Date:	Sun, 1 May 2016 18:54:58 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Crestez Dan Leonard <leonard.crestez@...el.com>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>
Cc:	linux-kernel@...r.kernel.org, Hartmut Knaack <knaack.h@....de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Peter Meerwald-Stadler <pmeerw@...erw.net>,
	Daniel Baluta <daniel.baluta@...el.com>,
	Ge Gao <GGao@...ensense.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Mark Rutland <Mark.Rutland@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Linux I2C <linux-i2c@...r.kernel.org>,
	Wolfram Sang <wsa@...-dreams.de>,
	Peter Rosin <peda@...ntia.se>,
	Wolfram Sang <wsa@...-dreams.de>,
	Mark Brown <broonie@...nel.org>
Subject: Re: [RFC 7/7] iio: inv_mpu6050: Add support for external sensors

On 29/04/16 20:02, Crestez Dan Leonard wrote:
> This works by copying their channels at probe time.
> 
> Signed-off-by: Crestez Dan Leonard <leonard.crestez@...el.com>
This is some mixture of deeply nefarious and rather cool.
Great bit of work all in all.

Cc'd device tree list and maintainers - to them - this is downright mad
hardware - basically a partial i2c offload engine sitting on either i2c or
spi buses with lots of iritating restrictions but useful stuff like a fifo
between it and the rest of the world - upshot it's much much faster than
actually addressing these devices directly... 

First time I read the datasheet for this part - I thought no one would ever
figure out / go to the effort of supporting this functionality.  Turns
out I was wrong!

It is the only part I know of allowing this level of flexibility as a sensor
hub.  Most sensor hubs hide entirely what is going on behind them whereas this
one exposes what is going on...

Also cc'd the i2c list / Peter, Wolfram and Mark (just because they might be
interested!)

Jonathan
> ---
>  .../devicetree/bindings/iio/imu/inv_mpu6050.txt    |  37 +-
>  drivers/iio/imu/inv_mpu6050/inv_mpu_core.c         | 420 ++++++++++++++++++++-
>  drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h          |  63 +++-
>  drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c         |  48 ++-
>  4 files changed, 555 insertions(+), 13 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/iio/imu/inv_mpu6050.txt b/Documentation/devicetree/bindings/iio/imu/inv_mpu6050.txt
> index aaf12b4..79b8326 100644
> --- a/Documentation/devicetree/bindings/iio/imu/inv_mpu6050.txt
> +++ b/Documentation/devicetree/bindings/iio/imu/inv_mpu6050.txt
> @@ -23,6 +23,28 @@ Devices connected in "bypass" mode must be listed behind i2c@0 with the address
>  
>  Devices connected in "master" mode must be listed behind i2c@1 with the address 1
>  
> +It is possible to configure the MPU to automatically fetch reading for
> +devices connected on the auxiliary i2c bus without CPU intervention. When this
> +is done the driver will present the channels of the slaved devices as if they
> +belong to the MPU device itself.
> +
> +Reads and write to config values (like scaling factors) are forwarded to the
> +other driver while data reads are handled from MPU registers. The channels are
> +also available through the MPU's iio triggered buffer mechanism. This feature
> +can allow sampling up to 24 bytes from 4 slaves at a much higher rate.
> +
> +This is be specified in device tree as an "inv,external-sensors" node which
> +have children indexed by slave ids 0 to 3. The child node identifying each
> +external sensor reading has the following properties:
> + - reg: 0 to 3 slave index
> + - inv,addr : the I2C address to read from
> + - inv,reg : the I2C register to read from
> + - inv,len : read length from the device
> + - inv,external-channels : list of slaved channels specified by config index.
I wonder if we want to try and make these generic - probably not until we have
at least one more device doing something similar...
> +
> +The sum of storagebits for external channels must equal the read length. Only
> +16bit channels are currently supported.
