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Subject[PATCH v8 18/22] docs: counter: Document character device interface
Date
This patch adds high-level documentation about the Counter subsystem
character device interface.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
---
Documentation/driver-api/generic-counter.rst | 243 +++++++++++++++---
.../userspace-api/ioctl/ioctl-number.rst | 1 +
2 files changed, 203 insertions(+), 41 deletions(-)

diff --git a/Documentation/driver-api/generic-counter.rst b/Documentation/driver-api/generic-counter.rst
index f6397218aa4c..3be109dc81bb 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
==========

@@ -388,16 +375,16 @@ 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::
@@ -428,23 +415,30 @@ 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
+--------------------------------------------------+
@@ -453,7 +447,7 @@ driver and Counter userspace interface::
\ Count: "42" /
--------------------------------------------------

-There are three primary components involved:
+There are four primary components involved:

Counter device driver
---------------------
@@ -473,3 +467,170 @@ 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::
+
+ void 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_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.
+
+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_ENABLE_EVENTS_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_ENABLE_EVENTS_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].status),
+ (unsigned long long)event_data[1].timestamp,
+ (unsigned long long)event_data[1].value,
+ strerror(event_data[1].status));
+ }
+
+ return 0;
+ }
diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
index a4c75a28c839..8ddca931ec4e 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
--
2.30.0
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