[PATCH v5 4/5] docs: counter: Document character device interface

[William Breathitt Gray <vilhelm.gray@gmail.com>]

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   |  18 ++
 Documentation/driver-api/generic-counter.rst  | 228 ++++++++++++++----
 .../userspace-api/ioctl/ioctl-number.rst      |   1 +
 3 files changed, 206 insertions(+), 41 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-bus-counter b/Documentation/ABI/testing/sysfs-bus-counter
index 566bd99fe0a5..b7fdb14ae891 100644
--- a/Documentation/ABI/testing/sysfs-bus-counter
+++ b/Documentation/ABI/testing/sysfs-bus-counter
@@ -99,6 +99,24 @@ 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:	5.11
+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/extensionZ_width
+What:		/sys/bus/counter/devices/counterX/extensionZ_width
+What:		/sys/bus/counter/devices/counterX/signalY/extensionZ_width
+KernelVersion:	5.11
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Read-only attribute that indicates the data width of value 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..6077bf162ac3 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 +466,155 @@ 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_SET_WATCH_IOCTL:
+  Set a Counter watch for the specified event
+
+* COUNTER_LOAD_WATCHES_IOCTL:
+  Activates the Counter watches set 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_SET_WATCH_IOCTL` ioctl command.
+
+The `COUNTER_SET_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 <poll.h>
+        #include <stdio.h>
+        #include <sys/ioctl.h>
+        #include <unistd.h>
+
+        struct counter_watch watches[2] = {
+                {
+                        .event = COUNTER_EVENT_INDEX,
+                        .channel = 0,
+                        .component.scope = COUNTER_SCOPE_COUNT,
+                        .component.parent = 0,
+                        .component.type = COUNTER_COMPONENT_COUNT,
+                },
+                {
+                        .event = COUNTER_EVENT_INDEX,
+                        .channel = 0,
+                        .component.scope = COUNTER_SCOPE_COUNT,
+                        .component.parent = 1,
+                        .component.type = COUNTER_COMPONENT_COUNT,
+                },
+        };
+
+        int main(void)
+        {
+                struct pollfd pfd = { .events = POLLIN };
+                struct counter_event event_data[2];
+
+                pfd.fd = open("/dev/counter0", O_RDWR);
+
+                ioctl(pfd.fd, COUNTER_SET_WATCH_IOCTL, watches);
+                ioctl(pfd.fd, COUNTER_SET_WATCH_IOCTL, watches + 1);
+                ioctl(pfd.fd, COUNTER_LOAD_WATCHES_IOCTL);
+
+                for (;;) {
+                        poll(&pfd, 1, -1);
+
+                        read(pfd.fd, event_data, sizeof(event_data));
+
+                        printf("Timestamp 0: %llu\nCount 0: %llu\n"
+                               "Timestamp 1: %llu\nCount 1: %llu\n",
+                               (unsigned long long)event_data[0].timestamp,
+                               (unsigned long long)event_data[0].value_u64,
+                               (unsigned long long)event_data[1].timestamp,
+                               (unsigned long long)event_data[1].value_u64);
+                }
+
+                return 0;
+        }
diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
index 2a198838fca9..f6e96bb780cd 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
-- 
2.28.0