linux-kernel - [PATCH v2 10/14] staging: iio: ad7746: Add comments

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Date:   Fri, 13 Apr 2018 13:36:47 -0300
From:   Hernán Gonzalez <hernan@...guardiasur.com.ar>
To:     jic23@...nel.org, knaack.h@....de, lars@...afoo.de,
        pmeerw@...erw.net, gregkh@...uxfoundation.org,
        Michael.Hennerich@...log.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Hernán Gonzalez <hernan@...guardiasur.com.ar>
Subject: [PATCH v2 10/14] staging: iio: ad7746: Add comments

Add comments to clarify some of the calculations made, specially when
reading or writing values.

Signed-off-by: Hernán Gonzalez <hernan@...guardiasur.com.ar>
---
 drivers/staging/iio/cdc/ad7746.c | 32 +++++++++++++++++++++++++++-----
 1 file changed, 27 insertions(+), 5 deletions(-)

diff --git a/drivers/staging/iio/cdc/ad7746.c b/drivers/staging/iio/cdc/ad7746.c
index 05506bf9..ef0ebb5 100644
--- a/drivers/staging/iio/cdc/ad7746.c
+++ b/drivers/staging/iio/cdc/ad7746.c
@@ -429,6 +429,7 @@ static int ad7746_write_raw(struct iio_dev *indio_dev,
 			goto out;
 		}
 
+		/* 2^16 in micro */
 		val = (val2 * 1024) / 15625;
 
 		switch (chan->type) {
@@ -554,6 +555,13 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
 		if (ret < 0)
 			goto out;
 
+		/*
+		 * Either for Capacitance, Voltage or Temperature,
+		 * the 0x000000 code represents negative full scale,
+		 * the 0x800000 code represents zero scale, and
+		 * the 0xFFFFFF code represents positive full scale.
+		 */
+
 		*val = (be32_to_cpu(chip->data.d32) & 0xFFFFFF) - 0x800000;
 
 		switch (chan->type) {
@@ -565,7 +573,13 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
 			*val = (*val * 125) / 256;
 			break;
 		case IIO_VOLTAGE:
-			if (chan->channel == 1) /* supply_raw*/
+
+			/*
+			 * The voltage from the VDD pin is internally
+			 * attenuated by 6.
+			 */
+
+			if (chan->channel == 1) /* supply_raw */
 				*val = *val * 6;
 			break;
 		default:
@@ -606,6 +620,13 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
 		ret = IIO_VAL_INT;
 		break;
 	case IIO_CHAN_INFO_OFFSET:
+
+		/*
+		 * CAPDAC Scale = 21pF_typ / 127
+		 * CIN Scale = 8.192pF / 2^24
+		 * Offset Scale = CAPDAC Scale / CIN Scale = 338646
+		 */
+
 		*val = AD7746_CAPDAC_DACP(chip->capdac[chan->channel]
 					  [chan->differential]) * 338646;
 
@@ -614,13 +635,13 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
 	case IIO_CHAN_INFO_SCALE:
 		switch (chan->type) {
 		case IIO_CAPACITANCE:
-			/* 8.192pf / 2^24 */
+			/* CIN Scale: 8.192pf / 2^24 */
 			*val =  0;
 			*val2 = 488;
 			ret = IIO_VAL_INT_PLUS_NANO;
 			break;
 		case IIO_VOLTAGE:
-			/* 1170mV / 2^23 */
+			/* VIN Scale: 1170mV / 2^23 */
 			*val = 1170;
 			*val2 = 23;
 			ret = IIO_VAL_FRACTIONAL_LOG2;
@@ -674,7 +695,8 @@ static struct ad7746_platform_data *ad7746_parse_dt(struct device *dev)
 	unsigned int tmp;
 	int ret;
 
-	/* The default excitation outputs are not inverted, it should be stated
+	/*
+	 * The default excitation outputs are not inverted, it should be stated
 	 * in the dt if needed.
 	 */
 
@@ -685,7 +707,7 @@ static struct ad7746_platform_data *ad7746_parse_dt(struct device *dev)
 	ret = of_property_read_u32(np, "adi,exclvl", &tmp);
 	if (ret || tmp > 3) {
 		dev_warn(dev, "Wrong exclvl value, using default\n");
-		pdata->exclvl = 3; /* default value */
+		pdata->exclvl = 3;
 	} else {
 		pdata->exclvl = tmp;
 	}
-- 
2.7.4