iio: adc: ad_sigma_delta: Add sequencer support

Some sigma-delta chips support sampling of multiple
channels in continuous mode.

When the operating with more than one channel enabled,
the channel sequencer cycles through the enabled channels
in sequential order, from first channel to the last one.
If a channel is disabled, it is skipped by the sequencer.

If more than one channel is used in continuous mode,
instruct the device to append the status to the SPI transfer
(1 extra byte) every time we receive a sample.
All sigma-delta chips possessing a sampling sequencer have
this ability. Inside the status register there will be
the number of the converted channel. In this way, even
if the CPU won't keep up with the sampling rate, it won't
send to userspace wrong channel samples.

When multiple channels are enabled in continuous mode,
the device needs to perform a measurement on all slots
before we can push to userspace the sample.

If, during sequencing and data reading, a channel measurement
is lost, a desync occurred. In this case, ad_sigma_delta drops
the incomplete sample and waits for the device to send the
measurement on the first active slot.

Co-developed-by: Alexandru Tachici <[e-mail]>
Signed-off-by: Alexandru Tachici <[e-mail]>
Signed-off-by: Lars-Peter Clausen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad_sigma_delta.c

···
  *  Author: Lars-Peter Clausen <[e-mail]>
  */
 
+#include <linux/align.h>
 #include <linux/interrupt.h>
 #include <linux/device.h>
 #include <linux/kernel.h>
···
 static int ad_sd_buffer_postenable(struct iio_dev *indio_dev)
 {
 	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
+	unsigned int i, slot, samples_buf_size;
 	unsigned int channel;
+	uint8_t *samples_buf;
 	int ret;
 
-	channel = find_first_bit(indio_dev->active_scan_mask,
-				 indio_dev->masklength);
-	ret = ad_sigma_delta_set_channel(sigma_delta,
-		indio_dev->channels[channel].address);
-	if (ret)
-		return ret;
+	if (sigma_delta->num_slots == 1) {
+		channel = find_first_bit(indio_dev->active_scan_mask,
+					 indio_dev->masklength);
+		ret = ad_sigma_delta_set_channel(sigma_delta,
+						 indio_dev->channels[channel].address);
+		if (ret)
+			return ret;
+		slot = 1;
+	} else {
+		/*
+		 * At this point update_scan_mode already enabled the required channels.
+		 * For sigma-delta sequencer drivers with multiple slots, an update_scan_mode
+		 * implementation is mandatory.
+		 */
+		slot = 0;
+		for_each_set_bit(i, indio_dev->active_scan_mask, indio_dev->masklength) {
+			sigma_delta->slots[slot] = indio_dev->channels[i].address;
+			slot++;
+		}
+	}
+
+	sigma_delta->active_slots = slot;
+	sigma_delta->current_slot = 0;
+
+	if (sigma_delta->active_slots > 1) {
+		ret = ad_sigma_delta_append_status(sigma_delta, true);
+		if (ret)
+			return ret;
+	}
+
+	samples_buf_size = ALIGN(slot * indio_dev->channels[0].scan_type.storagebits, 8);
+	samples_buf_size += sizeof(int64_t);
+	samples_buf = devm_krealloc(&sigma_delta->spi->dev, sigma_delta->samples_buf,
+				    samples_buf_size, GFP_KERNEL);
+	if (!samples_buf)
+		return -ENOMEM;
+
+	sigma_delta->samples_buf = samples_buf;
 
 	spi_bus_lock(sigma_delta->spi->master);
 	sigma_delta->bus_locked = true;
···
 	sigma_delta->keep_cs_asserted = false;
 	ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_IDLE);
 
+	if (sigma_delta->status_appended)
+		ad_sigma_delta_append_status(sigma_delta, false);
+
+	ad_sigma_delta_disable_all(sigma_delta);
 	sigma_delta->bus_locked = false;
 	return spi_bus_unlock(sigma_delta->spi->master);
 }
···
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
 	uint8_t *data = sigma_delta->rx_buf;
+	unsigned int transfer_size;
+	unsigned int sample_size;
+	unsigned int sample_pos;
+	unsigned int status_pos;
 	unsigned int reg_size;
 	unsigned int data_reg;
 
