diff mbox series

[v7,4/5] docs: counter: Document character device interface

Message ID 1e69b7beae4cf352bddb379220d0d52b20db0634.1608935587.git.vilhelm.gray@gmail.com (mailing list archive)
State New, archived
Headers show
Series Introduce the Counter character device interface | expand

Commit Message

William Breathitt Gray Dec. 26, 2020, 12:15 a.m. UTC 
This patch adds high-level documentation about the Counter subsystem
character device interface.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
---
 Documentation/ABI/testing/sysfs-bus-counter   |   9 +
 Documentation/driver-api/generic-counter.rst  | 236 +++++++++++++++---
 .../userspace-api/ioctl/ioctl-number.rst      |   1 +
 3 files changed, 205 insertions(+), 41 deletions(-)

Comments

Jonathan Cameron Dec. 30, 2020, 2:47 p.m. UTC | #1 
On Fri, 25 Dec 2020 19:15:37 -0500
William Breathitt Gray <vilhelm.gray@gmail.com> wrote:

> This patch adds high-level documentation about the Counter subsystem
> character device interface.
> 
> Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
> ---
>  Documentation/ABI/testing/sysfs-bus-counter   |   9 +
>  Documentation/driver-api/generic-counter.rst  | 236 +++++++++++++++---
>  .../userspace-api/ioctl/ioctl-number.rst      |   1 +
>  3 files changed, 205 insertions(+), 41 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-bus-counter b/Documentation/ABI/testing/sysfs-bus-counter
> index 1820ce2f9183..8f6ea0a50b75 100644
> --- a/Documentation/ABI/testing/sysfs-bus-counter
> +++ b/Documentation/ABI/testing/sysfs-bus-counter
> @@ -99,6 +99,15 @@ Description:
>  		Read-only attribute that indicates whether excessive noise is
>  		present at the channel Y counter inputs.
>  
> +What:		/sys/bus/counter/devices/counterX/countY/extensionZ_name
> +What:		/sys/bus/counter/devices/counterX/extensionZ_name
> +What:		/sys/bus/counter/devices/counterX/signalY/extensionZ_name
> +KernelVersion:	5.12
> +Contact:	linux-iio@vger.kernel.org
> +Description:
> +		Read-only attribute that indicates the component name of
> +		Extension Z.

Dumb question, but why is this only related to character device introduction?