> +
>  Example:
>  	mpu6050@68 {
>  		compatible = "invensense,mpu6050";
> @@ -61,7 +83,8 @@ Example describing mpu9150 (which includes an ak9875 on chip):
>  		};
>  	};
>  
> -Example describing a mpu6500 via SPI with an hmc5883l on auxiliary i2c:
> +Example describing a mpu6500 via SPI with an hmc5883l on aux i2c configured for
> +automatic external readings as slave 0:
>  	mpu6500@0 {
>  		compatible = "inv,mpu6500";
>  		reg = <0x0>;
> @@ -80,4 +103,16 @@ Example describing a mpu6500 via SPI with an hmc5883l on auxiliary i2c:
>  				reg = <0x1e>;
>  			};
>  		};
> +
> +		inv,external-sensors {
> +			#address-cells = <1>;
> +			#size-cells = <0>;
> +			hmc5833l@0 {
> +				reg = <0>;
> +				inv,addr = <0x1e>;
> +				inv,reg = <3>;
> +				inv,len = <6>;
> +				inv,external-channels = <0 1 2>;
> +			};
> +		};
>  	};
> diff --git a/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c b/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
> index 712e901..224be3c 100644
> --- a/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
> +++ b/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
> @@ -41,6 +41,8 @@ static const int gyro_scale_6050[] = {133090, 266181, 532362, 1064724};
>   */
>  static const int accel_scale[] = {598, 1196, 2392, 4785};
>  
> +#define INV_MPU6050_NUM_INT_CHAN 8
> +
>  static const struct inv_mpu6050_reg_map reg_set_6500 = {
>  	.sample_rate_div	= INV_MPU6050_REG_SAMPLE_RATE_DIV,
>  	.lpf                    = INV_MPU6050_REG_CONFIG,
> @@ -136,6 +138,9 @@ static bool inv_mpu6050_volatile_reg(struct device *dev, unsigned int reg)
>  		return true;
>  	if (reg >= INV_MPU6050_REG_RAW_GYRO && reg < INV_MPU6050_REG_RAW_GYRO + 6)
>  		return true;
> +	if (reg < INV_MPU6050_REG_EXT_SENS_DATA_00 + INV_MPU6050_CNT_EXT_SENS_DATA &&
> +			reg >= INV_MPU6050_REG_EXT_SENS_DATA_00)
> +		return true;
>  	switch (reg) {
>  	case INV_MPU6050_REG_TEMPERATURE:
>  	case INV_MPU6050_REG_TEMPERATURE + 1:
> @@ -340,13 +345,115 @@ static int inv_mpu6050_sensor_show(struct inv_mpu6050_state  *st, int reg,
>  	return IIO_VAL_INT;
>  }
>  
> +static bool iio_chan_needs_swab(const struct iio_chan_spec *chan)
> +{
> +#if defined(__LITTLE_ENDIAN)
> +	return chan->scan_type.endianness == IIO_BE;
> +#elif defined(__BIG_ENDIAN)
> +	return chan->scan_type.endianness == IIO_LE;
> +#else
> +	#error undefined endianness
> +#endif
> +}
> +
> +/* Convert iio buffer format to IIO_CHAN_INFO_RAW value */
> +static int iio_bufval_to_rawval(const struct iio_chan_spec *chan, void *buf, int *val)
> +{
> +	switch (chan->scan_type.storagebits) {
> +	case 8: *val = *((u8*)buf); break;
> +	case 16: *val = *(u16*)buf; break;
> +	case 32: *val = *(u32*)buf; break;
> +	default: return -EINVAL;
> +	}
> +
> +	*val >>= chan->scan_type.shift;
> +	*val &= (1 << chan->scan_type.realbits) - 1;
> +	if (iio_chan_needs_swab(chan)) {
> +		if (chan->scan_type.storagebits == 16)
> +			*val = swab16(*val);
> +		else if (chan->scan_type.storagebits == 32)
> +			*val = swab32(*val);
This needs some explanatory comments!
> +	}
> +	if (chan->scan_type.sign == 's')
> +		*val = sign_extend32(*val, chan->scan_type.storagebits - 1);
> +
> +	return 0;
> +}
> +
> +static int
> +inv_mpu6050_read_raw_external(struct iio_dev *indio_dev,
> +			      struct iio_chan_spec const *chan,
> +			      int *val, int *val2, long mask)
> +{
> +	int result;
> +	struct inv_mpu6050_state *st = iio_priv(indio_dev);
> +	int chan_index = chan - indio_dev->channels;
> +	struct inv_mpu6050_ext_chan_state *ext_chan_state =
> +			&st->ext_chan[chan_index - INV_MPU6050_NUM_INT_CHAN];
> +	struct inv_mpu6050_ext_sens_state *ext_sens_state =
> +			&st->ext_sens[ext_chan_state->ext_sens_index];
> +	struct iio_dev *orig_dev = ext_sens_state->orig_dev;
> +	const struct iio_chan_spec *orig_chan = &orig_dev->channels[ext_chan_state->orig_chan_index];
> +	int data_offset;
> +	int data_length;
> +	u8 buf[4];
> +
> +	/* Everything but raw data reads is forwarded. */
> +	if (mask != IIO_CHAN_INFO_RAW)
> +		return orig_dev->info->read_raw(orig_dev, orig_chan, val, val2, mask);
Obviously this is useful for testing, but on a polled interface do we actually
care that it is quicker to go directly?  I'd be tempted to not bother except
for the buffered mode.
