[PATCH 2/2 v4] 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, 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>
---
ChangeLog v3->v4:
- Split out the new iio_read_channel_processed_scale()
  API addition to a separate patch.
- My suggestion is to apply both patches to the hwmon
  tree.
ChangeLog v2->v3:
- After discussion about v2 we concludes that
  iio_read_channel_processed() could loose precision
  so we introduce a new API to read processed and
  scale.
- Include a link to the v2 discussion for reference.
- For ease of applying to the hwmon tree, keep it all
  in one patch.
- This needs Jonathans ACK to be merged through hwmon.
ChangeLog v1->v2:
- Fix the patch to multiply the processed value by
  1000 to get to microvolts from millivolts.
- Fix up the confusion in the commit message.
- Drop pointless comments about the code, we keep the
  original code path around if processed reads don't
  work, nothing bad with that.
---
 drivers/hwmon/ntc_thermistor.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/drivers/hwmon/ntc_thermistor.c b/drivers/hwmon/ntc_thermistor.c
index 3aad62a0e661..8587189c7f15 100644
--- a/drivers/hwmon/ntc_thermistor.c
+++ b/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;
-- 
2.29.2