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From: Kent Gibson <warthog618@gmail.com>
To: linux-kernel@vger.kernel.org,
	linux-gpio@vger.kernel.org,
	linux-doc@vger.kernel.org,
	brgl@bgdev.pl,
	linus.walleij@linaro.org,
	andy@kernel.org,
	corbet@lwn.net
Cc: Kent Gibson <warthog618@gmail.com>
Subject: [PATCH 1/7] Documentation: gpio: add chardev userspace API documentation
Date: Tue,  9 Jan 2024 21:59:46 +0800
Message-Id: <20240109135952.77458-2-warthog618@gmail.com>
In-Reply-To: <20240109135952.77458-1-warthog618@gmail.com>
References: <20240109135952.77458-1-warthog618@gmail.com>
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Add documentation for the GPIO character device userspace API.

Added to the userspace-api book, but also provide a link from the
admin-guide book, as historically the GPIO documentation has been
there.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
---
 Documentation/admin-guide/gpio/index.rst      |   1 +
 Documentation/userspace-api/gpio/chardev.rst  | 114 ++++++++++++++++++
 .../userspace-api/gpio/error-codes.rst        |  78 ++++++++++++
 .../gpio/gpio-get-chipinfo-ioctl.rst          |  41 +++++++
 .../gpio/gpio-get-lineinfo-unwatch-ioctl.rst  |  47 ++++++++
 .../gpio/gpio-v2-get-line-ioctl.rst           |  99 +++++++++++++++
 .../gpio/gpio-v2-get-lineinfo-ioctl.rst       |  50 ++++++++
 .../gpio/gpio-v2-get-lineinfo-watch-ioctl.rst |  67 ++++++++++
 .../gpio/gpio-v2-line-event-read.rst          |  83 +++++++++++++
 .../gpio/gpio-v2-line-get-values-ioctl.rst    |  51 ++++++++
 .../gpio/gpio-v2-line-set-config-ioctl.rst    |  57 +++++++++
 .../gpio/gpio-v2-line-set-values-ioctl.rst    |  47 ++++++++
 .../gpio/gpio-v2-lineinfo-changed-read.rst    |  81 +++++++++++++
 Documentation/userspace-api/gpio/index.rst    |  17 +++
 Documentation/userspace-api/index.rst         |   1 +
 15 files changed, 834 insertions(+)
 create mode 100644 Documentation/userspace-api/gpio/chardev.rst
 create mode 100644 Documentation/userspace-api/gpio/error-codes.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-get-chipinfo-ioctl.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-get-lineinfo-unwatch-ioctl.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-v2-get-line-ioctl.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-v2-get-lineinfo-ioctl.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-v2-get-lineinfo-watch-ioctl.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-v2-line-get-values-ioctl.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-v2-line-set-config-ioctl.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-v2-line-set-values-ioctl.rst
 create mode 100644 Documentation/userspace-api/gpio/gpio-v2-lineinfo-changed-read.rst
 create mode 100644 Documentation/userspace-api/gpio/index.rst

diff --git a/Documentation/admin-guide/gpio/index.rst b/Documentation/admin-guide/gpio/index.rst
index f6861ca16ffe..b40f0a2a6822 100644
--- a/Documentation/admin-guide/gpio/index.rst
+++ b/Documentation/admin-guide/gpio/index.rst
@@ -7,6 +7,7 @@ gpio
 .. toctree::
     :maxdepth: 1
 
+    Character Device Userspace API <../../userspace-api/gpio/chardev>
     gpio-aggregator
     sysfs
     gpio-mockup
diff --git a/Documentation/userspace-api/gpio/chardev.rst b/Documentation/userspace-api/gpio/chardev.rst
new file mode 100644
index 000000000000..af5f1753e565
--- /dev/null
+++ b/Documentation/userspace-api/gpio/chardev.rst
@@ -0,0 +1,114 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+GPIO Character Device Userspace API
+===================================
+
+This is latest version (v2) of the character device API, as defined in
+``include/uapi/linux/gpio.h.``
+
+.. note::
+   Do NOT abuse userspace APIs to control hardware that has proper kernel
+   drivers. There may already be a driver for your use case, and an existing
+   kernel driver is sure to provide a superior solution to bitbashing
+   from userspace.
+
+   Read Documentation/driver-api/gpio/drivers-on-gpio.rst to avoid reinventing
+   kernel wheels in userspace.