> +
> +	/* Raw reads are handled directly. */
> +	data_offset = INV_MPU6050_REG_EXT_SENS_DATA_00 + ext_chan_state->data_offset;
> +	data_length = chan->scan_type.storagebits / 8;
> +	if (data_length > sizeof(buf)) {
> +		return -EINVAL;
> +	}
> +
> +	mutex_lock(&indio_dev->mlock);
> +	if (!st->chip_config.enable) {
> +		result = inv_mpu6050_set_power_itg(st, true);
> +		if (result)
> +			goto error_unlock;
> +	}
> +	result = regmap_bulk_read(st->map, data_offset, buf, data_length);
> +	if (result)
> +		goto error_unpower;
> +	if (!st->chip_config.enable) {
> +		result = inv_mpu6050_set_power_itg(st, false);
> +		if (result)
> +			goto error_unlock;
> +	}
> +	mutex_unlock(&indio_dev->mlock);
> +
> +	/* Convert buf to val and return success */
> +	result = iio_bufval_to_rawval(chan, buf, val);
> +	if (result)
> +		return result;
> +	return IIO_VAL_INT;
> +
> +error_unpower:
> +	if (!st->chip_config.enable)
> +		inv_mpu6050_set_power_itg(st, false);
> +error_unlock:
> +	mutex_unlock(&indio_dev->mlock);
> +	return result;
> +}
> +
>  static int
>  inv_mpu6050_read_raw(struct iio_dev *indio_dev,
>  		     struct iio_chan_spec const *chan,
>  		     int *val, int *val2, long mask)
>  {
> -	struct inv_mpu6050_state  *st = iio_priv(indio_dev);
> +	struct inv_mpu6050_state *st = iio_priv(indio_dev);
>  	int ret = 0;
> +	int chan_index;
> +
> +	if (chan < indio_dev->channels || chan > indio_dev->channels + indio_dev->num_channels) {
> +		return -EINVAL;
> +	}
> +	chan_index = chan - indio_dev->channels;
> +	if (chan_index >= INV_MPU6050_NUM_INT_CHAN)
> +		return inv_mpu6050_read_raw_external(indio_dev, chan, val, val2, mask);
>  
>  	switch (mask) {
>  	case IIO_CHAN_INFO_RAW:
> @@ -819,6 +926,309 @@ static const struct iio_info mpu_info = {
>  	.validate_trigger = inv_mpu6050_validate_trigger,
>  };
>  
> +extern struct device_type iio_device_type;
> +
> +static int iio_device_from_i2c_client_match(struct device *dev, void *data)
> +{
> +	return dev->type == &iio_device_type;
> +}
> +
> +static struct iio_dev* iio_device_from_i2c_client(struct i2c_client* i2c)
> +{
> +	struct device *child;
> +
> +	child = device_find_child(&i2c->dev, NULL, iio_device_from_i2c_client_match);
> +
> +	return (child ? dev_to_iio_dev(child) : NULL);
> +}
> +
> +static int inv_mpu_set_external_reads_enabled(struct inv_mpu6050_state *st, bool en)
> +{
> +	int i, result, error = 0;
> +
> +	/* Even if there are errors try to disable all slaves. */
> +	for (i = 0; i < INV_MPU6050_MAX_EXT_SENSORS; ++i) {
> +		result = regmap_update_bits(st->map, INV_MPU6050_REG_I2C_SLV_CTRL(i),
> +					    INV_MPU6050_BIT_I2C_SLV_EN,
> +					    en ?  INV_MPU6050_BIT_I2C_SLV_EN : 0);
> +		if (result) {
> +			error = result;
> +		}
> +	}
> +
> +	return error;
> +}
> +
> +static int inv_mpu_parse_one_ext_sens(struct device *dev,
> +				      struct device_node *np,
> +				      struct inv_mpu6050_ext_sens_spec *spec)
> +{
> +	int result;
> +	u32 addr, reg, len;
> +
> +	result = of_property_read_u32(np, "inv,addr", &addr);
> +	if (result)
> +		return result;
> +	result = of_property_read_u32(np, "inv,reg", &reg);
> +	if (result)
> +		return result;
> +	result = of_property_read_u32(np, "inv,len", &len);
> +	if (result)
> +		return result;
> +
> +	spec->addr = addr;
> +	spec->reg = reg;
> +	spec->len = len;
> +
> +	result = of_property_count_u32_elems(np, "inv,external-channels");
> +	if (result < 0)
> +		return result;
> +	spec->num_ext_channels = result;
> +	spec->ext_channels = devm_kmalloc(dev, spec->num_ext_channels * sizeof(*spec->ext_channels), GFP_KERNEL);
