docs: IIO documentation sphinx conversion

This is a manual conversion of the existing DocBook documentation
for IIO.  The intent is not to substantially change any of the
content in this patch, but to give a base to build upon.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

6 files changed, 506 insertions, 0 deletions

diff --git a/Documentation/driver-api/iio/buffers.rst b/Documentation/driver-api/iio/buffers.rst
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+=======
+Buffers
+=======
+
+* struct :c:type:`iio_buffer` — general buffer structure
+* :c:func:`iio_validate_scan_mask_onehot` — Validates that exactly one channel
+  is selected
+* :c:func:`iio_buffer_get` — Grab a reference to the buffer
+* :c:func:`iio_buffer_put` — Release the reference to the buffer
+
+The Industrial I/O core offers a way for continuous data capture based on a
+trigger source. Multiple data channels can be read at once from
+:file:`/dev/iio:device{X}` character device node, thus reducing the CPU load.
+
+IIO buffer sysfs interface
+==========================
+An IIO buffer has an associated attributes directory under
+:file:`/sys/bus/iio/iio:device{X}/buffer/*`. Here are some of the existing
+attributes:
+
+* :file:`length`, the total number of data samples (capacity) that can be
+  stored by the buffer.
+* :file:`enable`, activate buffer capture.
+
+IIO buffer setup
+================
+
+The meta information associated with a channel reading placed in a buffer is
+called a scan element . The important bits configuring scan elements are
+exposed to userspace applications via the
+:file:`/sys/bus/iio/iio:device{X}/scan_elements/*` directory. This file contains
+attributes of the following form:
+
+* :file:`enable`, used for enabling a channel. If and only if its attribute
+  is non *zero*, then a triggered capture will contain data samples for this
+  channel.
+* :file:`type`, description of the scan element data storage within the buffer
+  and hence the form in which it is read from user space.
+  Format is [be|le]:[s|u]bits/storagebitsXrepeat[>>shift] .
+  * *be* or *le*, specifies big or little endian.
+  * *s* or *u*, specifies if signed (2's complement) or unsigned.
+  * *bits*, is the number of valid data bits.
+  * *storagebits*, is the number of bits (after padding) that it occupies in the
+  buffer.
+  * *shift*, if specified, is the shift that needs to be applied prior to
+  masking out unused bits.
+  * *repeat*, specifies the number of bits/storagebits repetitions. When the
+  repeat element is 0 or 1, then the repeat value is omitted.
+
+For example, a driver for a 3-axis accelerometer with 12 bit resolution where
+data is stored in two 8-bits registers as follows::
+
+        7   6   5   4   3   2   1   0
+      +---+---+---+---+---+---+---+---+
+      |D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06)
+      +---+---+---+---+---+---+---+---+
+
+        7   6   5   4   3   2   1   0
+      +---+---+---+---+---+---+---+---+
+      |D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07)
+      +---+---+---+---+---+---+---+---+
+
+will have the following scan element type for each axis::
+
+      $ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type
+      le:s12/16>>4
+
+A user space application will interpret data samples read from the buffer as
+two byte little endian signed data, that needs a 4 bits right shift before
+masking out the 12 valid bits of data.
+
+For implementing buffer support a driver should initialize the following
+fields in iio_chan_spec definition::
+
+   struct iio_chan_spec {
+   /* other members */
+           int scan_index
+           struct {
+                   char sign;
+                   u8 realbits;
+                   u8 storagebits;
+                   u8 shift;
+                   u8 repeat;
+                   enum iio_endian endianness;
+                  } scan_type;
+          };
+
+The driver implementing the accelerometer described above will have the
+following channel definition::
+
+   struct struct iio_chan_spec accel_channels[] = {
+           {
+                   .type = IIO_ACCEL,
+		   .modified = 1,
+		   .channel2 = IIO_MOD_X,
+		   /* other stuff here */
+		   .scan_index = 0,
+		   .scan_type = {
+		           .sign = 's',
+			   .realbits = 12,
+			   .storagebits = 16,
+			   .shift = 4,
+			   .endianness = IIO_LE,
+		   },
+           }
+           /* similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1)
+            * and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis
+            */
+    }
+
+Here **scan_index** defines the order in which the enabled channels are placed
+inside the buffer. Channels with a lower **scan_index** will be placed before
+channels with a higher index. Each channel needs to have a unique
+**scan_index**.
