iio: adc: stm32: add support for differential channels

STM32H7 ADC channels can be configured either as single ended or
differential with 'st,adc-channels' or 'st,adc-diff-channels'
(positive and negative input pair: <vinp vinn>, ...).

Differential channels have different offset and scale, from spec:
raw value = (full_scale / 2) * (1 + (vinp - vinn) / vref).
Add offset attribute.

Differential channels are selected by DIFSEL register. Negative
inputs must be added to pre-selected channels as well (PCSEL).
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc.c

@@ -92,6 +92,7 @@
 #define STM32H7_ADC_SQR3		0x38
 #define STM32H7_ADC_SQR4		0x3C
 #define STM32H7_ADC_DR			0x40
+#define STM32H7_ADC_DIFSEL		0xC0
 #define STM32H7_ADC_CALFACT		0xC4
 #define STM32H7_ADC_CALFACT2		0xC8
 
@@ -154,7 +155,7 @@ enum stm32h7_adc_dmngt {
 #define STM32H7_BOOST_CLKRATE		20000000UL
 
 #define STM32_ADC_CH_MAX		20	/* max number of channels */
-#define STM32_ADC_CH_SZ			5	/* max channel name size */
+#define STM32_ADC_CH_SZ			10	/* max channel name size */
 #define STM32_ADC_MAX_SQ		16	/* SQ1..SQ16 */
 #define STM32_ADC_MAX_SMP		7	/* SMPx range is [0..7] */
 #define STM32_ADC_TIMEOUT_US		100000
@@ -299,6 +300,7 @@ struct stm32_adc_cfg {
  * @rx_buf:		dma rx buffer cpu address
  * @rx_dma_buf:		dma rx buffer bus address
  * @rx_buf_sz:		dma rx buffer size
+ * @difsel		bitmask to set single-ended/differential channel
  * @pcsel		bitmask to preselect channels on some devices
  * @smpr_val:		sampling time settings (e.g. smpr1 / smpr2)
  * @cal:		optional calibration data on some devices
@@ -321,12 +323,18 @@ struct stm32_adc {
 	u8			*rx_buf;
 	dma_addr_t		rx_dma_buf;
 	unsigned int		rx_buf_sz;
+	u32			difsel;
 	u32			pcsel;
 	u32			smpr_val[2];
 	struct stm32_adc_calib	cal;
 	char			chan_name[STM32_ADC_CH_MAX][STM32_ADC_CH_SZ];
 };
 
+struct stm32_adc_diff_channel {
+	u32 vinp;
+	u32 vinn;
+};
+
 /**
  * struct stm32_adc_info - stm32 ADC, per instance config data
  * @max_channels:	Number of channels
@@ -945,15 +953,19 @@ static int stm32h7_adc_selfcalib(struct stm32_adc *adc)
  * stm32h7_adc_prepare() - Leave power down mode to enable ADC.
  * @adc: stm32 adc instance
  * Leave power down mode.
+ * Configure channels as single ended or differential before enabling ADC.
  * Enable ADC.
  * Restore calibration data.
- * Pre-select channels that may be used in PCSEL (required by input MUX / IO).
+ * Pre-select channels that may be used in PCSEL (required by input MUX / IO):
+ * - Only one input is selected for single ended (e.g. 'vinp')
+ * - Two inputs are selected for differential channels (e.g. 'vinp' & 'vinn')
  */
 static int stm32h7_adc_prepare(struct stm32_adc *adc)
 {
 	int ret;
 
 	stm32h7_adc_exit_pwr_down(adc);
+	stm32_adc_writel(adc, STM32H7_ADC_DIFSEL, adc->difsel);
 
 	ret = stm32h7_adc_enable(adc);
 	if (ret)
@@ -1225,10 +1237,23 @@ static int stm32_adc_read_raw(struct iio_dev *indio_dev,
 		return ret;
 
 	case IIO_CHAN_INFO_SCALE:
-		*val = adc->common->vref_mv;
-		*val2 = chan->scan_type.realbits;
+		if (chan->differential) {
+			*val = adc->common->vref_mv * 2;
+			*val2 = chan->scan_type.realbits;
+		} else {
+			*val = adc->common->vref_mv;
+			*val2 = chan->scan_type.realbits;
+		}
 		return IIO_VAL_FRACTIONAL_LOG2;
 
+	case IIO_CHAN_INFO_OFFSET:
+		if (chan->differential)
+			/* ADC_full_scale / 2 */
+			*val = -((1 << chan->scan_type.realbits) / 2);
+		else
+			*val = 0;
+		return IIO_VAL_INT;
+
 	default:
 		return -EINVAL;
 	}
@@ -1591,29 +1616,39 @@ static void stm32_adc_smpr_init(struct stm32_adc *adc, int channel, u32 smp_ns)
 
 static void stm32_adc_chan_init_one(struct iio_dev *indio_dev,
 				    struct iio_chan_spec *chan, u32 vinp,
-				    int scan_index, u32 smp)
+				    u32 vinn, int scan_index, bool differential)
 {
 	struct stm32_adc *adc = iio_priv(indio_dev);
 	char *name = adc->chan_name[vinp];
 