> +	if (!spec->ext_channels)
> +		return -ENOMEM;
> +	result = of_property_read_u32_array(np, "inv,external-channels",
> +					    spec->ext_channels,
> +					    spec->num_ext_channels);
> +	if (result)
> +		return result;
> +
> +	return 0;
> +}
> +
> +static int inv_mpu_parse_ext_sens(struct device *dev,
> +				  struct device_node *node,
> +				  struct inv_mpu6050_ext_sens_spec *specs)
> +{
> +	struct device_node *child;
> +	int result;
> +	u32 reg;
> +
> +	for_each_available_child_of_node(node, child) {
> +		result = of_property_read_u32(child, "reg", &reg);
> +		if (result)
> +			return result;
> +		if (reg > INV_MPU6050_MAX_EXT_SENSORS) {
> +			dev_err(dev, "External sensor index %u out of range, max %d\n",
> +				reg, INV_MPU6050_MAX_EXT_SENSORS);
> +			return -EINVAL;
> +		}
> +		result = inv_mpu_parse_one_ext_sens(dev, child, &specs[reg]);
> +		if (result)
> +			return result;
> +	}
> +
> +	return 0;
> +}
> +
> +static int inv_mpu_get_ext_sens_spec(struct iio_dev *indio_dev)
> +{
> +	struct inv_mpu6050_state *st = iio_priv(indio_dev);
> +	struct device *dev = regmap_get_device(st->map);
> +	struct device_node *node;
> +	int result;
> +
> +	node = of_get_child_by_name(dev->of_node, "inv,external-sensors");
> +	if (node) {
> +		result = inv_mpu_parse_ext_sens(dev, node, st->ext_sens_spec);
> +		if (result)
> +			dev_err(dev, "Failed to parsing external-sensors devicetree data\n");
> +		return result;
> +	}
> +
> +	/* Maybe some other way to deal with this? */
Probably not ;)
> +
> +	return 0;
> +}
> +
> +/* Struct used while enumerating devices and matching them */
> +struct inv_mpu_handle_ext_sensor_fnarg
> +{
> +	struct iio_dev *indio_dev;
> +
> +	/* Next scan index */
> +	int scan_index;
> +	/* Index among external channels */
> +	int chan_index;
> +	/* Offset in EXTDATA */
> +	int data_offset;
> +	struct iio_chan_spec *channels;
> +};
> +
> +static int inv_mpu_config_external_read(struct inv_mpu6050_state *st, int index,
> +					const struct inv_mpu6050_ext_sens_spec *spec)
> +{
> +	int result;
> +
> +	result = regmap_write(st->map, INV_MPU6050_REG_I2C_SLV_ADDR(index),
> +			      INV_MPU6050_BIT_I2C_SLV_RW | spec->addr);
> +	if (result)
> +		return result;
> +	result = regmap_write(st->map, INV_MPU6050_REG_I2C_SLV_REG(index), spec->reg);
> +	if (result)
> +		return result;
> +	result = regmap_write(st->map, INV_MPU6050_REG_I2C_SLV_CTRL(index),
> +			      INV_MPU6050_BIT_I2C_SLV_EN | spec->len);
> +
> +	return result;
> +}
> +
> +static int inv_mpu_handle_ext_sensor_fn(struct device *dev, void *data)
> +{
> +	struct inv_mpu_handle_ext_sensor_fnarg *fnarg = data;
> +	struct iio_dev *indio_dev = fnarg->indio_dev;
> +	struct inv_mpu6050_state *st = iio_priv(indio_dev);
> +	struct device *mydev = regmap_get_device(st->map);
> +	struct i2c_client *client;
> +	struct iio_dev *orig_dev;
> +	int i, j;
> +
> +	client = i2c_verify_client(dev);
> +	if (!client)
> +		return 0;
> +	orig_dev = iio_device_from_i2c_client(client);
> +	if (!orig_dev)
> +		return 0;
> +	for (i = 0; i < INV_MPU6050_MAX_EXT_SENSORS; ++i) {
> +		int start_data_offset;
> +		struct inv_mpu6050_ext_sens_spec *ext_sens_spec = &st->ext_sens_spec[i];
> +		struct inv_mpu6050_ext_sens_state *ext_sens_state = &st->ext_sens[i];
> +
> +		if (ext_sens_spec->addr != client->addr)
> +			continue;
> +		if (ext_sens_state->orig_dev) {
> +			dev_warn(&indio_dev->dev, "already found slave with at addr %d\n", client->addr);
> +			continue;
> +		}