+
+   Similarly, for multi-function lines there may be other subsystems, such as
+   Documentation/spi/index.rst, Documentation/i2c/index.rst,
+   Documentation/driver-api/pwm.rst, Documentation/w1/index.rst etc, that
+   provide suitable drivers and APIs for your hardware.
+
+Basic examples using the character device API can be found in ``tools/gpio/*``.
+
+The API is based around two major objects, the :ref:`gpio-v2-chip` and the
+:ref:`gpio-v2-line-request`.
+
+.. _gpio-v2-chip:
+
+Chip
+====
+
+The Chip represents a single GPIO chip and is exposed to userspace using device
+files of the form ``/dev/gpiochipX``.
+
+Each chip supports a number of GPIO lines,
+:c:type:`chip.lines<gpiochip_info>`. Lines on the chip are identified by an
+``offset`` in the range from 0 to ``chip.lines - 1``, i.e. `[0,chip.lines)`.
+
+Lines are requested from the chip using gpio-v2-get-line-ioctl.rst
+and the resulting line request is used to access the GPIO chip's lines or
+monitor the lines for edge events.
+
+Within this documentation, the file descriptor returned by calling `open()`
+on the GPIO device file is referred to as ``chip_fd``.
+
+Operations
+----------
+
+The following operations may be performed on the chip:
+
+.. toctree::
+   :titlesonly:
+
+   Get Line <gpio-v2-get-line-ioctl>
+   Get Chip Info <gpio-get-chipinfo-ioctl>
+   Get Line Info <gpio-v2-get-lineinfo-ioctl>
+   Watch Line Info <gpio-v2-get-lineinfo-watch-ioctl>
+   Unwatch Line Info <gpio-get-lineinfo-unwatch-ioctl>
+   Read Line Info Changed Events <gpio-v2-lineinfo-changed-read>
+
+.. _gpio-v2-line-request:
+
+Line Request
+============
+
+Line requests are created by gpio-v2-get-line-ioctl.rst and provide
+access to a set of requested lines.  The line request is exposed to userspace
+via the anonymous file descriptor returned in
+:c:type:`request.fd<gpio_v2_line_request>` by gpio-v2-get-line-ioctl.rst.
+
+Within this documentation, the line request file descriptor is referred to
+as ``req_fd``.
+
+Operations
+----------
+
+The following operations may be performed on the line request:
+
+.. toctree::
+   :titlesonly:
+
+   Get Line Values <gpio-v2-line-get-values-ioctl>
+   Set Line Values <gpio-v2-line-set-values-ioctl>
+   Read Line Edge Events <gpio-v2-line-event-read>
+   Reconfigure Lines <gpio-v2-line-set-config-ioctl>
+
+Types
+=====
+
+This section contains the structs and enums that are referenced by the API v2,
+as defined in ``include/uapi/linux/gpio.h``.
+
+.. kernel-doc:: include/uapi/linux/gpio.h
+   :identifiers:
+    gpio_v2_line_attr_id
+    gpio_v2_line_attribute
+    gpio_v2_line_changed_type
+    gpio_v2_line_config
+    gpio_v2_line_config_attribute
+    gpio_v2_line_event
+    gpio_v2_line_event_id
+    gpio_v2_line_flag
+    gpio_v2_line_info
+    gpio_v2_line_info_changed
+    gpio_v2_line_request
+    gpio_v2_line_values
+    gpiochip_info
+
+.. toctree::
+   :hidden:
+
+   error-codes
diff --git a/Documentation/userspace-api/gpio/error-codes.rst b/Documentation/userspace-api/gpio/error-codes.rst
new file mode 100644
index 000000000000..edf01f2cf9d2
--- /dev/null
+++ b/Documentation/userspace-api/gpio/error-codes.rst
@@ -0,0 +1,78 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _gpio_errors:
+
+*******************
+GPIO Error Codes
+*******************
+
+.. _gpio-errors:
+
+.. tabularcolumns:: |p{2.5cm}|p{15.0cm}|
+
+.. flat-table:: Common GPIO error codes
+    :header-rows:  0
+    :stub-columns: 0
+    :widths: 1 16
+
+    -  -  ``EAGAIN`` (aka ``EWOULDBLOCK``)
+
+       -  The device was opened in non-blocking mode and a read can't
+          be performed as there is no data available.
+
+    -  -  ``EBADF``
+
+       -  The file descriptor is not valid.
+
+    -  -  ``EBUSY``
+
+       -  The ioctl can't be handled because the device is busy. Typically
+          returned when an ioctl attempts something that would require the
+          usage of a resource that was already allocated. The ioctl must not
+          be retried without performing another action to fix the problem
+          first.
