iio: adc: at91-sama5d2_adc: add support for oversampling resolution

This implements oversampling support for the SAMA5d2 ADC device.
Enabling oversampling : OSR can improve resolution from 12 bits to
13 or 14 bits.
Changing the channel specification to have 14 bits, and we shift the value
1 bit to the left if we have oversampling for just one extra bit, and two
bits to the left if we have no oversampling (old support).
From this commit on, the converted values for all the voltage channels
change to 14 bits real data, with most insignificant two bits always zero
if oversampling is not enabled.
sysfs object oversampling_ratio has been enabled and
oversampling_ratio_available will list possible values (1 or 4 or 16) having
1 as default (no oversampling, 1 sample for each conversion).
Special care was required for the triggered buffer scenario (+ DMA), to
adjust the values accordingly.
Touchscreen measurements supported by this driver are not affected by
oversampling, they are still on 12 bits (scale handing is already included
in the driver).

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/at91-sama5d2_adc.c

@@ -130,6 +130,15 @@
 #define AT91_SAMA5D2_OVER	0x3c
 /* Extended Mode Register */
 #define AT91_SAMA5D2_EMR	0x40
+/* Extended Mode Register - Oversampling rate */
+#define AT91_SAMA5D2_EMR_OSR(V)			((V) << 16)
+#define AT91_SAMA5D2_EMR_OSR_MASK		GENMASK(17, 16)
+#define AT91_SAMA5D2_EMR_OSR_1SAMPLES		0
+#define AT91_SAMA5D2_EMR_OSR_4SAMPLES		1
+#define AT91_SAMA5D2_EMR_OSR_16SAMPLES		2
+
+/* Extended Mode Register - Averaging on single trigger event */
+#define AT91_SAMA5D2_EMR_ASTE(V)		((V) << 20)
 /* Compare Window Register */
 #define AT91_SAMA5D2_CWR	0x44
 /* Channel Gain Register */
@@ -248,6 +257,11 @@
 #define AT91_HWFIFO_MAX_SIZE_STR	"128"
 #define AT91_HWFIFO_MAX_SIZE		128
 
+/* Possible values for oversampling ratio */
+#define AT91_OSR_1SAMPLES		1
+#define AT91_OSR_4SAMPLES		4
+#define AT91_OSR_16SAMPLES		16
+
 #define AT91_SAMA5D2_CHAN_SINGLE(num, addr)				\
 	{								\
 		.type = IIO_VOLTAGE,					\
@@ -256,12 +270,13 @@
 		.scan_index = num,					\
 		.scan_type = {						\
 			.sign = 'u',					\
-			.realbits = 12,					\
+			.realbits = 14,					\
 			.storagebits = 16,				\
 		},							\
 		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),		\
 		.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),	\
-		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ),\
+		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ)|\
+				BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO),	\
 		.datasheet_name = "CH"#num,				\
 		.indexed = 1,						\
 	}
@@ -276,12 +291,13 @@
 		.scan_index = num + AT91_SAMA5D2_SINGLE_CHAN_CNT,	\
 		.scan_type = {						\
 			.sign = 's',					\
-			.realbits = 12,					\
+			.realbits = 14,					\
 			.storagebits = 16,				\
 		},							\
 		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),		\
 		.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),	\
-		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ),\
+		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ)|\
+				BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO),	\
 		.datasheet_name = "CH"#num"-CH"#num2,			\
 		.indexed = 1,						\
 	}
@@ -299,7 +315,8 @@
 			.storagebits = 16,				\
 		},							\
 		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),		\
-		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ),\
+		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ)|\
+				BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO),	\
 		.datasheet_name = name,					\
 	}
 #define AT91_SAMA5D2_CHAN_PRESSURE(num, name)				\
@@ -313,7 +330,8 @@
 			.storagebits = 16,				\
 		},							\
 		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),		\
-		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ),\
+		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ)|\
+				BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO),	\
 		.datasheet_name = name,					\
 	}
 