> +		dev_info(mydev, "external sensor %d is device %s\n",
> +				i, orig_dev->name ?: dev_name(&orig_dev->dev));
> +		ext_sens_state->orig_dev = orig_dev;
> +		ext_sens_state->scan_mask = 0;
> +		ext_sens_state->data_len = ext_sens_spec->len;
> +		start_data_offset = fnarg->data_offset;
> +
> +		/* Matched the device (by address). Now process channels: */
> +		for (j = 0; j < ext_sens_spec->num_ext_channels; ++j) {
> +			int orig_chan_index;
> +			const struct iio_chan_spec *orig_chan_spec;
> +			struct iio_chan_spec *chan_spec;
> +			struct inv_mpu6050_ext_chan_state *chan_state;
> +
> +			orig_chan_index = ext_sens_spec->ext_channels[j];
> +			orig_chan_spec = &orig_dev->channels[orig_chan_index];
> +			chan_spec = &fnarg->channels[INV_MPU6050_NUM_INT_CHAN + fnarg->chan_index];
> +			chan_state = &st->ext_chan[fnarg->chan_index];
> +
> +			chan_state->ext_sens_index = i;
> +			chan_state->orig_chan_index = orig_chan_index;
> +			chan_state->data_offset = fnarg->data_offset;
> +			memcpy(chan_spec, orig_chan_spec, sizeof(struct iio_chan_spec));
> +			chan_spec->scan_index = fnarg->scan_index;
> +			ext_sens_state->scan_mask |= (1 << chan_spec->scan_index);
> +			if (orig_chan_spec->scan_type.storagebits != 16) {
> +				/*
> +				 * Supporting other channels sizes would require data read from the
> +				 * hardware fifo to comply with iio alignment.
> +				 */
Or a demux/mux before iio_push_to_buffers.
> +				dev_err(&indio_dev->dev, "All external channels must have 16 storage bits\n");
> +				return -EINVAL;
> +			}
> +
> +			fnarg->scan_index++;
> +			fnarg->chan_index++;
> +			fnarg->data_offset += chan_spec->scan_type.storagebits / 8;
> +			dev_info(mydev, "Reading external channel #%d scan_index %d data_offset %d"
> +					" from original device %s chan #%d scan_index %d\n",
> +					fnarg->chan_index, chan_spec->scan_index, chan_state->data_offset,
> +					orig_dev->name ?: dev_name(&orig_dev->dev), orig_chan_index, orig_chan_spec->scan_index);
> +		}
> +		if (start_data_offset + ext_sens_spec->len != fnarg->data_offset) {
> +			dev_warn(mydev, "length mismatch between i2c slave read length and slave channel spec\n");
> +			return -EINVAL;
> +		}
> +
> +		return inv_mpu_config_external_read(st, i, ext_sens_spec);
> +	}
> +	return 0;
> +}
> +
> +static int inv_mpu6050_handle_ext_sensors(struct iio_dev *indio_dev)
> +{
> +	struct inv_mpu6050_state *st = iio_priv(indio_dev);
> +	struct device *dev = regmap_get_device(st->map);
> +	struct inv_mpu_handle_ext_sensor_fnarg fnarg = {
> +		.indio_dev = indio_dev,
> +		.chan_index = 0,
> +		.data_offset = 0,
> +		.scan_index = INV_MPU6050_SCAN_TIMESTAMP,
> +	};
> +	int i, result;
> +	int num_ext_chan = 0, sum_data_len = 0;
> +
> +	inv_mpu_get_ext_sens_spec(indio_dev);
> +	for (i = 0; i < INV_MPU6050_MAX_EXT_SENSORS; ++i) {
> +		num_ext_chan += st->ext_sens_spec[i].num_ext_channels;
> +		sum_data_len += st->ext_sens_spec[i].len;
> +	}
> +	if (sum_data_len > INV_MPU6050_CNT_EXT_SENS_DATA) {
> +		dev_err(dev, "Too many bytes from external sensors:"
> +			      " requested=%d max=%d\n",
> +			      sum_data_len, INV_MPU6050_CNT_EXT_SENS_DATA);
> +		return -EINVAL;
> +	}
> +
> +	/* Zero length means nothing to do */
> +	if (!sum_data_len) {
> +		indio_dev->channels = inv_mpu_channels;
> +		indio_dev->num_channels = INV_MPU6050_NUM_INT_CHAN;
> +		BUILD_BUG_ON(ARRAY_SIZE(inv_mpu_channels) != INV_MPU6050_NUM_INT_CHAN);
> +		return 0;
> +	}
> +
> +	fnarg.channels = devm_kmalloc(indio_dev->dev.parent,