+
+    -  -  ``EFAULT``
+
+       -  There was a failure while copying data from/to userspace, probably
+	  caused by an invalid pointer reference.
+
+    -  -  ``EINVAL``
+
+       -  One or more of the ioctl parameters are invalid or out of the
+          allowed range. This is a widely used error code.
+
+    -  -  ``ENODEV``
+
+       -  Device not found or was removed.
+
+    -  -  ``ENOMEM``
+
+       -  There's not enough memory to handle the desired operation.
+
+    -  -  ``EPERM``
+
+       -  Permission denied. Typically returned in response to an attempt
+          to perform an action incompatible with the current line
+          configuration.
+
+    -  -  ``EIO``
+
+       -  I/O error. Typically returned when there are problems communicating
+          with a hardware device or requesting features that hardware does not
+          support. This could indicate broken or flaky hardware.
+          It's a 'Something is wrong, I give up!' type of error.
+
+    -  - ``ENXIO``
+
+       -  No device corresponding to this device special file exists.
+
+.. note::
+
+  #. This list is not exhaustive; ioctls may return other error codes.
+     Since errors may have side effects such as a driver reset,
+     applications should abort on unexpected errors, or otherwise
+     assume that the device is in a bad state.
+
+  #. Request-specific error codes are listed in the individual
+     requests descriptions.
diff --git a/Documentation/userspace-api/gpio/gpio-get-chipinfo-ioctl.rst b/Documentation/userspace-api/gpio/gpio-get-chipinfo-ioctl.rst
new file mode 100644
index 000000000000..05f07fdefe2f
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-get-chipinfo-ioctl.rst
@@ -0,0 +1,41 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_GET_CHIPINFO_IOCTL:
+
+***********************
+GPIO_GET_CHIPINFO_IOCTL
+***********************
+
+Name
+====
+
+GPIO_GET_CHIPINFO_IOCTL - Get the publicly available information for a chip.
+
+Synopsis
+========
+
+.. c:macro:: GPIO_GET_CHIPINFO_IOCTL
+
+``int ioctl(int chip_fd, GPIO_GET_CHIPINFO_IOCTL, struct gpiochip_info *info)``
+
+Arguments
+=========
+
+``chip_fd``
+    The file descriptor of the GPIO character device returned by `open()`.
+
+``info``
+    The :c:type:`chip_info<gpiochip_info>` to be populated.
+
+Description
+===========
+
+Gets the publicly available information for a particular GPIO chip.
+
+Return Value
+============
+
+On success 0 and ``info`` is populated with the chip info.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/gpio-get-lineinfo-unwatch-ioctl.rst b/Documentation/userspace-api/gpio/gpio-get-lineinfo-unwatch-ioctl.rst
new file mode 100644
index 000000000000..ba6f9d00a40b
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-get-lineinfo-unwatch-ioctl.rst
@@ -0,0 +1,47 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_GET_LINEINFO_UNWATCH_IOCTL:
+
+*******************************
+GPIO_GET_LINEINFO_UNWATCH_IOCTL
+*******************************
+
+Name
+====
+
+GPIO_GET_LINEINFO_UNWATCH_IOCTL - Disable watching a line for changes to its
+requested state and configuration information.
+
+Synopsis
+========
+
+.. c:macro:: GPIO_GET_LINEINFO_UNWATCH_IOCTL
+
+``int ioctl(int chip_fd, GPIO_GET_LINEINFO_UNWATCH_IOCTL, u32 *offset)``
+
+Arguments
+=========
+
+``chip_fd``
+    The file descriptor of the GPIO character device returned by `open()`.
+
+``offset``
+    The offset of the line to no longer watch.
+
+Description
+===========
+
+Remove the line from the list of lines being watched on this ``chip_fd``.
+
+This is the opposite of gpio-v2-get-lineinfo-watch-ioctl.rst (v2) and
+gpio-get-lineinfo-watch-ioctl.rst (v1).
+
+Unwatching a line that is not watched is an error (**EBUSY**).
+
+Return Value
+============
+
+On success 0.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/gpio-v2-get-line-ioctl.rst b/Documentation/userspace-api/gpio/gpio-v2-get-line-ioctl.rst
new file mode 100644
index 000000000000..4259c08779c1
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-v2-get-line-ioctl.rst
@@ -0,0 +1,99 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_V2_GET_LINE_IOCTL:
+
+**********************
+GPIO_V2_GET_LINE_IOCTL
+**********************
+
+Name
+====
+
+GPIO_V2_GET_LINE_IOCTL - Request a line or lines from the kernel.