> +
>  What:		/sys/bus/counter/devices/counterX/countY/function
>  KernelVersion:	5.2
>  Contact:	linux-iio@vger.kernel.org
> diff --git a/Documentation/driver-api/generic-counter.rst b/Documentation/driver-api/generic-counter.rst
> index b842ddbbd8a0..4775dcaff557 100644
> --- a/Documentation/driver-api/generic-counter.rst
> +++ b/Documentation/driver-api/generic-counter.rst
> @@ -223,19 +223,6 @@ whether an input line is differential or single-ended) and instead focus
>  on the core idea of what the data and process represent (e.g. position
>  as interpreted from quadrature encoding data).
>  
> -Userspace Interface
> -===================
> -
> -Several sysfs attributes are generated by the Generic Counter interface,
> -and reside under the /sys/bus/counter/devices/counterX directory, where
> -counterX refers to the respective counter device. Please see
> -Documentation/ABI/testing/sysfs-bus-counter for detailed
> -information on each Generic Counter interface sysfs attribute.
> -
> -Through these sysfs attributes, programs and scripts may interact with
> -the Generic Counter paradigm Counts, Signals, and Synapses of respective
> -counter devices.
> -
>  Driver API
>  ==========
>  
> @@ -387,16 +374,16 @@ userspace interface components::
>                          / driver callbacks /
>                          -------------------
>                                  |
> -                +---------------+
> -                |
> -                V
> -        +--------------------+
> -        | Counter sysfs      |
> -        +--------------------+
> -        | Translates to the  |
> -        | standard Counter   |
> -        | sysfs output       |
> -        +--------------------+
> +                +---------------+---------------+
> +                |                               |
> +                V                               V
> +        +--------------------+          +---------------------+
> +        | Counter sysfs      |          | Counter chrdev      |
> +        +--------------------+          +---------------------+
> +        | Translates to the  |          | Translates to the   |
> +        | standard Counter   |          | standard Counter    |
> +        | sysfs output       |          | character device    |
> +        +--------------------+          +---------------------+
>  
>  Thereafter, data can be transferred directly between the Counter device
>  driver and Counter userspace interface::
> @@ -427,23 +414,30 @@ driver and Counter userspace interface::
>                          / u64     /
>                          ----------
>                                  |
> -                +---------------+
> -                |
> -                V
> -        +--------------------+
> -        | Counter sysfs      |
> -        +--------------------+
> -        | Translates to the  |
> -        | standard Counter   |
> -        | sysfs output       |
> -        |--------------------|
> -        | Type: const char * |
> -        | Value: "42"        |
> -        +--------------------+
> -                |
> -         ---------------
> -        / const char * /
> -        ---------------
> +                +---------------+---------------+
> +                |                               |
> +                V                               V
> +        +--------------------+          +---------------------+
> +        | Counter sysfs      |          | Counter chrdev      |
> +        +--------------------+          +---------------------+
> +        | Translates to the  |          | Translates to the   |
> +        | standard Counter   |          | standard Counter    |
> +        | sysfs output       |          | character device    |
> +        |--------------------|          |---------------------|
> +        | Type: const char * |          | Type: u64           |
> +        | Value: "42"        |          | Value: 42           |
> +        +--------------------+          +---------------------+
> +                |                               |
> +         ---------------                 -----------------------
> +        / const char * /                / struct counter_event /
> +        ---------------                 -----------------------
> +                |                               |
> +                |                               V
> +                |                       +-----------+
> +                |                       | read      |
