[v2 6/8] iio: adc: aspeed: Add compensation phase.

[Billy Tsai <billy_tsai@xxxxxxxxxxxxxx>]

This patch adds a compensation phase to improve the accurate of adc
measurement. This is the builtin function though input half of the
reference voltage to get the adc offset.

Signed-off-by: Billy Tsai <billy_tsai@xxxxxxxxxxxxxx>
---
 drivers/iio/adc/aspeed_adc.c | 52 ++++++++++++++++++++++++++++++++++--
 1 file changed, 50 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/adc/aspeed_adc.c b/drivers/iio/adc/aspeed_adc.c
index bb6100228cae..0153b28b83b7 100644
--- a/drivers/iio/adc/aspeed_adc.c
+++ b/drivers/iio/adc/aspeed_adc.c
@@ -61,6 +61,7 @@
  * rate for most user case.
  */
 #define ASPEED_ADC_DEF_SAMPLING_RATE	65000
+#define ASPEED_ADC_MAX_RAW_DATA		GENMASK(9, 0)
 
 enum aspeed_adc_version {
 	aspeed_adc_ast2400,
@@ -84,6 +85,7 @@ struct aspeed_adc_data {
 	struct reset_control	*rst;
 	int			vref;
 	u32			sample_period_ns;
+	int			cv;
 };
 
 #define ASPEED_CHAN(_idx, _data_reg_addr) {			\
@@ -115,6 +117,48 @@ static const struct iio_chan_spec aspeed_adc_iio_channels[] = {
 	ASPEED_CHAN(15, 0x2E),
 };
 
+static int aspeed_adc_compensation(struct platform_device *pdev)
+{
+	struct iio_dev *indio_dev = platform_get_drvdata(pdev);
+	struct aspeed_adc_data *data = iio_priv(indio_dev);
+	u32 index, adc_raw = 0;
+	u32 adc_engine_control_reg_val =
+		readl(data->base + ASPEED_REG_ENGINE_CONTROL);
+	adc_engine_control_reg_val |=
+		(ASPEED_ADC_OPERATION_MODE_NORMAL | ASPEED_ADC_ENGINE_ENABLE);
+
+	/*
+	 * Enable compensating sensing:
+	 * After that, the input voltage of adc will force to half of the reference
+	 * voltage. So the expected reading raw data will become half of the max
+	 * value. We can get compensating value = 0x200 - adc read raw value.
+	 * It is recommended to average at least 10 samples to get a final CV.
+	 */
+	writel(adc_engine_control_reg_val | ASPEED_ADC_CTRL_COMPENSATION |
+		       ASPEED_ADC_CTRL_CHANNEL_ENABLE(0),
+	       data->base + ASPEED_REG_ENGINE_CONTROL);
+	/*
+	 * After enable compensating sensing mode need to wait some time for adc stable
+	 * Experiment result is 1ms.
+	 */
+	mdelay(1);
+
+	for (index = 0; index < 16; index++) {
+		/*
+		 * Waiting for the sampling period ensures that the value acquired
+		 * is fresh each time.
+		 */
+		ndelay(data->sample_period_ns);
+		adc_raw += readw(data->base + aspeed_adc_iio_channels[0].address);
+	}
+	adc_raw >>= 4;
+	data->cv = BIT(ASPEED_RESOLUTION_BITS - 1) - adc_raw;
+	writel(adc_engine_control_reg_val,
+	       data->base + ASPEED_REG_ENGINE_CONTROL);
+	dev_dbg(data->dev, "compensating value = %d\n", data->cv);
+	return 0;
+}
+
 static int aspeed_adc_set_sampling_rate(struct iio_dev *indio_dev, u32 rate)
 {
 	struct aspeed_adc_data *data = iio_priv(indio_dev);
@@ -143,7 +187,11 @@ static int aspeed_adc_read_raw(struct iio_dev *indio_dev,
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW:
-		*val = readw(data->base + chan->address);
+		*val = readw(data->base + chan->address) + data->cv;
+		if (*val < 0)
+			*val = 0;
+		else if (*val >= ASPEED_ADC_MAX_RAW_DATA)
+			*val = ASPEED_ADC_MAX_RAW_DATA;
 		return IIO_VAL_INT;
 
 	case IIO_CHAN_INFO_SCALE:
@@ -347,7 +395,7 @@ static int aspeed_adc_probe(struct platform_device *pdev)
 		if (ret)
 			goto poll_timeout_error;
 	}
-
+	aspeed_adc_compensation(pdev);
 	adc_engine_control_reg_val =
 		readl(data->base + ASPEED_REG_ENGINE_CONTROL);
 	/* Start all channels in normal mode. */
-- 
2.25.1