···
 	else
 		data_reg = AD_SD_REG_DATA;
 
+	/* Status word will be appended to the sample during transfer */
+	if (sigma_delta->status_appended)
+		transfer_size = reg_size + 1;
+	else
+		transfer_size = reg_size;
+
 	switch (reg_size) {
 	case 4:
 	case 2:
 	case 1:
-		ad_sd_read_reg_raw(sigma_delta, data_reg, reg_size, &data[0]);
+		status_pos = reg_size;
+		ad_sd_read_reg_raw(sigma_delta, data_reg, transfer_size, &data[0]);
 		break;
 	case 3:
+		/*
+		 * Data array after transfer will look like (if status is appended):
+		 * data[] = { [0][sample][sample][sample][status] }
+		 * Keeping the first byte 0 shifts the status postion by 1 byte to the right.
+		 */
+		status_pos = reg_size + 1;
+
 		/* We store 24 bit samples in a 32 bit word. Keep the upper
 		 * byte set to zero. */
-		ad_sd_read_reg_raw(sigma_delta, data_reg, reg_size, &data[1]);
+		ad_sd_read_reg_raw(sigma_delta, data_reg, transfer_size, &data[1]);
 		break;
 	}
 
-	iio_push_to_buffers_with_timestamp(indio_dev, data, pf->timestamp);
+	/*
+	 * For devices sampling only one channel at
+	 * once, there is no need for sample number tracking.
+	 */
+	if (sigma_delta->active_slots == 1) {
+		iio_push_to_buffers_with_timestamp(indio_dev, data, pf->timestamp);
+		goto irq_handled;
+	}
 
+	if (sigma_delta->status_appended) {
+		u8 converted_channel;
+
+		converted_channel = data[status_pos] & sigma_delta->info->status_ch_mask;
+		if (converted_channel != sigma_delta->slots[sigma_delta->current_slot]) {
+			/*
+			 * Desync occurred during continuous sampling of multiple channels.
+			 * Drop this incomplete sample and start from first channel again.
+			 */
+
+			sigma_delta->current_slot = 0;
+			goto irq_handled;
+		}
+	}
+
+	sample_size = indio_dev->channels[0].scan_type.storagebits / 8;
+	sample_pos = sample_size * sigma_delta->current_slot;
+	memcpy(&sigma_delta->samples_buf[sample_pos], data, sample_size);
+	sigma_delta->current_slot++;
+
+	if (sigma_delta->current_slot == sigma_delta->active_slots) {
+		sigma_delta->current_slot = 0;
+		iio_push_to_buffers_with_timestamp(indio_dev, sigma_delta->samples_buf,
+						   pf->timestamp);
+	}
+
+irq_handled:
 	iio_trigger_notify_done(indio_dev->trig);
 	sigma_delta->irq_dis = false;
 	enable_irq(sigma_delta->spi->irq);
···
 	return IRQ_HANDLED;
 }
 
+static bool ad_sd_validate_scan_mask(struct iio_dev *indio_dev, const unsigned long *mask)
+{
+	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
+
+	return bitmap_weight(mask, indio_dev->masklength) <= sigma_delta->num_slots;
+}
+
 static const struct iio_buffer_setup_ops ad_sd_buffer_setup_ops = {
 	.postenable = &ad_sd_buffer_postenable,
 	.postdisable = &ad_sd_buffer_postdisable,
-	.validate_scan_mask = &iio_validate_scan_mask_onehot,
+	.validate_scan_mask = &ad_sd_validate_scan_mask,
 };
 
 static irqreturn_t ad_sd_data_rdy_trig_poll(int irq, void *private)
···
  */
 int devm_ad_sd_setup_buffer_and_trigger(struct device *dev, struct iio_dev *indio_dev)
 {
+	struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
 	int ret;
+
+	sigma_delta->slots = devm_kcalloc(dev, sigma_delta->num_slots,
+					  sizeof(*sigma_delta->slots), GFP_KERNEL);
+	if (!sigma_delta->slots)
+		return -ENOMEM;
 
