[PATCH v16 08/14] docs: counter: Document character device interface
Jonathan Cameron
jic23 at kernel.org
Sun Sep 12 09:18:02 PDT 2021
On Fri, 27 Aug 2021 12:47:52 +0900
William Breathitt Gray <vilhelm.gray at gmail.com> wrote:
> This patch adds high-level documentation about the Counter subsystem
> character device interface.
>
> Signed-off-by: William Breathitt Gray <vilhelm.gray at gmail.com>
One trivial suggestion inline. This document is very good.
Jonathan
> ---
> Documentation/driver-api/generic-counter.rst | 177 ++++++++++++++----
> .../userspace-api/ioctl/ioctl-number.rst | 1 +
> 2 files changed, 137 insertions(+), 41 deletions(-)
>
> diff --git a/Documentation/driver-api/generic-counter.rst b/Documentation/driver-api/generic-counter.rst
> index f6397218aa4c..6ecb5d1942c7 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 +467,104 @@ 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
Whilst it is clear when you read on, perhaps change "data" to "Counter
events" here.
> +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 7 -- 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:
> +
> +* :c:macro:`COUNTER_ADD_WATCH_IOCTL`
> +
> +* :c:macro:`COUNTER_ENABLE_EVENTS_IOCTL`
> +
> +* :c:macro:`COUNTER_DISABLE_EVENTS_IOCTL`
> +
> +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.
> diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
> index 1409e40e6345..fc4ccc79b1b8 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 at vger.kernel.org>
> '!' 00-1F uapi/linux/seccomp.h
> '#' 00-3F IEEE 1394 Subsystem
> Block for the entire subsystem
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