> +                |                       +-----------+
> +                |                       \ Count: 42 /
> +                |                        -----------
>                  |
>                  V
>          +--------------------------------------------------+
> @@ -452,7 +446,7 @@ driver and Counter userspace interface::
>          \ Count: "42"                                      /
>           --------------------------------------------------
>  
> -There are three primary components involved:
> +There are four primary components involved:
>  
>  Counter device driver
>  ---------------------
> @@ -472,3 +466,163 @@ and vice versa.
>  Please refer to the `Documentation/ABI/testing/sysfs-bus-counter` file
>  for a detailed breakdown of the available Generic Counter interface
>  sysfs attributes.
> +
> +Counter chrdev
> +--------------
> +Translates counter data to the standard Counter character device; data
> +is transferred via standard character device read calls, while Counter
> +events are configured via ioctl calls.
> +
> +Sysfs Interface
> +===============
> +
> +Several sysfs attributes are generated by the Generic Counter interface,
> +and reside under the `/sys/bus/counter/devices/counterX` directory,
> +where `X` is to the respective counter device id. Please see
> +`Documentation/ABI/testing/sysfs-bus-counter` for detailed information
> +on each Generic Counter interface sysfs attribute.
> +
> +Through these sysfs attributes, programs and scripts may interact with
> +the Generic Counter paradigm Counts, Signals, and Synapses of respective
> +counter devices.
> +
> +Counter Character Device
> +========================
> +
> +Counter character device nodes are created under the `/dev` directory as
> +`counterX`, where `X` is the respective counter device id. Defines for
> +the standard Counter data types are exposed via the userspace
> +`include/uapi/linux/counter.h` file.
> +
> +Counter events
> +--------------
> +Counter device drivers can support Counter events by utilizing the
> +`counter_push_event` function::
> +
> +        int counter_push_event(struct counter_device *const counter, const u8 event,
> +                               const u8 channel);
> +
> +The event id is specified by the `event` parameter; the event channel id
> +is specified by the `channel` parameter. When this function is called,
> +the Counter data associated with the respective event is gathered, and a
> +`struct counter_event` is generated for each datum and pushed to
> +userspace.
> +
> +Counter events can be configured by users to report various Counter
> +data of interest. This can be conceptualized as a list of Counter
> +component read calls to perform. For example::
> +
> +        +~~~~~~~~~~~~~~~~~~~~~~~~+~~~~~~~~~~~~~~~~~~~~~~~~+
> +        | COUNTER_EVENT_OVERFLOW | COUNTER_EVENT_INDEX    |
> +        +~~~~~~~~~~~~~~~~~~~~~~~~+~~~~~~~~~~~~~~~~~~~~~~~~+
> +        | Channel 0              | Channel 0              |
> +        +------------------------+------------------------+
> +        | * Count 0              | * Signal 0             |
> +        | * Count 1              | * Signal 0 Extension 0 |
> +        | * Signal 3             | * Extension 4          |
> +        | * Count 4 Extension 2  +------------------------+
> +        | * Signal 5 Extension 0 | Channel 1              |
> +        |                        +------------------------+
> +        |                        | * Signal 4             |
> +        |                        | * Signal 4 Extension 0 |
> +        |                        | * Count 7              |
> +        +------------------------+------------------------+
> +
> +When `counter_push_event(counter, COUNTER_EVENT_INDEX, 1)` is called for
> +example, it will go down the list for the `COUNTER_EVENT_INDEX` event
> +channel 1 and execute the read callbacks for Signal 4, Signal 4
> +Extension 0, and Count 4 -- the data returned for each is pushed to a
> +kfifo as a `struct counter_event`, which userspace can retrieve via a
> +standard read operation on the respective character device node.
> +
> +Userspace
> +---------
> +Userspace applications can configure Counter events via ioctl operations
> +on the Counter character device node. There following ioctl codes are
> +supported and provided by the `linux/counter.h` userspace header file:
> +
> +* COUNTER_CLEAR_WATCHES_IOCTL:
> +  Clear all Counter watches from all events
> +
> +* COUNTER_ADD_WATCH_IOCTL:
> +  Add a Counter watch for the specified event