 	ret = devm_iio_triggered_buffer_setup(dev, indio_dev,
 					      &iio_pollfunc_store_time,
···
 {
 	sigma_delta->spi = spi;
 	sigma_delta->info = info;
+
+	/* If the field is unset in ad_sigma_delta_info, asume there can only be 1 slot. */
+	if (!info->num_slots)
+		sigma_delta->num_slots = 1;
+	else
+		sigma_delta->num_slots = info->num_slots;
+
+	if (sigma_delta->num_slots > 1) {
+		if (!indio_dev->info->update_scan_mode) {
+			dev_err(&spi->dev, "iio_dev lacks update_scan_mode().\n");
+			return -EINVAL;
+		}
+
+		if (!info->disable_all) {
+			dev_err(&spi->dev, "ad_sigma_delta_info lacks disable_all().\n");
+			return -EINVAL;
+		}
+	}
+
 	iio_device_set_drvdata(indio_dev, sigma_delta);
 
 	return 0;

include/linux/iio/adc/ad_sigma_delta.h

···
 /**
  * struct ad_sigma_delta_info - Sigma Delta driver specific callbacks and options
  * @set_channel: Will be called to select the current channel, may be NULL.
+ * @append_status: Will be called to enable status append at the end of the sample, may be NULL.
  * @set_mode: Will be called to select the current mode, may be NULL.
+ * @disable_all: Will be called to disable all channels, may be NULL.
  * @postprocess_sample: Is called for each sampled data word, can be used to
  *		modify or drop the sample data, it, may be NULL.
  * @has_registers: true if the device has writable and readable registers, false
  *		if there is just one read-only sample data shift register.
  * @addr_shift: Shift of the register address in the communications register.
  * @read_mask: Mask for the communications register having the read bit set.
+ * @status_ch_mask: Mask for the channel number stored in status register.
  * @data_reg: Address of the data register, if 0 the default address of 0x3 will
  *   be used.
  * @irq_flags: flags for the interrupt used by the triggered buffer
+ * @num_slots: Number of sequencer slots
  */
 struct ad_sigma_delta_info {
 	int (*set_channel)(struct ad_sigma_delta *, unsigned int channel);
+	int (*append_status)(struct ad_sigma_delta *, bool append);
 	int (*set_mode)(struct ad_sigma_delta *, enum ad_sigma_delta_mode mode);
+	int (*disable_all)(struct ad_sigma_delta *);
 	int (*postprocess_sample)(struct ad_sigma_delta *, unsigned int raw_sample);
 	bool has_registers;
 	unsigned int addr_shift;
 	unsigned int read_mask;
+	unsigned int status_ch_mask;
 	unsigned int data_reg;
 	unsigned long irq_flags;
+	unsigned int num_slots;
 };
 
 /**
···
 	uint8_t			comm;
 
 	const struct ad_sigma_delta_info *info;
+	unsigned int		active_slots;
+	unsigned int		current_slot;
+	unsigned int		num_slots;
+	bool			status_appended;
+	/* map slots to channels in order to know what to expect from devices */
+	unsigned int		*slots;
+	uint8_t			*samples_buf;
 
 	/*
 	 * DMA (thus cache coherency maintenance) requires the
···
 {
 	if (sd->info->set_channel)
 		return sd->info->set_channel(sd, channel);
+
+	return 0;
+}
+
+static inline int ad_sigma_delta_append_status(struct ad_sigma_delta *sd, bool append)
+{
+	int ret;
+
+	if (sd->info->append_status) {
+		ret = sd->info->append_status(sd, append);
+		if (ret < 0)
+			return ret;
+
+		sd->status_appended = append;
+	}
+
+	return 0;
+}
+
+static inline int ad_sigma_delta_disable_all(struct ad_sigma_delta *sd)
+{
+	if (sd->info->disable_all)
+		return sd->info->disable_all(sd);
 
 	return 0;
 }