@@ -384,6 +402,7 @@ struct at91_adc_state {
 	const struct iio_chan_spec	*chan;
 	bool				conversion_done;
 	u32				conversion_value;
+	unsigned int			oversampling_ratio;
 	struct at91_adc_soc_info	soc_info;
 	wait_queue_head_t		wq_data_available;
 	struct at91_adc_dma		dma_st;
@@ -475,6 +494,77 @@ static inline int at91_adc_of_xlate(struct iio_dev *indio_dev,
 	return at91_adc_chan_xlate(indio_dev, iiospec->args[0]);
 }
 
+static void at91_adc_config_emr(struct at91_adc_state *st)
+{
+	/* configure the extended mode register */
+	unsigned int emr = at91_adc_readl(st, AT91_SAMA5D2_EMR);
+
+	/* select oversampling per single trigger event */
+	emr |= AT91_SAMA5D2_EMR_ASTE(1);
+
+	/* delete leftover content if it's the case */
+	emr &= ~AT91_SAMA5D2_EMR_OSR_MASK;
+
+	/* select oversampling ratio from configuration */
+	switch (st->oversampling_ratio) {
+	case AT91_OSR_1SAMPLES:
+		emr |= AT91_SAMA5D2_EMR_OSR(AT91_SAMA5D2_EMR_OSR_1SAMPLES) &
+		       AT91_SAMA5D2_EMR_OSR_MASK;
+		break;
+	case AT91_OSR_4SAMPLES:
+		emr |= AT91_SAMA5D2_EMR_OSR(AT91_SAMA5D2_EMR_OSR_4SAMPLES) &
+		       AT91_SAMA5D2_EMR_OSR_MASK;
+		break;
+	case AT91_OSR_16SAMPLES:
+		emr |= AT91_SAMA5D2_EMR_OSR(AT91_SAMA5D2_EMR_OSR_16SAMPLES) &
+		       AT91_SAMA5D2_EMR_OSR_MASK;
+		break;
+	}
+
+	at91_adc_writel(st, AT91_SAMA5D2_EMR, emr);
+}
+
+static int at91_adc_adjust_val_osr(struct at91_adc_state *st, int *val)
+{
+	if (st->oversampling_ratio == AT91_OSR_1SAMPLES) {
+		/*
+		 * in this case we only have 12 bits of real data, but channel
+		 * is registered as 14 bits, so shift left two bits
+		 */
+		*val <<= 2;
+	} else if (st->oversampling_ratio == AT91_OSR_4SAMPLES) {
+		/*
+		 * in this case we have 13 bits of real data, but channel
+		 * is registered as 14 bits, so left shift one bit
+		 */
+		*val <<= 1;
+	}
+
+	return IIO_VAL_INT;
+}
+
+static void at91_adc_adjust_val_osr_array(struct at91_adc_state *st, void *buf,
+					  int len)
+{
+	int i = 0, val;
+	u16 *buf_u16 = (u16 *) buf;
+
+	/*
+	 * We are converting each two bytes (each sample).
+	 * First convert the byte based array to u16, and convert each sample
+	 * separately.
+	 * Each value is two bytes in an array of chars, so to not shift
+	 * more than we need, save the value separately.
+	 * len is in bytes, so divide by two to get number of samples.
+	 */
+	while (i < len / 2) {
+		val = buf_u16[i];
+		at91_adc_adjust_val_osr(st, &val);
+		buf_u16[i] = val;
+		i++;
+	}
+}
+
 static int at91_adc_configure_touch(struct at91_adc_state *st, bool state)
 {
 	u32 clk_khz = st->current_sample_rate / 1000;
@@ -916,6 +1006,7 @@ static void at91_adc_trigger_handler_nodma(struct iio_dev *indio_dev,
 {
 	struct at91_adc_state *st = iio_priv(indio_dev);
 	int i = 0;
+	int val;
 	u8 bit;
 