+
+Synopsis
+========
+
+.. c:macro:: GPIO_V2_GET_LINE_IOCTL
+
+``int ioctl(int chip_fd, GPIO_V2_GET_LINE_IOCTL, struct gpio_v2_line_request *request)``
+
+Arguments
+=========
+
+``chip_fd``
+    The file descriptor of the GPIO character device returned by `open()`.
+
+``request``
+    The :c:type:`line_request<gpio_v2_line_request>` specifying the lines
+    to request and their configuration.
+
+Description
+===========
+
+On success, the requesting process is granted exclusive access to the line
+value, write access to the line configuration, and may receive events when
+edges are detected on the line, all of which are described in more detail in
+:ref:`gpio-v2-line-request`.
+
+A number of lines may be requested in the one line request, and request
+operations are performed on the requested lines by the kernel as atomically
+as possible. e.g. gpio-v2-line-get-values-ioctl.rst will read all the
+requested lines at once.
+
+The state of a line, including the value of output lines, is guaranteed to
+remain as requested until the returned file descriptor is closed. Once the
+file descriptor is closed, the state of the line becomes uncontrolled from
+the userspace perspective, and may revert to its default state.
+
+Closing the ``chip_fd`` has no effect on existing line requests.
+
+.. _gpio-v2-get-line-config-rules:
+
+Configuration Rules
+-------------------
+
+For any given requested line, the following configuration rules apply:
+
+The direction flags, ``GPIO_V2_LINE_FLAG_INPUT`` and
+``GPIO_V2_LINE_FLAG_OUTPUT``, cannot be combined. If neither are set then
+the only other flag that may be set is ``GPIO_V2_LINE_FLAG_ACTIVE_LOW``
+and the line is requested "as-is" to allow reading of the line value
+without altering the electrical configuration.
+
+The drive flags, ``GPIO_V2_LINE_FLAG_OPEN_xxx``, require the
+``GPIO_V2_LINE_FLAG_OUTPUT`` to be set.
+Only one drive flag may be set.
+If none are set then the line is assumed push-pull.
+
+Only one bias flag, ``GPIO_V2_LINE_FLAG_BIAS_xxx``, may be set, and it
+requires a direction flag to also be set.
+If no bias flags are set then the bias configuration is not changed.
+
+The edge flags, ``GPIO_V2_LINE_FLAG_EDGE_xxx``, require
+``GPIO_V2_LINE_FLAG_INPUT`` to be set and may be combined to detect both rising
+and falling edges.
+
+Only one event clock flag, ``GPIO_V2_LINE_FLAG_EVENT_CLOCK_xxx``, may be set.
+If none are set then the event clock defaults to ``CLOCK_MONOTONIC``.
+The ``GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE`` flag requires supporting hardware
+and a kernel with ``CONFIG_HTE`` set.  Requesting HTE from a device that
+doesn't support it is an error (**EOPNOTSUP**).
+
+The :c:type:`debounce_period_us<gpio_v2_line_attribute>` attribute may only
+be applied to lines with ``GPIO_V2_LINE_FLAG_INPUT`` set. When set, debounce
+applies to both the values returned by gpio-v2-line-get-values-ioctl.rst and
+the edges returned by gpio-v2-line-event-read.rst.  If not
+supported directly by hardware, the debouncing is performed in software by the
+kernel.
+
+Requesting an invalid configuration is an error (**EINVAL**).
+
+Return Value
+============
+
+On success 0 and the :c:type:`request.fd<gpio_v2_line_request>` contains the
+file descriptor for the request.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/gpio-v2-get-lineinfo-ioctl.rst b/Documentation/userspace-api/gpio/gpio-v2-get-lineinfo-ioctl.rst
new file mode 100644
index 000000000000..bc4d8df887d4
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-v2-get-lineinfo-ioctl.rst
@@ -0,0 +1,50 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_V2_GET_LINEINFO_IOCTL:
+
+**************************
+GPIO_V2_GET_LINEINFO_IOCTL
+**************************
+
+Name
+====
+
+GPIO_V2_GET_LINEINFO_IOCTL - Get the publicly available information for a line.
+
+Synopsis
+========
+
+.. c:macro:: GPIO_V2_GET_LINEINFO_IOCTL
+
+``int ioctl(int chip_fd, GPIO_V2_GET_LINEINFO_IOCTL, struct gpio_v2_line_info *info)``
+
+Arguments
+=========
+
+``chip_fd``
+    The file descriptor of the GPIO character device returned by `open()`.