> +				      (num_ext_chan + INV_MPU6050_NUM_INT_CHAN) *
> +				      sizeof(struct iio_chan_spec),
> +				      GFP_KERNEL);
> +	if (!fnarg.channels)
> +		return -ENOMEM;
> +	memcpy(fnarg.channels, inv_mpu_channels, sizeof(inv_mpu_channels));
> +	memset(fnarg.channels + INV_MPU6050_NUM_INT_CHAN, 0,
> +	       num_ext_chan * sizeof(struct iio_chan_spec));
> +
> +	st->ext_chan = devm_kzalloc(indio_dev->dev.parent,
> +			(num_ext_chan) * sizeof(*st->ext_chan),
> +			GFP_KERNEL);
> +	if (!st->ext_chan)
> +		return -ENOMEM;
> +
> +	result = inv_mpu6050_set_power_itg(st, true);
> +	if (result < 0)
> +		return result;
> +
> +	result = device_for_each_child(&st->aux_master_adapter.dev, &fnarg,
> +				       inv_mpu_handle_ext_sensor_fn);
> +	if (result)
> +		goto out_disable;
> +	/* Timestamp channel has index 0 and last scan_index */
> +	fnarg.channels[0].scan_index = fnarg.scan_index;
> +
> +	if (fnarg.chan_index != num_ext_chan) {
> +		dev_err(&indio_dev->dev, "Failed to match all external channels!\n");
> +		result = -EINVAL;
> +		goto out_disable;
> +	}
> +
> +	result = inv_mpu6050_set_power_itg(st, false);
> +	indio_dev->channels = fnarg.channels;
> +	indio_dev->num_channels = INV_MPU6050_NUM_INT_CHAN + num_ext_chan;
> +	return result;
> +
> +out_disable:
> +	inv_mpu6050_set_power_itg(st, false);
> +	inv_mpu_set_external_reads_enabled(st, false);
> +	return result;
> +}
> +
>  /**
>   *  inv_check_and_setup_chip() - check and setup chip.
>   */
> @@ -1121,8 +1531,6 @@ int inv_mpu_core_probe(struct regmap *regmap, int irq, const char *name,
>  		indio_dev->name = name;
>  	else
>  		indio_dev->name = dev_name(dev);
> -	indio_dev->channels = inv_mpu_channels;
> -	indio_dev->num_channels = ARRAY_SIZE(inv_mpu_channels);
>  
>  	indio_dev->info = &mpu_info;
>  	indio_dev->modes = INDIO_BUFFER_TRIGGERED;
> @@ -1160,6 +1568,12 @@ int inv_mpu_core_probe(struct regmap *regmap, int irq, const char *name,
>  	}
>  #endif
>  
> +	result = inv_mpu6050_handle_ext_sensors(indio_dev);
> +	if (result < 0) {
> +		dev_err(dev, "failed to configure channels %d\n", result);
> +		goto out_remove_trigger;
> +	}
> +
>  	INIT_KFIFO(st->timestamps);
>  	spin_lock_init(&st->time_stamp_lock);
>  	result = iio_device_register(indio_dev);
> diff --git a/drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h b/drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h
> index 9d406df..007fe75 100644
> --- a/drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h
> +++ b/drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h
> @@ -124,6 +124,40 @@ struct inv_mpu6050_hw {
>  	const struct inv_mpu6050_chip_config *config;
>  };
>  
> +#define INV_MPU6050_MAX_EXT_SENSORS 4
> +
> +/* Specification for an external sensor */
> +struct inv_mpu6050_ext_sens_spec {
> +	unsigned short addr;
> +	u8 reg, len;
> +
> +	int num_ext_channels;
> +	int *ext_channels;
> +};
> +
> +/* Driver state for each external sensor */
> +struct inv_mpu6050_ext_sens_state {
> +	struct iio_dev *orig_dev;
> +
> +	/* Mask of all channels in this sensor */
> +	unsigned long scan_mask;
> +
> +	/* Length of reading. */
> +	int data_len;
> +};
> +
> +/* Driver state for each external channel */
> +struct inv_mpu6050_ext_chan_state {
> +	/* Index inside state->ext_sens */
> +	int ext_sens_index;
> +
> +	/* Index inside orig_dev->channels */
> +	int orig_chan_index;
> +
> +	/* Relative to EXT_SENS_DATA_00 */
> +	int data_offset;
> +};
> +
>  /*
>   *  struct inv_mpu6050_state - Driver state variables.
>   *  @TIMESTAMP_FIFO_SIZE: fifo size for timestamp.