+
+Setting **scan_index** to -1 can be used to indicate that the specific channel
+does not support buffered capture. In this case no entries will be created for
+the channel in the scan_elements directory.
+
+More details
+============
+.. kernel-doc:: include/linux/iio/buffer.h
+.. kernel-doc:: drivers/iio/industrialio-buffer.c
+   :export:
+
diff --git a/Documentation/driver-api/iio/core.rst b/Documentation/driver-api/iio/core.rst
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+=============
+Core elements
+=============
+
+The Industrial I/O core offers a unified framework for writing drivers for
+many different types of embedded sensors. a standard interface to user space
+applications manipulating sensors. The implementation can be found under
+:file:`drivers/iio/industrialio-*`
+
+Industrial I/O Devices
+----------------------
+
+* struct :c:type:`iio_dev` - industrial I/O device
+* :c:func:`iio_device_alloc()` - alocate an :c:type:`iio_dev` from a driver
+* :c:func:`iio_device_free()` - free an :c:type:`iio_dev` from a driver
+* :c:func:`iio_device_register()` - register a device with the IIO subsystem
+* :c:func:`iio_device_unregister()` - unregister a device from the IIO
+  subsystem
+
+An IIO device usually corresponds to a single hardware sensor and it
+provides all the information needed by a driver handling a device.
+Let's first have a look at the functionality embedded in an IIO device
+then we will show how a device driver makes use of an IIO device.
+
+There are two ways for a user space application to interact with an IIO driver.
+
+1. :file:`/sys/bus/iio/iio:device{X}/`, this represents a hardware sensor
+   and groups together the data channels of the same chip.
+2. :file:`/dev/iio:device{X}`, character device node interface used for
+   buffered data transfer and for events information retrieval.
+
+A typical IIO driver will register itself as an :doc:`I2C <../i2c>` or
+:doc:`SPI <../spi>` driver and will create two routines, probe and remove.
+
+At probe:
+
+1. Call :c:func:`iio_device_alloc()`, which allocates memory for an IIO device.
+2. Initialize IIO device fields with driver specific information (e.g.
+   device name, device channels).
+3. Call :c:func:`iio_device_register()`, this registers the device with the
+   IIO core. After this call the device is ready to accept requests from user
+   space applications.
+
+At remove, we free the resources allocated in probe in reverse order:
+
+1. :c:func:`iio_device_unregister()`, unregister the device from the IIO core.
+2. :c:func:`iio_device_free()`, free the memory allocated for the IIO device.
+
+IIO device sysfs interface
+==========================
+
+Attributes are sysfs files used to expose chip info and also allowing
+applications to set various configuration parameters. For device with
+index X, attributes can be found under /sys/bus/iio/iio:deviceX/ directory.
+Common attributes are:
+
+* :file:`name`, description of the physical chip.
+* :file:`dev`, shows the major:minor pair associated with
+  :file:`/dev/iio:deviceX` node.
+* :file:`sampling_frequency_available`, available discrete set of sampling
+  frequency values for device.
+* Available standard attributes for IIO devices are described in the
+  :file:`Documentation/ABI/testing/sysfs-bus-iio` file in the Linux kernel
+  sources.
+
+IIO device channels
+===================
+
+struct :c:type:`iio_chan_spec` - specification of a single channel
+
+An IIO device channel is a representation of a data channel. An IIO device can
+have one or multiple channels. For example:
+
+* a thermometer sensor has one channel representing the temperature measurement.
+* a light sensor with two channels indicating the measurements in the visible
+  and infrared spectrum.