> +
> +* COUNTER_LOAD_WATCHES_IOCTL:
> +  Activates the Counter watches added earlier
> +
> +To configure events to gather Counter data, users first populate a
> +`struct counter_watch` with the relevant event id, event channel id, and
> +the information for the desired Counter component from which to read,
> +and then pass it via the `COUNTER_ADD_WATCH_IOCTL` ioctl command.
> +
> +Note that an event can be watched without gathering Counter data by
> +setting the `component.type` member equal to `COUNTER_COMPONENT_NONE`.
> +With this configuration the Counter character device will simply
> +populate the event timestamps for those respective
> +`struct counter_event` elements and ignore the component value.
> +
> +The `COUNTER_ADD_WATCH_IOCTL` command will buffer these Counter watches.
> +When ready, the `COUNTER_LOAD_WATCHES_IOCTL` ioctl command may be used
> +to activate these Counter watches.
> +
> +Userspace applications can then execute a `read` operation (optionally
> +calling `poll` first) on the Counter character device node to retrieve
> +`struct counter_event` elements with the desired data.
> +
> +For example, the following userspace code opens `/dev/counter0`,
> +configures the `COUNTER_EVENT_INDEX` event channel 0 to gather Count 0
> +and Count 1, and prints out the data as it becomes available on the
> +character device node::
> +
> +        #include <fcntl.h>
> +        #include <linux/counter.h>
> +        #include <stdio.h>
> +        #include <string.h>
> +        #include <sys/ioctl.h>
> +        #include <unistd.h>
> +
> +        struct counter_watch watches[2] = {
> +                {
> +                        .component.type = COUNTER_COMPONENT_COUNT,
> +                        .component.scope = COUNTER_SCOPE_COUNT,
> +                        .component.parent = 0,
> +                        .event = COUNTER_EVENT_INDEX,
> +                        .channel = 0,
> +                },
> +                {
> +                        .component.type = COUNTER_COMPONENT_COUNT,
> +                        .component.scope = COUNTER_SCOPE_COUNT,
> +                        .component.parent = 1,
> +                        .event = COUNTER_EVENT_INDEX,
> +                        .channel = 0,
> +                },
> +        };
> +
> +        int main(void)
> +        {
> +                int fd;
> +                struct counter_event event_data[2];
> +
> +                fd = open("/dev/counter0", O_RDWR);
> +
> +                ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches);
> +                ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches + 1);
> +                ioctl(fd, COUNTER_LOAD_WATCHES_IOCTL);
> +
> +                for (;;) {
> +                        read(fd, event_data, sizeof(event_data));
> +
> +                        printf("Timestamp 0: %llu\tCount 0: %llu\n"
> +                               "Error Message 0: %s\n"
> +                               "Timestamp 1: %llu\tCount 1: %llu\n"
> +                               "Error Message 1: %s\n",
> +                               (unsigned long long)event_data[0].timestamp,
> +                               (unsigned long long)event_data[0].value,
> +                               strerror(event_data[0].errno),
> +                               (unsigned long long)event_data[1].timestamp,
> +                               (unsigned long long)event_data[1].value,
> +                               strerror(event_data[1].errno));
> +                }
> +
> +                return 0;
> +        }
> diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
> index 55a2d9b2ce33..b7761ceb82a7 100644
> --- a/Documentation/userspace-api/ioctl/ioctl-number.rst
> +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
> @@ -88,6 +88,7 @@ Code  Seq#    Include File                                           Comments
>                                                                       <http://infiniband.sourceforge.net/>
>  0x20  all    drivers/cdrom/cm206.h
>  0x22  all    scsi/sg.h
> +0x3E  00-0F  linux/counter.h                                         <mailto:linux-iio@vger.kernel.org>
>  '!'   00-1F  uapi/linux/seccomp.h
>  '#'   00-3F                                                          IEEE 1394 Subsystem
>                                                                       Block for the entire subsystem
David Lechner Dec. 30, 2020, 6:18 p.m. UTC | #2 
On 12/25/20 6:15 PM, William Breathitt Gray wrote:

> +Userspace
> +---------
> +Userspace applications can configure Counter events via ioctl operations
> +on the Counter character device node. There following ioctl codes are
> +supported and provided by the `linux/counter.h` userspace header file:
> +
> +* COUNTER_CLEAR_WATCHES_IOCTL:
> +  Clear all Counter watches from all events
> +
> +* COUNTER_ADD_WATCH_IOCTL:
> +  Add a Counter watch for the specified event
> +
> +* COUNTER_LOAD_WATCHES_IOCTL:
> +  Activates the Counter watches added earlier

Would it make more sense to call the last and first ones
COUNTER_ENABLE_EVENTS_IOCTL and COUNTER_DISABLE_EVENTS_IOCTL?
In any case, more explanation would be helpful.

* COUNTER_ADD_WATCH_IOCTL:
   Queues a Counter watch for the specified event. The queued watches
   will not be applied until COUNTER_ENABLE_EVENTS_IOCTL is called.

* COUNTER_ENABLE_EVENTS_IOCTL:
   Enables monitoring the events specified by the Counter watches that were
   queued by COUNTER_ADD_WATCH_IOCTL. If events are already enabled, the new
   set of watches replaces the old one. Calling this ioctl also has the
   effect of clearing the queue of watches added by COUNTER_ADD_WATCH_IOCTL.

* COUNTER_DISABLE_EVENTS_IOCTL:
   Stops monitoring the previously enabled events.