 	for_each_set_bit(bit, indio_dev->active_scan_mask,
@@ -936,7 +1027,9 @@ static void at91_adc_trigger_handler_nodma(struct iio_dev *indio_dev,
 		 * Thus, emit a warning.
 		 */
 		if (chan->type == IIO_VOLTAGE) {
-			st->buffer[i] = at91_adc_readl(st, chan->address);
+			val = at91_adc_readl(st, chan->address);
+			at91_adc_adjust_val_osr(st, &val);
+			st->buffer[i] = val;
 		} else {
 			st->buffer[i] = 0;
 			WARN(true, "This trigger cannot handle this type of channel");
@@ -972,6 +1065,14 @@ static void at91_adc_trigger_handler_dma(struct iio_dev *indio_dev)
 	interval = div_s64((ns - st->dma_st.dma_ts), sample_count);
 
 	while (transferred_len >= sample_size) {
+		/*
+		 * for all the values in the current sample,
+		 * adjust the values inside the buffer for oversampling
+		 */
+		at91_adc_adjust_val_osr_array(st,
+					&st->dma_st.rx_buf[st->dma_st.buf_idx],
+					sample_size);
+
 		iio_push_to_buffers_with_timestamp(indio_dev,
 				(st->dma_st.rx_buf + st->dma_st.buf_idx),
 				(st->dma_st.dma_ts + interval * sample_index));
@@ -1212,7 +1313,7 @@ static int at91_adc_read_info_raw(struct iio_dev *indio_dev,
 		mutex_unlock(&st->lock);
 		iio_device_release_direct_mode(indio_dev);
 
-		return ret;
+		return at91_adc_adjust_val_osr(st, val);
 	}
 	if (chan->type == IIO_PRESSURE) {
 		ret = iio_device_claim_direct_mode(indio_dev);
@@ -1225,7 +1326,7 @@ static int at91_adc_read_info_raw(struct iio_dev *indio_dev,
 		mutex_unlock(&st->lock);
 		iio_device_release_direct_mode(indio_dev);
 
-		return ret;
+		return at91_adc_adjust_val_osr(st, val);
 	}
 
 	/* in this case we have a voltage channel */
@@ -1254,9 +1355,9 @@ static int at91_adc_read_info_raw(struct iio_dev *indio_dev,
 
 	if (ret > 0) {
 		*val = st->conversion_value;
+		ret = at91_adc_adjust_val_osr(st, val);
 		if (chan->scan_type.sign == 's')
 			*val = sign_extend32(*val, 11);
-		ret = IIO_VAL_INT;
 		st->conversion_done = false;
 	}
 
@@ -1292,6 +1393,10 @@ static int at91_adc_read_raw(struct iio_dev *indio_dev,
 		*val = at91_adc_get_sample_freq(st);
 		return IIO_VAL_INT;
 
+	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
+		*val = st->oversampling_ratio;
+		return IIO_VAL_INT;
+
 	default:
 		return -EINVAL;
 	}
@@ -1303,16 +1408,28 @@ static int at91_adc_write_raw(struct iio_dev *indio_dev,
 {
 	struct at91_adc_state *st = iio_priv(indio_dev);
 
-	if (mask != IIO_CHAN_INFO_SAMP_FREQ)
-		return -EINVAL;
+	switch (mask) {
+	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
+		if ((val != AT91_OSR_1SAMPLES) && (val != AT91_OSR_4SAMPLES) &&
+		    (val != AT91_OSR_16SAMPLES))
+			return -EINVAL;
+		/* if no change, optimize out */
+		if (val == st->oversampling_ratio)
+			return 0;
+		st->oversampling_ratio = val;
+		/* update ratio */
+		at91_adc_config_emr(st);
+		return 0;
+	case IIO_CHAN_INFO_SAMP_FREQ:
+		if (val < st->soc_info.min_sample_rate ||
+		    val > st->soc_info.max_sample_rate)
+			return -EINVAL;
 
