hwmon: (ntc_thermistor): try reading processed

Before trying the custom method of reading the sensor
as raw and then converting, we want to use
iio_read_channel_processed_scale() 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. We need to
multiply the scaled value with 1000 to get to
microvolts from millivolts which is what processed
IIO channels returns.

Keep the code that assumes 12bit ADC around as a
fallback.

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@kernel.org>
Cc: linux-iio@vger.kernel.org
Link: https://lore.kernel.org/linux-iio/20201224011607.1059534-1-linus.walleij@linaro.org/Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Chris Lesiak <chris.lesiak@licor.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210308100219.2732156-2-linus.walleij@linaro.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/hwmon/ntc_thermistor.c

@@ -326,18 +326,27 @@ 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);
+	ret = iio_read_channel_processed_scale(channel, &uv, 1000);
 	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;
+		/*
+		 * This fallback uses a raw read and then
+		 * assumes the ADC is 12 bits, scaling with
+		 * a factor 1000 to get to microvolts.
+		 */
+		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;