> @@ -164,6 +198,10 @@ struct inv_mpu6050_state {
>  	struct i2c_adapter aux_master_adapter;
>  	struct completion slv4_done;
>  #endif
> +
> +	struct inv_mpu6050_ext_sens_spec ext_sens_spec[INV_MPU6050_MAX_EXT_SENSORS];
> +	struct inv_mpu6050_ext_sens_state ext_sens[INV_MPU6050_MAX_EXT_SENSORS];
> +	struct inv_mpu6050_ext_chan_state *ext_chan;
>  };
>  
>  /*register and associated bit definition*/
> @@ -178,6 +216,24 @@ struct inv_mpu6050_state {
>  #define INV_MPU6050_REG_FIFO_EN             0x23
>  #define INV_MPU6050_BIT_ACCEL_OUT           0x08
>  #define INV_MPU6050_BITS_GYRO_OUT           0x70
> +#define INV_MPU6050_BIT_SLV0_FIFO_EN        0x01
> +#define INV_MPU6050_BIT_SLV1_FIFO_EN        0x02
> +#define INV_MPU6050_BIT_SLV2_FIFO_EN        0x04
> +#define INV_MPU6050_BIT_SLV2_FIFO_EN        0x04
> +
> +/* SLV3 fifo enabling is not in REG_FIFO_EN */
> +#define INV_MPU6050_REG_MST_CTRL	    0x24
> +#define INV_MPU6050_BIT_SLV3_FIFO_EN        0x10
> +
> +/* For slaves 0-3 */
> +#define INV_MPU6050_REG_I2C_SLV_ADDR(i)     (0x25 + 3 * (i))
> +#define INV_MPU6050_REG_I2C_SLV_REG(i)      (0x26 + 3 * (i))
> +#define INV_MPU6050_REG_I2C_SLV_CTRL(i)     (0x27 + 3 * (i))
> +#define INV_MPU6050_BIT_I2C_SLV_RW          0x80
> +#define INV_MPU6050_BIT_I2C_SLV_EN          0x80
> +#define INV_MPU6050_BIT_I2C_SLV_BYTE_SW     0x40
> +#define INV_MPU6050_BIT_I2C_SLV_REG_DIS     0x20
> +#define INV_MPU6050_BIT_I2C_SLV_REG_GRP     0x10
>  
>  #define INV_MPU6050_REG_I2C_SLV4_ADDR       0x31
>  #define INV_MPU6050_BIT_I2C_SLV4_R          0x80
> @@ -252,8 +308,8 @@ struct inv_mpu6050_state {
>  #define INV_MPU6050_GYRO_CONFIG_FSR_SHIFT    3
>  #define INV_MPU6050_ACCL_CONFIG_FSR_SHIFT    3
>  
> -/* 6 + 6 round up and plus 8 */
> -#define INV_MPU6050_OUTPUT_DATA_SIZE         24
> +/* max is 3*2 accel + 3*2 gyro + 24 external + 8 ts */
> +#define INV_MPU6050_OUTPUT_DATA_SIZE         44
>  
>  #define INV_MPU6050_REG_INT_PIN_CFG	0x37
>  #define INV_MPU6050_BIT_BYPASS_EN	0x2
> @@ -266,6 +322,9 @@ struct inv_mpu6050_state {
>  #define INV_MPU6050_MIN_FIFO_RATE            4
>  #define INV_MPU6050_ONE_K_HZ                 1000
>  
> +#define INV_MPU6050_REG_EXT_SENS_DATA_00	0x49
> +#define INV_MPU6050_CNT_EXT_SENS_DATA		24
> +
>  #define INV_MPU6050_REG_WHOAMI			117
>  
>  #define INV_MPU6000_WHOAMI_VALUE		0x68
> diff --git a/drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c b/drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c
> index e8bda7f..8fa5c3d 100644
> --- a/drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c
> +++ b/drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c
> @@ -52,6 +52,10 @@ int inv_reset_fifo(struct iio_dev *indio_dev)
>  	result = regmap_write(st->map, st->reg->fifo_en, 0);
>  	if (result)
>  		goto reset_fifo_fail;
> +	result = regmap_update_bits(st->map, INV_MPU6050_REG_MST_CTRL,
> +				    INV_MPU6050_BIT_SLV3_FIFO_EN, 0);
> +	if (result)
> +		goto reset_fifo_fail;
>  	/* disable fifo reading */
>  	st->chip_config.user_ctrl &= ~INV_MPU6050_BIT_FIFO_EN;
>  	result = regmap_write(st->map, st->reg->user_ctrl,
> @@ -70,7 +74,8 @@ int inv_reset_fifo(struct iio_dev *indio_dev)
>  
>  	/* enable interrupt */
>  	if (st->chip_config.accl_fifo_enable ||
> -	    st->chip_config.gyro_fifo_enable) {