> +
> +To configure events to gather Counter data, users first populate a
> +`struct counter_watch` with the relevant event id, event channel id, and
> +the information for the desired Counter component from which to read,
> +and then pass it via the `COUNTER_ADD_WATCH_IOCTL` ioctl command.

for restructured text, two backticks are needed for ``code`` formatting
diff mbox series

Patch

diff --git a/Documentation/ABI/testing/sysfs-bus-counter b/Documentation/ABI/testing/sysfs-bus-counter
index 1820ce2f9183..8f6ea0a50b75 100644
--- a/Documentation/ABI/testing/sysfs-bus-counter
+++ b/Documentation/ABI/testing/sysfs-bus-counter
@@ -99,6 +99,15 @@  Description:
 		Read-only attribute that indicates whether excessive noise is
 		present at the channel Y counter inputs.
 
+What:		/sys/bus/counter/devices/counterX/countY/extensionZ_name
+What:		/sys/bus/counter/devices/counterX/extensionZ_name
+What:		/sys/bus/counter/devices/counterX/signalY/extensionZ_name
+KernelVersion:	5.12
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Read-only attribute that indicates the component name of
+		Extension Z.
+
 What:		/sys/bus/counter/devices/counterX/countY/function
 KernelVersion:	5.2
 Contact:	linux-iio@vger.kernel.org
diff --git a/Documentation/driver-api/generic-counter.rst b/Documentation/driver-api/generic-counter.rst
index b842ddbbd8a0..4775dcaff557 100644
--- a/Documentation/driver-api/generic-counter.rst
+++ b/Documentation/driver-api/generic-counter.rst
@@ -223,19 +223,6 @@  whether an input line is differential or single-ended) and instead focus
 on the core idea of what the data and process represent (e.g. position
 as interpreted from quadrature encoding data).
 
-Userspace Interface
-===================
-
-Several sysfs attributes are generated by the Generic Counter interface,
-and reside under the /sys/bus/counter/devices/counterX directory, where
-counterX refers to the respective counter device. Please see
-Documentation/ABI/testing/sysfs-bus-counter for detailed
-information on each Generic Counter interface sysfs attribute.
-
-Through these sysfs attributes, programs and scripts may interact with
-the Generic Counter paradigm Counts, Signals, and Synapses of respective
-counter devices.
-
 Driver API
 ==========
 
@@ -387,16 +374,16 @@  userspace interface components::
                         / driver callbacks /
                         -------------------
                                 |
-                +---------------+
-                |
-                V
-        +--------------------+
-        | Counter sysfs      |
-        +--------------------+
-        | Translates to the  |
-        | standard Counter   |
-        | sysfs output       |
-        +--------------------+
+                +---------------+---------------+
+                |                               |
+                V                               V
+        +--------------------+          +---------------------+
+        | Counter sysfs      |          | Counter chrdev      |
+        +--------------------+          +---------------------+
+        | Translates to the  |          | Translates to the   |
+        | standard Counter   |          | standard Counter    |
+        | sysfs output       |          | character device    |
+        +--------------------+          +---------------------+
 
 Thereafter, data can be transferred directly between the Counter device
 driver and Counter userspace interface::
@@ -427,23 +414,30 @@  driver and Counter userspace interface::
                         / u64     /
                         ----------
                                 |
-                +---------------+
-                |
-                V
-        +--------------------+
-        | Counter sysfs      |
-        +--------------------+
-        | Translates to the  |
-        | standard Counter   |
-        | sysfs output       |
-        |--------------------|
-        | Type: const char * |
-        | Value: "42"        |
-        +--------------------+
-                |
-         ---------------
-        / const char * /
-        ---------------
+                +---------------+---------------+
+                |                               |
+                V                               V
+        +--------------------+          +---------------------+
+        | Counter sysfs      |          | Counter chrdev      |
+        +--------------------+          +---------------------+
+        | Translates to the  |          | Translates to the   |
+        | standard Counter   |          | standard Counter    |
+        | sysfs output       |          | character device    |
+        |--------------------|          |---------------------|
+        | Type: const char * |          | Type: u64           |
+        | Value: "42"        |          | Value: 42           |
+        +--------------------+          +---------------------+
+                |                               |
+         ---------------                 -----------------------
+        / const char * /                / struct counter_event /
+        ---------------                 -----------------------
+                |                               |
+                |                               V
+                |                       +-----------+
+                |                       | read      |
+                |                       +-----------+
+                |                       \ Count: 42 /
+                |                        -----------
                 |
                 V
         +--------------------------------------------------+
@@ -452,7 +446,7 @@  driver and Counter userspace interface::
         \ Count: "42"                                      /
          --------------------------------------------------
 
