Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

7 files changed, 557 insertions, 0 deletions

diff --git a/Documentation/driver-api/iio/buffers.rst b/Documentation/driver-api/iio/buffers.rst
new file mode 100644
index 000000000..8779022e3
--- /dev/null
+++ b/Documentation/driver-api/iio/buffers.rst
@@ -0,0 +1,127 @@
+=======
+Buffers
+=======
+
+* struct 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/devices/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/devices/iio:device{X}/scan_elements/` directory. This
+directory 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:`index`, the scan_index of the 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]:[f|s|u]bits/storagebits[Xrepeat][>>shift] .
+
+  * *be* or *le*, specifies big or little endian.
+  * *f*, specifies if floating-point.
+  * *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.
+  * *repeat*, specifies the number of bits/storagebits repetitions. When the
+    repeat element is 0 or 1, then the repeat value is omitted.
+  * *shift*, if specified, is the shift that needs to be applied prior to
+    masking out unused bits.
+
+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 format;
+                   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 iio_chan_spec accel_channels[] = {
+           {
+                   .type = IIO_ACCEL,
+		   .modified = 1,
+		   .channel2 = IIO_MOD_X,
+		   /* other stuff here */
+		   .scan_index = 0,
+		   .scan_type = {
+		           .format = IIO_SCAN_FORMAT_SIGNED_INT,
+			   .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
new file mode 100644
index 000000000..42b580fb2
--- /dev/null
+++ b/Documentation/driver-api/iio/core.rst
@@ -0,0 +1,182 @@
+=============
+Core elements
+=============
+
+The Industrial I/O core offers both a unified framework for writing drivers for
+many different types of embedded sensors and a standard interface to user space
+applications manipulating sensors. The implementation can be found under
+:file:`drivers/iio/industrialio-*`
+
+Industrial I/O Devices
+----------------------
+
+* struct iio_dev - industrial I/O device
+* iio_device_alloc() - allocate an :c:type:`iio_dev` from a driver
+* iio_device_free() - free an :c:type:`iio_dev` from a driver
+* iio_device_register() - register a device with the IIO subsystem
+* 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/devices/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 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 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. iio_device_unregister(), unregister the device from the IIO core.
+2. 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/devices/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 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 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/devices/iio:device{X}/in_intensity_ir_raw`
+* :file:`/sys/bus/iio/devices/iio:device{X}/in_intensity_both_raw`
+
+one file for processed data:
+
+* :file:`/sys/bus/iio/devices/iio:device{X}/in_illuminance_input`
+
+and one shared sysfs file for sampling frequency:
+
+* :file:`/sys/bus/iio/devices/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/hw-consumer.rst b/Documentation/driver-api/iio/hw-consumer.rst
new file mode 100644
index 000000000..76133a379
--- /dev/null
+++ b/Documentation/driver-api/iio/hw-consumer.rst
@@ -0,0 +1,50 @@
+===========
+HW consumer
+===========
+An IIO device can be directly connected to another device in hardware. In this
+case the buffers between IIO provider and IIO consumer are handled by hardware.
+The Industrial I/O HW consumer offers a way to bond these IIO devices without
+software buffer for data. The implementation can be found under
+:file:`drivers/iio/buffer/hw-consumer.c`
+
+
+* struct iio_hw_consumer — Hardware consumer structure
+* :c:func:`iio_hw_consumer_alloc` — Allocate IIO hardware consumer
+* :c:func:`iio_hw_consumer_free` — Free IIO hardware consumer
+* :c:func:`iio_hw_consumer_enable` — Enable IIO hardware consumer
+* :c:func:`iio_hw_consumer_disable` — Disable IIO hardware consumer
+
+
+HW consumer setup
+=================
+
+As standard IIO device the implementation is based on IIO provider/consumer.
+A typical IIO HW consumer setup looks like this::
+
+	static struct iio_hw_consumer *hwc;
+
+	static const struct iio_info adc_info = {
+		.read_raw = adc_read_raw,
+	};
+
+	static int adc_read_raw(struct iio_dev *indio_dev,
+				struct iio_chan_spec const *chan, int *val,
+				int *val2, long mask)
+	{
+		ret = iio_hw_consumer_enable(hwc);
+
+		/* Acquire data */
+
+		ret = iio_hw_consumer_disable(hwc);
+	}
+
+	static int adc_probe(struct platform_device *pdev)
+	{
+		hwc = devm_iio_hw_consumer_alloc(&iio->dev);
+	}
+
+More details
+============
+.. kernel-doc:: drivers/iio/buffer/industrialio-hw-consumer.c
+   :export:
+
diff --git a/Documentation/driver-api/iio/index.rst b/Documentation/driver-api/iio/index.rst
new file mode 100644
index 000000000..7fba341bd
--- /dev/null
+++ b/Documentation/driver-api/iio/index.rst
@@ -0,0 +1,18 @@
+.. include:: <isonum.txt>
+
+Industrial I/O
+==============
+
+**Copyright** |copy| 2015 Intel Corporation
+
+Contents:
+
+.. toctree::
+   :maxdepth: 2
+
+   intro
+   core
+   buffers
+   triggers
+   triggered-buffers
+   hw-consumer
diff --git a/Documentation/driver-api/iio/intro.rst b/Documentation/driver-api/iio/intro.rst
new file mode 100644
index 000000000..3653fbd57
--- /dev/null
+++ b/Documentation/driver-api/iio/intro.rst
@@ -0,0 +1,33 @@
+.. 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
index 000000000..23762b06f
--- /dev/null
+++ b/Documentation/driver-api/iio/triggered-buffers.rst
@@ -0,0 +1,69 @@
+=================
+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 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)
+    {
+        IIO_DECLARE_BUFFER_WITH_TS(u16, buf, 3) = { };
+        int i = 0;
+
+        /* read data for each active channel */
+        iio_for_each_active_channel(indio_dev, bit)
+            buf[i++] = sensor_get_data(bit);
+
+        iio_push_to_buffers_with_ts(indio_dev, buf, sizeof(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_pollfunc,
+                               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 000000000..288625e40
--- /dev/null
+++ b/Documentation/driver-api/iio/triggers.rst
@@ -0,0 +1,78 @@
+========
+Triggers
+========
+
+* struct iio_trigger — industrial I/O trigger device
+* :c:func:`devm_iio_trigger_alloc` — Resource-managed iio_trigger_alloc
+* :c:func:`devm_iio_trigger_register` — Resource-managed iio_trigger_register
+  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 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: