iio: adc: meson-saradc: add calibration

This patch adds calibration for the Meson SAR ADC.
Points 25% vref and 75% vref are used for calibration.
It uses a simple linear calibration function: SCALE * val + BIAS

Successfully tested on a Odroid C2.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/meson_saradc.c

@@ -166,6 +166,8 @@
 
 #define MESON_SAR_ADC_MAX_FIFO_SIZE				32
 #define MESON_SAR_ADC_TIMEOUT					100 /* ms */
+/* for use with IIO_VAL_INT_PLUS_MICRO */
+#define MILLION							1000000
 
 #define MESON_SAR_ADC_CHAN(_chan) {					\
 	.type = IIO_VOLTAGE,						\
@@ -173,7 +175,9 @@
 	.channel = _chan,						\
 	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |			\
 				BIT(IIO_CHAN_INFO_AVERAGE_RAW),		\
-	.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),		\
+	.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE) |		\
+				BIT(IIO_CHAN_INFO_CALIBBIAS) |		\
+				BIT(IIO_CHAN_INFO_CALIBSCALE),		\
 	.datasheet_name = "SAR_ADC_CH"#_chan,				\
 }
 
@@ -233,6 +237,8 @@ struct meson_sar_adc_priv {
 	struct clk				*adc_div_clk;
 	struct clk_divider			clk_div;
 	struct completion			done;
+	int					calibbias;
+	int					calibscale;
 };
 
 static const struct regmap_config meson_sar_adc_regmap_config = {
@@ -252,6 +258,17 @@ static unsigned int meson_sar_adc_get_fifo_count(struct iio_dev *indio_dev)
 	return FIELD_GET(MESON_SAR_ADC_REG0_FIFO_COUNT_MASK, regval);
 }
 
+static int meson_sar_adc_calib_val(struct iio_dev *indio_dev, int val)
+{
+	struct meson_sar_adc_priv *priv = iio_priv(indio_dev);
+	int tmp;
+
+	/* use val_calib = scale * val_raw + offset calibration function */
+	tmp = div_s64((s64)val * priv->calibscale, MILLION) + priv->calibbias;
+
+	return clamp(tmp, 0, (1 << priv->data->resolution) - 1);
+}
+
 static int meson_sar_adc_wait_busy_clear(struct iio_dev *indio_dev)
 {
 	struct meson_sar_adc_priv *priv = iio_priv(indio_dev);
@@ -302,7 +319,7 @@ static int meson_sar_adc_read_raw_sample(struct iio_dev *indio_dev,
 
 	fifo_val = FIELD_GET(MESON_SAR_ADC_FIFO_RD_SAMPLE_VALUE_MASK, regval);
 	fifo_val &= GENMASK(priv->data->resolution - 1, 0);
-	*val = fifo_val;
+	*val = meson_sar_adc_calib_val(indio_dev, fifo_val);
 
 	return 0;
 }
@@ -527,6 +544,15 @@ static int meson_sar_adc_iio_info_read_raw(struct iio_dev *indio_dev,
 		*val2 = priv->data->resolution;
 		return IIO_VAL_FRACTIONAL_LOG2;
 
+	case IIO_CHAN_INFO_CALIBBIAS:
+		*val = priv->calibbias;
+		return IIO_VAL_INT;
+
+	case IIO_CHAN_INFO_CALIBSCALE:
+		*val = priv->calibscale / MILLION;
+		*val2 = priv->calibscale % MILLION;
+		return IIO_VAL_INT_PLUS_MICRO;
+
 	default:
 		return -EINVAL;
 	}
@@ -762,6 +788,47 @@ static irqreturn_t meson_sar_adc_irq(int irq, void *data)
 	return IRQ_HANDLED;
 }
 
+static int meson_sar_adc_calib(struct iio_dev *indio_dev)
+{
+	struct meson_sar_adc_priv *priv = iio_priv(indio_dev);
+	int ret, nominal0, nominal1, value0, value1;
+
+	/* use points 25% and 75% for calibration */
+	nominal0 = (1 << priv->data->resolution) / 4;
+	nominal1 = (1 << priv->data->resolution) * 3 / 4;
+
+	meson_sar_adc_set_chan7_mux(indio_dev, CHAN7_MUX_VDD_DIV4);
+	usleep_range(10, 20);
+	ret = meson_sar_adc_get_sample(indio_dev,
+				       &meson_sar_adc_iio_channels[7],
+				       MEAN_AVERAGING, EIGHT_SAMPLES, &value0);
+	if (ret < 0)
+		goto out;
+
+	meson_sar_adc_set_chan7_mux(indio_dev, CHAN7_MUX_VDD_MUL3_DIV4);
+	usleep_range(10, 20);
+	ret = meson_sar_adc_get_sample(indio_dev,
+				       &meson_sar_adc_iio_channels[7],
+				       MEAN_AVERAGING, EIGHT_SAMPLES, &value1);
+	if (ret < 0)
+		goto out;
+
+	if (value1 <= value0) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	priv->calibscale = div_s64((nominal1 - nominal0) * (s64)MILLION,
+				   value1 - value0);
+	priv->calibbias = nominal0 - div_s64((s64)value0 * priv->calibscale,
+					     MILLION);
+	ret = 0;
+out:
+	meson_sar_adc_set_chan7_mux(indio_dev, CHAN7_MUX_CH7_INPUT);
+
+	return ret;
+}
+
 static const struct iio_info meson_sar_adc_iio_info = {
 	.read_raw = meson_sar_adc_iio_info_read_raw,
 	.driver_module = THIS_MODULE,
@@ -901,6 +968,8 @@ static int meson_sar_adc_probe(struct platform_device *pdev)
 		return PTR_ERR(priv->vref);
 	}
 
+	priv->calibscale = MILLION;
+
 	ret = meson_sar_adc_init(indio_dev);
 	if (ret)
 		goto err;
@@ -909,6 +978,10 @@ static int meson_sar_adc_probe(struct platform_device *pdev)
 	if (ret)
 		goto err;
 
+	ret = meson_sar_adc_calib(indio_dev);
+	if (ret)
+		dev_warn(&pdev->dev, "calibration failed\n");
+
 	platform_set_drvdata(pdev, indio_dev);
 
 	ret = iio_device_register(indio_dev);