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SubjectRe: [PATCH v8 18/22] docs: counter: Document character device interface
On Fri, 12 Feb 2021 21:13:42 +0900
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/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.

Is there a way to remove a watch?

> +
> +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::
> +

Consider adding an example program under tools/

> + #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,

Good to add comments on what these elements actually are?

> + .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

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