+* an accelerometer can have up to 3 channels representing acceleration on X, Y
+  and Z axes.
+
+An IIO channel is described by the struct :c:type:`iio_chan_spec`.
+A thermometer driver for the temperature sensor in the example above would
+have to describe its channel as follows::
+
+   static const struct iio_chan_spec temp_channel[] = {
+        {
+            .type = IIO_TEMP,
+            .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
+        },
+   };
+
+Channel sysfs attributes exposed to userspace are specified in the form of
+bitmasks. Depending on their shared info, attributes can be set in one of the
+following masks:
+
+* **info_mask_separate**, attributes will be specific to
+  this channel
+* **info_mask_shared_by_type**, attributes are shared by all channels of the
+  same type
+* **info_mask_shared_by_dir**, attributes are shared by all channels of the same
+  direction
+* **info_mask_shared_by_all**, attributes are shared by all channels
+
+When there are multiple data channels per channel type we have two ways to
+distinguish between them:
+
+* set **.modified** field of :c:type:`iio_chan_spec` to 1. Modifiers are
+  specified using **.channel2** field of the same :c:type:`iio_chan_spec`
+  structure and are used to indicate a physically unique characteristic of the
+  channel such as its direction or spectral response. For example, a light
+  sensor can have two channels, one for infrared light and one for both
+  infrared and visible light.
+* set **.indexed** field of :c:type:`iio_chan_spec` to 1. In this case the
+  channel is simply another instance with an index specified by the **.channel**
+  field.
+
+Here is how we can make use of the channel's modifiers::
+
+   static const struct iio_chan_spec light_channels[] = {
+           {
+                   .type = IIO_INTENSITY,
+                   .modified = 1,
+                   .channel2 = IIO_MOD_LIGHT_IR,
+                   .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
+                   .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
+           },
+           {
+                   .type = IIO_INTENSITY,
+                   .modified = 1,
+                   .channel2 = IIO_MOD_LIGHT_BOTH,
+                   .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
+                   .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
+           },
+           {
+                   .type = IIO_LIGHT,
+                   .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
+                   .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ),
+           },
+      }
+
+This channel's definition will generate two separate sysfs files for raw data
+retrieval:
+
+* :file:`/sys/bus/iio/iio:device{X}/in_intensity_ir_raw`
+* :file:`/sys/bus/iio/iio:device{X}/in_intensity_both_raw`
+
+one file for processed data:
+
+* :file:`/sys/bus/iio/iio:device{X}/in_illuminance_input`
+
+and one shared sysfs file for sampling frequency:
+
+* :file:`/sys/bus/iio/iio:device{X}/sampling_frequency`.
+
+Here is how we can make use of the channel's indexing::
+
+   static const struct iio_chan_spec light_channels[] = {
+           {
+                   .type = IIO_VOLTAGE,
+		   .indexed = 1,
+		   .channel = 0,
+		   .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
+	   },
+           {
+	           .type = IIO_VOLTAGE,
+                   .indexed = 1,
+                   .channel = 1,
+                   .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
+           },
+   }
+
+This will generate two separate attributes files for raw data retrieval:
+
+* :file:`/sys/bus/iio/devices/iio:device{X}/in_voltage0_raw`, representing
+  voltage measurement for channel 0.
+* :file:`/sys/bus/iio/devices/iio:device{X}/in_voltage1_raw`, representing
+  voltage measurement for channel 1.