-	if (val < st->soc_info.min_sample_rate ||
-	    val > st->soc_info.max_sample_rate)
+		at91_adc_setup_samp_freq(st, val);
+		return 0;
+	default:
 		return -EINVAL;
-
-	at91_adc_setup_samp_freq(st, val);
-
-	return 0;
+	};
 }
 
 static void at91_adc_dma_init(struct platform_device *pdev)
@@ -1446,14 +1563,6 @@ static int at91_adc_update_scan_mode(struct iio_dev *indio_dev,
 	return 0;
 }
 
-static const struct iio_info at91_adc_info = {
-	.read_raw = &at91_adc_read_raw,
-	.write_raw = &at91_adc_write_raw,
-	.update_scan_mode = &at91_adc_update_scan_mode,
-	.of_xlate = &at91_adc_of_xlate,
-	.hwfifo_set_watermark = &at91_adc_set_watermark,
-};
-
 static void at91_adc_hw_init(struct at91_adc_state *st)
 {
 	at91_adc_writel(st, AT91_SAMA5D2_CR, AT91_SAMA5D2_CR_SWRST);
@@ -1466,6 +1575,9 @@ static void at91_adc_hw_init(struct at91_adc_state *st)
 			AT91_SAMA5D2_MR_TRANSFER(2) | AT91_SAMA5D2_MR_ANACH);
 
 	at91_adc_setup_samp_freq(st, st->soc_info.min_sample_rate);
+
+	/* configure extended mode register */
+	at91_adc_config_emr(st);
 }
 
 static ssize_t at91_adc_get_fifo_state(struct device *dev,
@@ -1496,6 +1608,20 @@ static IIO_DEVICE_ATTR(hwfifo_watermark, 0444,
 static IIO_CONST_ATTR(hwfifo_watermark_min, "2");
 static IIO_CONST_ATTR(hwfifo_watermark_max, AT91_HWFIFO_MAX_SIZE_STR);
 
+static IIO_CONST_ATTR(oversampling_ratio_available,
+		      __stringify(AT91_OSR_1SAMPLES) " "
+		      __stringify(AT91_OSR_4SAMPLES) " "
+		      __stringify(AT91_OSR_16SAMPLES));
+
+static struct attribute *at91_adc_attributes[] = {
+	&iio_const_attr_oversampling_ratio_available.dev_attr.attr,
+	NULL,
+};
+
+static const struct attribute_group at91_adc_attribute_group = {
+	.attrs = at91_adc_attributes,
+};
+
 static const struct attribute *at91_adc_fifo_attributes[] = {
 	&iio_const_attr_hwfifo_watermark_min.dev_attr.attr,
 	&iio_const_attr_hwfifo_watermark_max.dev_attr.attr,
@@ -1504,6 +1630,15 @@ static const struct attribute *at91_adc_fifo_attributes[] = {
 	NULL,
 };
 
+static const struct iio_info at91_adc_info = {
+	.attrs = &at91_adc_attribute_group,
+	.read_raw = &at91_adc_read_raw,
+	.write_raw = &at91_adc_write_raw,
+	.update_scan_mode = &at91_adc_update_scan_mode,
+	.of_xlate = &at91_adc_of_xlate,
+	.hwfifo_set_watermark = &at91_adc_set_watermark,
+};
+
 static int at91_adc_probe(struct platform_device *pdev)
 {
 	struct iio_dev *indio_dev;
@@ -1532,6 +1667,8 @@ static int at91_adc_probe(struct platform_device *pdev)
 	bitmap_set(&st->touch_st.channels_bitmask,
 		   AT91_SAMA5D2_TOUCH_P_CHAN_IDX, 1);
 
+	st->oversampling_ratio = AT91_OSR_1SAMPLES;
+
 	ret = of_property_read_u32(pdev->dev.of_node,
 				   "atmel,min-sample-rate-hz",
 				   &st->soc_info.min_sample_rate);