hardcodes scaling to 1000 and assumes
a 12bit ADC is very dubious. I keep it around here
but I have a strong urge to just delete it.

This gives correct readings on the AB8500 thermistor
inputs used in the Ux500 HREFP520 platform for reading
battery and board temperature.

Cc: Peter Rosin <peda@axentia.se>
Cc: Chris Lesiak <chris.lesiak@licor.com>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/hwmon/ntc_thermistor.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/drivers/hwmon/ntc_thermistor.c b/drivers/hwmon/ntc_thermistor.c
index 3aad62a0e661..ac0d80faddf6 100644
--- a/drivers/hwmon/ntc_thermistor.c
+++ b/drivers/hwmon/ntc_thermistor.c
@@ -326,18 +326,29 @@ struct ntc_data {
 static int ntc_adc_iio_read(struct ntc_thermistor_platform_data *pdata)
 {
 	struct iio_channel *channel = pdata->chan;
-	int raw, uv, ret;
+	int uv, ret;
 
-	ret = iio_read_channel_raw(channel, &raw);
+	/* A processed voltage channel will return microvolts */
+	ret = iio_read_channel_processed(channel, &uv);
 	if (ret < 0) {
-		pr_err("read channel() error: %d\n", ret);
-		return ret;
-	}
+		int raw;
 
-	ret = iio_convert_raw_to_processed(channel, raw, &uv, 1000);
-	if (ret < 0) {
-		/* Assume 12 bit ADC with vref at pullup_uv */
-		uv = (pdata->pullup_uv * (s64)raw) >> 12;
+		/*
+		 * FIXME: This fallback to using a raw read and then right
+		 * out assume the ADC is 12 bits and hard-coding scale
+		 * to 1000 seems a bit dangerous. Should it simply be
+		 * deleted?
+		 */
+		ret = iio_read_channel_raw(channel, &raw);
+		if (ret < 0) {
+			pr_err("read channel() error: %d\n", ret);
+			return ret;
+		}
+		ret = iio_convert_raw_to_processed(channel, raw, &uv, 1000);
+		if (ret < 0) {
+			/* Assume 12 bit ADC with vref at pullup_uv */
+			uv = (pdata->pullup_uv * (s64)raw) >> 12;
+		}
 	}
 
 	return uv;
-- 
2.29.2