+
+More details
+============
+.. kernel-doc:: include/linux/iio/iio.h
+.. kernel-doc:: drivers/iio/industrialio-core.c
+   :export:
diff --git a/Documentation/driver-api/iio/index.rst b/Documentation/driver-api/iio/index.rst
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index 000000000000..e5c3922d1b6f
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+++ b/Documentation/driver-api/iio/index.rst
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+.. include:: <isonum.txt>
+
+Industrial I/O
+==============
+
+**Copyright** |copy| 2015 Intel Corporation
+
+Contents:
+
+.. toctree::
+   :maxdepth: 2
+
+   intro
+   core
+   buffers
+   triggers
+   triggered-buffers
diff --git a/Documentation/driver-api/iio/intro.rst b/Documentation/driver-api/iio/intro.rst
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+.. include:: <isonum.txt>
+
+============
+Introduction
+============
+
+The main purpose of the Industrial I/O subsystem (IIO) is to provide support
+for devices that in some sense perform either
+analog-to-digital conversion (ADC) or digital-to-analog conversion (DAC)
+or both. The aim is to fill the gap between the somewhat similar hwmon and
+:doc:`input <../input>` subsystems. Hwmon is directed at low sample rate
+sensors used to monitor and control the system itself, like fan speed control
+or temperature measurement. :doc:`Input <../input>` is, as its name suggests,
+focused on human interaction input devices (keyboard, mouse, touchscreen).
+In some cases there is considerable overlap between these and IIO.
+
+Devices that fall into this category include:
+
+* analog to digital converters (ADCs)
+* accelerometers
+* capacitance to digital converters (CDCs)
+* digital to analog converters (DACs)
+* gyroscopes
+* inertial measurement units (IMUs)
+* color and light sensors
+* magnetometers
+* pressure sensors
+* proximity sensors
+* temperature sensors
+
+Usually these sensors are connected via :doc:`SPI <../spi>` or
+:doc:`I2C <../i2c>`. A common use case of the sensors devices is to have
+combined functionality (e.g. light plus proximity sensor).
diff --git a/Documentation/driver-api/iio/triggered-buffers.rst b/Documentation/driver-api/iio/triggered-buffers.rst
new file mode 100644
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+=================
+Triggered Buffers
+=================
+
+Now that we know what buffers and triggers are let's see how they work together.
+
+IIO triggered buffer setup
+==========================
+
+* :c:func:`iio_triggered_buffer_setup` — Setup triggered buffer and pollfunc
+* :c:func:`iio_triggered_buffer_cleanup` — Free resources allocated by
+  :c:func:`iio_triggered_buffer_setup`
+* struct :c:type:`iio_buffer_setup_ops` — buffer setup related callbacks
+
+A typical triggered buffer setup looks like this::
+
+    const struct iio_buffer_setup_ops sensor_buffer_setup_ops = {
+      .preenable    = sensor_buffer_preenable,
+      .postenable   = sensor_buffer_postenable,
+      .postdisable  = sensor_buffer_postdisable,
+      .predisable   = sensor_buffer_predisable,
+    };
+
+    irqreturn_t sensor_iio_pollfunc(int irq, void *p)
+    {
+        pf->timestamp = iio_get_time_ns((struct indio_dev *)p);
+        return IRQ_WAKE_THREAD;
+    }
+
+    irqreturn_t sensor_trigger_handler(int irq, void *p)
+    {
+        u16 buf[8];
+        int i = 0;
+
+        /* read data for each active channel */
+        for_each_set_bit(bit, active_scan_mask, masklength)
+            buf[i++] = sensor_get_data(bit)
+
+        iio_push_to_buffers_with_timestamp(indio_dev, buf, timestamp);
+
+        iio_trigger_notify_done(trigger);
+        return IRQ_HANDLED;
+    }
+
+    /* setup triggered buffer, usually in probe function */
+    iio_triggered_buffer_setup(indio_dev, sensor_iio_polfunc,
+                               sensor_trigger_handler,
+                               sensor_buffer_setup_ops);
+
+The important things to notice here are:
+
+* :c:type:`iio_buffer_setup_ops`, the buffer setup functions to be called at
+  predefined points in the buffer configuration sequence (e.g. before enable,
+  after disable). If not specified, the IIO core uses the default
+  iio_triggered_buffer_setup_ops.