> +	    st->chip_config.gyro_fifo_enable ||
> +	    true) {
>  		result = regmap_update_bits(st->map, st->reg->int_enable,
>  					    INV_MPU6050_BIT_DATA_RDY_EN,
>  					    INV_MPU6050_BIT_DATA_RDY_EN);
> @@ -89,10 +94,23 @@ int inv_reset_fifo(struct iio_dev *indio_dev)
>  		d |= INV_MPU6050_BITS_GYRO_OUT;
>  	if (st->chip_config.accl_fifo_enable)
>  		d |= INV_MPU6050_BIT_ACCEL_OUT;
> +	if (*indio_dev->active_scan_mask & st->ext_sens[0].scan_mask)
> +		d |= INV_MPU6050_BIT_SLV0_FIFO_EN;
> +	if (*indio_dev->active_scan_mask & st->ext_sens[1].scan_mask)
> +		d |= INV_MPU6050_BIT_SLV1_FIFO_EN;
> +	if (*indio_dev->active_scan_mask & st->ext_sens[2].scan_mask)
> +		d |= INV_MPU6050_BIT_SLV2_FIFO_EN;
>  	result = regmap_write(st->map, st->reg->fifo_en, d);
>  	if (result)
>  		goto reset_fifo_fail;
> -
> +	if (*indio_dev->active_scan_mask & st->ext_sens[3].scan_mask)
> +	{
> +		result = regmap_update_bits(st->map, INV_MPU6050_REG_MST_CTRL,
> +					    INV_MPU6050_BIT_SLV3_FIFO_EN,
> +					    INV_MPU6050_BIT_SLV3_FIFO_EN);
> +		if (result)
> +			goto reset_fifo_fail;
> +	}
>  	return 0;
>  
>  reset_fifo_fail:
> @@ -129,8 +147,9 @@ irqreturn_t inv_mpu6050_read_fifo(int irq, void *p)
>  	struct iio_poll_func *pf = p;
>  	struct iio_dev *indio_dev = pf->indio_dev;
>  	struct inv_mpu6050_state *st = iio_priv(indio_dev);
> -	size_t bytes_per_datum;
> +	size_t bytes_per_datum = 0;
>  	int result;
> +	int i;
>  	u8 data[INV_MPU6050_OUTPUT_DATA_SIZE];
>  	u16 fifo_count;
>  	s64 timestamp;
> @@ -143,15 +162,18 @@ irqreturn_t inv_mpu6050_read_fifo(int irq, void *p)
>  	spi = i2c ? NULL: to_spi_device(regmap_dev);
>  
>  	mutex_lock(&indio_dev->mlock);
> -	if (!(st->chip_config.accl_fifo_enable |
> -		st->chip_config.gyro_fifo_enable))
> -		goto end_session;
> +
> +	/* Compute bytes_per_datum */
>  	bytes_per_datum = 0;
>  	if (st->chip_config.accl_fifo_enable)
>  		bytes_per_datum += INV_MPU6050_BYTES_PER_3AXIS_SENSOR;
> -
>  	if (st->chip_config.gyro_fifo_enable)
>  		bytes_per_datum += INV_MPU6050_BYTES_PER_3AXIS_SENSOR;
> +	for (i = 0; i < INV_MPU6050_MAX_EXT_SENSORS; ++i)
> +		if (st->ext_sens[i].scan_mask & *indio_dev->active_scan_mask)
> +			bytes_per_datum += st->ext_sens[i].data_len;
> +	if (!bytes_per_datum)
> +		return 0;
>  
>  	/*
>  	 * read fifo_count register to know how many bytes inside FIFO
> @@ -228,6 +250,7 @@ static int inv_mpu_buffer_preenable(struct iio_dev *indio_dev)
>  {
>  	struct inv_mpu6050_state *st = iio_priv(indio_dev);
>  	unsigned long mask = *indio_dev->active_scan_mask;
> +	int i;
>  
>  	if ((mask & INV_MPU6050_SCAN_MASK_GYRO) &&
>  	    (mask & INV_MPU6050_SCAN_MASK_GYRO) != INV_MPU6050_SCAN_MASK_GYRO)
> @@ -245,6 +268,17 @@ static int inv_mpu_buffer_preenable(struct iio_dev *indio_dev)
>  		return -EINVAL;
>  	}
>  
> +	for (i = 0; i < INV_MPU6050_MAX_EXT_SENSORS; ++i) {
> +		if ((mask & st->ext_sens[i].scan_mask) &&
> +		    (mask & st->ext_sens[i].scan_mask) != st->ext_sens[i].scan_mask)
> +		{
> +			dev_warn(regmap_get_device(st->map),
> +				 "External channels from the same reading "
> +				 "can only be enabled together\n");
> +			return -EINVAL;
> +		}
> +	}
> +
>  	return 0;
>  }
>  
> 

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