iio: adc: ina2xx: Actually align the loop with the conversion ready flag

Currently, the registers are read out once per conversion interval. If
the reading is delayed as the conversion has not yet finished, this extra
time is treated as being part of the readout, although it should delay
the start of the poll interval. This results in the interval starting
slightly earlier in each iteration, until all time between reads is
spent polling the status registers instead of sleeping.

To fix this, the delay has to account for the state of the conversion
ready flag. Whenever the conversion is already finished, schedule the next
read on the regular interval, otherwise schedule it one interval after the
flag bit has been set.

Split the work function in two functions, one for the status poll and one
for reading the values, to be able to note down the time when the flag
bit is raised.

Signed-off-by: Stefan Brüns <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/iio/adc/ina2xx-adc.c

···
 	IIO_CHAN_SOFT_TIMESTAMP(4),
 };
 
-static int ina2xx_work_buffer(struct iio_dev *indio_dev)
+static int ina2xx_conversion_ready(struct iio_dev *indio_dev)
 {
 	struct ina2xx_chip_info *chip = iio_priv(indio_dev);
-	/* data buffer needs space for channel data and timestap */
-	unsigned short data[4 + sizeof(s64)/sizeof(short)];
-	int bit, ret, i = 0;
-	s64 time;
+	int ret;
 	unsigned int alert;
 
 	/*
···
 	 * For now, we do an extra read of the MASK_ENABLE register (INA226)
 	 * resp. the BUS_VOLTAGE register (INA219).
 	 */
-	if (!chip->allow_async_readout)
-		do {
-			if (chip->config->chip_id == ina226) {
-				ret = regmap_read(chip->regmap,
-						  INA226_MASK_ENABLE, &alert);
-				alert &= INA226_CVRF;
-			} else {
-				ret = regmap_read(chip->regmap,
-						  INA2XX_BUS_VOLTAGE, &alert);
-				alert &= INA219_CNVR;
-			}
+	if (chip->config->chip_id == ina226) {
+		ret = regmap_read(chip->regmap,
+				  INA226_MASK_ENABLE, &alert);
+		alert &= INA226_CVRF;
+	} else {
+		ret = regmap_read(chip->regmap,
+				  INA2XX_BUS_VOLTAGE, &alert);
+		alert &= INA219_CNVR;
+	}
 
-			if (ret < 0)
-				return ret;
+	if (ret < 0)
+		return ret;
 
-		} while (!alert);
+	return !!alert;
+}
+
+static int ina2xx_work_buffer(struct iio_dev *indio_dev)
+{
+	struct ina2xx_chip_info *chip = iio_priv(indio_dev);
+	/* data buffer needs space for channel data and timestap */
+	unsigned short data[4 + sizeof(s64)/sizeof(short)];
+	int bit, ret, i = 0;
+	s64 time;
 
 	time = iio_get_time_ns(indio_dev);
 
···
 	ktime_get_ts64(&next);
 
 	do {
+		while (!chip->allow_async_readout) {
+			ret = ina2xx_conversion_ready(indio_dev);
+			if (ret < 0)
+				return ret;
+
+			/*
+			 * If the conversion was not yet finished,
+			 * reset the reference timestamp.
+			 */
+			if (ret == 0)
+				ktime_get_ts64(&next);
+			else
+				break;
+		}
+
 		ret = ina2xx_work_buffer(indio_dev);
 		if (ret < 0)
 			return ret;