+* **sensor_iio_pollfunc**, the function that will be used as top half of poll
+  function. It should do as little processing as possible, because it runs in
+  interrupt context. The most common operation is recording of the current
+  timestamp and for this reason one can use the IIO core defined
+  :c:func:`iio_pollfunc_store_time` function.
+* **sensor_trigger_handler**, the function that will be used as bottom half of
+  the poll function. This runs in the context of a kernel thread and all the
+  processing takes place here. It usually reads data from the device and
+  stores it in the internal buffer together with the timestamp recorded in the
+  top half.
+
+More details
+============
+.. kernel-doc:: drivers/iio/buffer/industrialio-triggered-buffer.c
diff --git a/Documentation/driver-api/iio/triggers.rst b/Documentation/driver-api/iio/triggers.rst
new file mode 100644
index 000000000000..f89d37e7dd82
--- /dev/null
+++ b/Documentation/driver-api/iio/triggers.rst
@@ -0,0 +1,80 @@
+========
+Triggers
+========
+
+* struct :c:type:`iio_trigger` — industrial I/O trigger device
+* :c:func:`devm_iio_trigger_alloc` — Resource-managed iio_trigger_alloc
+* :c:func:`devm_iio_trigger_free` — Resource-managed iio_trigger_free
+* :c:func:`devm_iio_trigger_register` — Resource-managed iio_trigger_register
+* :c:func:`devm_iio_trigger_unregister` — Resource-managed
+  iio_trigger_unregister
+* :c:func:`iio_trigger_validate_own_device` — Check if a trigger and IIO
+  device belong to the same device
+
+In many situations it is useful for a driver to be able to capture data based
+on some external event (trigger) as opposed to periodically polling for data.
+An IIO trigger can be provided by a device driver that also has an IIO device
+based on hardware generated events (e.g. data ready or threshold exceeded) or
+provided by a separate driver from an independent interrupt source (e.g. GPIO
+line connected to some external system, timer interrupt or user space writing
+a specific file in sysfs). A trigger may initiate data capture for a number of
+sensors and also it may be completely unrelated to the sensor itself.
+
+IIO trigger sysfs interface
+===========================
+
+There are two locations in sysfs related to triggers:
+
+* :file:`/sys/bus/iio/devices/trigger{Y}/*`, this file is created once an
+  IIO trigger is registered with the IIO core and corresponds to trigger
+  with index Y.
+  Because triggers can be very different depending on type there are few
+  standard attributes that we can describe here:
+
+  * :file:`name`, trigger name that can be later used for association with a
+    device.
+  * :file:`sampling_frequency`, some timer based triggers use this attribute to
+    specify the frequency for trigger calls.
+
+* :file:`/sys/bus/iio/devices/iio:device{X}/trigger/*`, this directory is
+  created once the device supports a triggered buffer. We can associate a
+  trigger with our  device by writing the trigger's name in the
+  :file:`current_trigger` file.
+
+IIO trigger setup
+=================
+
+Let's see a simple example of how to setup a trigger to be used by a driver::
+
+      struct iio_trigger_ops trigger_ops = {
+          .set_trigger_state = sample_trigger_state,
+          .validate_device = sample_validate_device,
+      }
+
+      struct iio_trigger *trig;
+
+      /* first, allocate memory for our trigger */
+      trig = iio_trigger_alloc(dev, "trig-%s-%d", name, idx);
+
+      /* setup trigger operations field */
+      trig->ops = &trigger_ops;
+
+      /* now register the trigger with the IIO core */
+      iio_trigger_register(trig);
+
+IIO trigger ops
+===============
+
+* struct :c:type:`iio_trigger_ops` — operations structure for an iio_trigger.
+
+Notice that a trigger has a set of operations attached:
+
+* :file:`set_trigger_state`, switch the trigger on/off on demand.
+* :file:`validate_device`, function to validate the device when the current
+  trigger gets changed.
+
+More details
+============
+.. kernel-doc:: include/linux/iio/trigger.h
+.. kernel-doc:: drivers/iio/industrialio-trigger.c
+   :export: