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Date:   Sun, 14 Feb 2021 17:48:19 +0000
From:   Jonathan Cameron <jic23@kernel.org>
To:     William Breathitt Gray <vilhelm.gray@gmail.com>
Cc:     kernel@pengutronix.de, linux-stm32@st-md-mailman.stormreply.com,
        a.fatoum@pengutronix.de, kamel.bouhara@bootlin.com,
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        syednwaris@gmail.com, patrick.havelange@essensium.com,
        fabrice.gasnier@st.com, mcoquelin.stm32@gmail.com,
        alexandre.torgue@st.com, o.rempel@pengutronix.de
Subject: Re: [PATCH v8 18/22] docs: counter: Document character device
 interface
Message-ID: <20210214174819.6757e2b0@archlinux>
In-Reply-To: <350cafba81d3220b64efdb019bd76c08eb1e5d10.1613131238.git.vilhelm.gray@gmail.com>
References: <cover.1613131238.git.vilhelm.gray@gmail.com>
        <350cafba81d3220b64efdb019bd76c08eb1e5d10.1613131238.git.vilhelm.gray@gmail.com>
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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