 	chan->type = IIO_VOLTAGE;
 	chan->channel = vinp;
-	snprintf(name, STM32_ADC_CH_SZ, "in%d", vinp);
+	if (differential) {
+		chan->differential = 1;
+		chan->channel2 = vinn;
+		snprintf(name, STM32_ADC_CH_SZ, "in%d-in%d", vinp, vinn);
+	} else {
+		snprintf(name, STM32_ADC_CH_SZ, "in%d", vinp);
+	}
 	chan->datasheet_name = name;
 	chan->scan_index = scan_index;
 	chan->indexed = 1;
 	chan->info_mask_separate = BIT(IIO_CHAN_INFO_RAW);
-	chan->info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE);
+	chan->info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE) |
+					 BIT(IIO_CHAN_INFO_OFFSET);
 	chan->scan_type.sign = 'u';
 	chan->scan_type.realbits = adc->cfg->adc_info->resolutions[adc->res];
 	chan->scan_type.storagebits = 16;
 	chan->ext_info = stm32_adc_ext_info;
 
-	/* Prepare sampling time settings */
-	stm32_adc_smpr_init(adc, chan->channel, smp);
-
 	/* pre-build selected channels mask */
 	adc->pcsel |= BIT(chan->channel);
+	if (differential) {
+		/* pre-build diff channels mask */
+		adc->difsel |= BIT(chan->channel);
+		/* Also add negative input to pre-selected channels */
+		adc->pcsel |= BIT(chan->channel2);
+	}
 }
 
 static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
@@ -1621,17 +1656,40 @@ static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
 	struct device_node *node = indio_dev->dev.of_node;
 	struct stm32_adc *adc = iio_priv(indio_dev);
 	const struct stm32_adc_info *adc_info = adc->cfg->adc_info;
+	struct stm32_adc_diff_channel diff[STM32_ADC_CH_MAX];
 	struct property *prop;
 	const __be32 *cur;
 	struct iio_chan_spec *channels;
-	int scan_index = 0, num_channels, ret;
+	int scan_index = 0, num_channels = 0, num_diff = 0, ret, i;
 	u32 val, smp = 0;
 
-	num_channels = of_property_count_u32_elems(node, "st,adc-channels");
-	if (num_channels < 0 ||
-	    num_channels > adc_info->max_channels) {
+	ret = of_property_count_u32_elems(node, "st,adc-channels");
+	if (ret > adc_info->max_channels) {
 		dev_err(&indio_dev->dev, "Bad st,adc-channels?\n");
-		return num_channels < 0 ? num_channels : -EINVAL;
+		return -EINVAL;
+	} else if (ret > 0) {
+		num_channels += ret;
+	}
+
+	ret = of_property_count_elems_of_size(node, "st,adc-diff-channels",
+					      sizeof(*diff));
+	if (ret > adc_info->max_channels) {
+		dev_err(&indio_dev->dev, "Bad st,adc-diff-channels?\n");
+		return -EINVAL;
+	} else if (ret > 0) {
+		int size = ret * sizeof(*diff) / sizeof(u32);
+
+		num_diff = ret;
+		num_channels += ret;
+		ret = of_property_read_u32_array(node, "st,adc-diff-channels",
+						 (u32 *)diff, size);
+		if (ret)
+			return ret;
+	}
+
+	if (!num_channels) {
+		dev_err(&indio_dev->dev, "No channels configured\n");
+		return -ENODATA;
 	}
 
 	/* Optional sample time is provided either for each, or all channels */
@@ -1652,6 +1710,33 @@ static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
 			return -EINVAL;
 		}
 
+		/* Channel can't be configured both as single-ended & diff */
+		for (i = 0; i < num_diff; i++) {
+			if (val == diff[i].vinp) {
+				dev_err(&indio_dev->dev,
+					"channel %d miss-configured\n",	val);
+				return -EINVAL;
+			}
+		}
+		stm32_adc_chan_init_one(indio_dev, &channels[scan_index], val,
+					0, scan_index, false);
+		scan_index++;
+	}
+
+	for (i = 0; i < num_diff; i++) {
+		if (diff[i].vinp >= adc_info->max_channels ||
+		    diff[i].vinn >= adc_info->max_channels) {
+			dev_err(&indio_dev->dev, "Invalid channel in%d-in%d\n",
+				diff[i].vinp, diff[i].vinn);
+			return -EINVAL;
+		}
+		stm32_adc_chan_init_one(indio_dev, &channels[scan_index],
+					diff[i].vinp, diff[i].vinn, scan_index,
+					true);
+		scan_index++;
+	}
+
+	for (i = 0; i < scan_index; i++) {
 		/*
 		 * Using of_property_read_u32_index(), smp value will only be
 		 * modified if valid u32 value can be decoded. This allows to
@@ -1659,11 +1744,9 @@ static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)
 		 * value per channel.
 		 */
 		of_property_read_u32_index(node, "st,min-sample-time-nsecs",
-					   scan_index, &smp);
-
-		stm32_adc_chan_init_one(indio_dev, &channels[scan_index],
-					val, scan_index, smp);
-		scan_index++;
+					   i, &smp);
+		/* Prepare sampling time settings */
+		stm32_adc_smpr_init(adc, channels[i].channel, smp);
 	}
 
 	indio_dev->num_channels = scan_index;