-There are three primary components involved:
+There are four primary components involved:
 
 Counter device driver
 ---------------------
@@ -472,3 +466,163 @@  and vice versa.
 Please refer to the `Documentation/ABI/testing/sysfs-bus-counter` file
 for a detailed breakdown of the available Generic Counter interface
 sysfs attributes.
+
+Counter chrdev
+--------------
+Translates counter data to the standard Counter character device; data
+is transferred via standard character device read calls, while Counter
+events are configured via ioctl calls.
+
+Sysfs Interface
+===============
+
+Several sysfs attributes are generated by the Generic Counter interface,
+and reside under the `/sys/bus/counter/devices/counterX` directory,
+where `X` is to the respective counter device id. Please see
+`Documentation/ABI/testing/sysfs-bus-counter` for detailed information
+on each Generic Counter interface sysfs attribute.
+
+Through these sysfs attributes, programs and scripts may interact with
+the Generic Counter paradigm Counts, Signals, and Synapses of respective
+counter devices.
+
+Counter Character Device
+========================
+
+Counter character device nodes are created under the `/dev` directory as
+`counterX`, where `X` is the respective counter device id. Defines for
+the standard Counter data types are exposed via the userspace
+`include/uapi/linux/counter.h` file.
+
+Counter events
+--------------
+Counter device drivers can support Counter events by utilizing the
+`counter_push_event` function::
+
+        int counter_push_event(struct counter_device *const counter, const u8 event,
+                               const u8 channel);
+
+The event id is specified by the `event` parameter; the event channel id
+is specified by the `channel` parameter. When this function is called,
+the Counter data associated with the respective event is gathered, and a
+`struct counter_event` is generated for each datum and pushed to
+userspace.
+
+Counter events can be configured by users to report various Counter
+data of interest. This can be conceptualized as a list of Counter
+component read calls to perform. For example::
+
+        +~~~~~~~~~~~~~~~~~~~~~~~~+~~~~~~~~~~~~~~~~~~~~~~~~+
+        | COUNTER_EVENT_OVERFLOW | COUNTER_EVENT_INDEX    |
+        +~~~~~~~~~~~~~~~~~~~~~~~~+~~~~~~~~~~~~~~~~~~~~~~~~+
+        | Channel 0              | Channel 0              |
+        +------------------------+------------------------+
+        | * Count 0              | * Signal 0             |
+        | * Count 1              | * Signal 0 Extension 0 |
+        | * Signal 3             | * Extension 4          |
+        | * Count 4 Extension 2  +------------------------+
+        | * Signal 5 Extension 0 | Channel 1              |
+        |                        +------------------------+
+        |                        | * Signal 4             |
+        |                        | * Signal 4 Extension 0 |
+        |                        | * Count 7              |
+        +------------------------+------------------------+
+
+When `counter_push_event(counter, COUNTER_EVENT_INDEX, 1)` is called for
+example, it will go down the list for the `COUNTER_EVENT_INDEX` event
+channel 1 and execute the read callbacks for Signal 4, Signal 4
+Extension 0, and Count 4 -- the data returned for each is pushed to a
+kfifo as a `struct counter_event`, which userspace can retrieve via a
+standard read operation on the respective character device node.
+
+Userspace
+---------
+Userspace applications can configure Counter events via ioctl operations
+on the Counter character device node. There following ioctl codes are
+supported and provided by the `linux/counter.h` userspace header file:
+
+* COUNTER_CLEAR_WATCHES_IOCTL:
+  Clear all Counter watches from all events
+
+* COUNTER_ADD_WATCH_IOCTL:
+  Add a Counter watch for the specified event
+
+* COUNTER_LOAD_WATCHES_IOCTL:
+  Activates the Counter watches added earlier
+
+To configure events to gather Counter data, users first populate a
+`struct counter_watch` with the relevant event id, event channel id, and
+the information for the desired Counter component from which to read,
+and then pass it via the `COUNTER_ADD_WATCH_IOCTL` ioctl command.
+
+Note that an event can be watched without gathering Counter data by
+setting the `component.type` member equal to `COUNTER_COMPONENT_NONE`.
+With this configuration the Counter character device will simply
+populate the event timestamps for those respective
+`struct counter_event` elements and ignore the component value.
+
+The `COUNTER_ADD_WATCH_IOCTL` command will buffer these Counter watches.
+When ready, the `COUNTER_LOAD_WATCHES_IOCTL` ioctl command may be used
+to activate these Counter watches.
+
+Userspace applications can then execute a `read` operation (optionally
+calling `poll` first) on the Counter character device node to retrieve
+`struct counter_event` elements with the desired data.
+
+For example, the following userspace code opens `/dev/counter0`,
+configures the `COUNTER_EVENT_INDEX` event channel 0 to gather Count 0
+and Count 1, and prints out the data as it becomes available on the
+character device node::
+
+        #include <fcntl.h>
+        #include <linux/counter.h>
+        #include <stdio.h>
+        #include <string.h>
+        #include <sys/ioctl.h>
+        #include <unistd.h>
+
+        struct counter_watch watches[2] = {
+                {
+                        .component.type = COUNTER_COMPONENT_COUNT,
+                        .component.scope = COUNTER_SCOPE_COUNT,
+                        .component.parent = 0,
+                        .event = COUNTER_EVENT_INDEX,
+                        .channel = 0,
+                },
+                {
+                        .component.type = COUNTER_COMPONENT_COUNT,
+                        .component.scope = COUNTER_SCOPE_COUNT,
+                        .component.parent = 1,
+                        .event = COUNTER_EVENT_INDEX,
+                        .channel = 0,
+                },
+        };
+
+        int main(void)
+        {
+                int fd;
+                struct counter_event event_data[2];
+
+                fd = open("/dev/counter0", O_RDWR);
+
+                ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches);
+                ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches + 1);
+                ioctl(fd, COUNTER_LOAD_WATCHES_IOCTL);
+
+                for (;;) {
+                        read(fd, event_data, sizeof(event_data));
+
+                        printf("Timestamp 0: %llu\tCount 0: %llu\n"
+                               "Error Message 0: %s\n"
+                               "Timestamp 1: %llu\tCount 1: %llu\n"
+                               "Error Message 1: %s\n",
+                               (unsigned long long)event_data[0].timestamp,
+                               (unsigned long long)event_data[0].value,
+                               strerror(event_data[0].errno),
+                               (unsigned long long)event_data[1].timestamp,
+                               (unsigned long long)event_data[1].value,
+                               strerror(event_data[1].errno));
+                }
+
+                return 0;
+        }
diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
index 55a2d9b2ce33..b7761ceb82a7 100644
--- a/Documentation/userspace-api/ioctl/ioctl-number.rst
+++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
@@ -88,6 +88,7 @@  Code  Seq#    Include File                                           Comments
                                                                      <http://infiniband.sourceforge.net/>
 0x20  all    drivers/cdrom/cm206.h
 0x22  all    scsi/sg.h
+0x3E  00-0F  linux/counter.h                                         <mailto:linux-iio@vger.kernel.org>
 '!'   00-1F  uapi/linux/seccomp.h
 '#'   00-3F                                                          IEEE 1394 Subsystem
                                                                      Block for the entire subsystem