+
+``info``
+    The :c:type:`line_info<gpio_v2_line_info>` to be populated, with the
+    ``offset`` field set to indicate the line to be collected.
+
+Description
+===========
+
+Get the publicly available information for a line.
+
+This information is available independent of whether the line is in use.
+
+.. note::
+    The line info does not include the line value.
+
+    The line must be requested using gpio-v2-get-line-ioctl.rst to access its
+    value.
+
+Return Value
+============
+
+On success 0 and ``info`` is populated with the chip info.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/gpio-v2-get-lineinfo-watch-ioctl.rst b/Documentation/userspace-api/gpio/gpio-v2-get-lineinfo-watch-ioctl.rst
new file mode 100644
index 000000000000..938ff85a9322
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-v2-get-lineinfo-watch-ioctl.rst
@@ -0,0 +1,67 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_V2_GET_LINEINFO_WATCH_IOCTL:
+
+********************************
+GPIO_V2_GET_LINEINFO_WATCH_IOCTL
+********************************
+
+Name
+====
+
+GPIO_V2_GET_LINEINFO_WATCH_IOCTL - Enable watching a line for changes to its
+request state and configuration information.
+
+Synopsis
+========
+
+.. c:macro:: GPIO_V2_GET_LINEINFO_WATCH_IOCTL
+
+``int ioctl(int chip_fd, GPIO_V2_GET_LINEINFO_WATCH_IOCTL, struct gpio_v2_line_info *info)``
+
+Arguments
+=========
+
+``chip_fd``
+    The file descriptor of the GPIO character device returned by `open()`.
+
+``info``
+    The :c:type:`line_info<gpio_v2_line_info>` struct to be populated, with
+    the ``offset`` set to indicate the line to watch
+
+Description
+===========
+
+Enable watching a line for changes to its request state and configuration
+information. Changes to line info include a line being requested, released
+or reconfigured.
+
+.. note::
+    Watching line info is not generally required, and would typically only be
+    used by a system monitoring component.
+
+    The line info does NOT include the line value.
+    The line must be requested using gpio-v2-get-line-ioctl.rst to access
+    its value, and the line request can monitor a line for events using
+    gpio-v2-line-event-read.rst.
+
+By default all lines are unwatched when the GPIO chip is opened.
+
+Multiple lines may be watched simultaneously by adding a watch for each.
+
+Once a watch is set, any changes to line info will generate events which can be
+read from the ``chip_fd`` as described in
+gpio-v2-lineinfo-changed-read.rst.
+
+Adding a watch to a line that is already watched is an error (**EBUSY**).
+
+Watches are specific to the ``chip_fd`` and are independent of watches
+on the same GPIO chip opened with a separate call to `open()`.
+
+Return Value
+============
+
+On success 0 and ``info`` is populated with the current line info.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst b/Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst
new file mode 100644
index 000000000000..6513c23fb7ca
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-v2-line-event-read.rst
@@ -0,0 +1,83 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_V2_LINE_EVENT_READ:
+
+***********************
+GPIO_V2_LINE_EVENT_READ
+***********************
+
+Name
+====
+
+GPIO_V2_LINE_EVENT_READ - Read edge detection events for lines from a request.
+
+Synopsis
+========
+
+``int read(int req_fd, void *buf, size_t count)``
+
+Arguments
+=========
+
+``req_fd``
+    The file descriptor of the GPIO character device, as returned in the
+    :c:type:`request.fd<gpio_v2_line_request>` by gpio-v2-get-line-ioctl.rst.
+
+``buf``
+    The buffer to contain the :c:type:`events<gpio_v2_line_event>`.
+
+``count``
+    The number of bytes available in ``buf``, which must be at
+    least the size of a :c:type:`gpio_v2_line_event`.
+
+Description
+===========
+
+Read edge detection events for lines from a request.
+
+Edge detection must be enabled for the input line using either
+``GPIO_V2_LINE_FLAG_EDGE_RISING`` or ``GPIO_V2_LINE_FLAG_EDGE_FALLING``, or
+both. Edge events are then generated whenever edge interrupts are detected on
+the input line.
+
+The kernel captures and timestamps edge events as close as possible to their
+occurrence and stores them in a buffer from where they can be read by
+userspace at its convenience using `read()`.
+
+Events read from the buffer are always in the same order that they were
+detected by the kernel, including when multiple lines are being monitored by
+the one request.
+
+The size of the kernel event buffer is fixed at the time of line request
+creation, and can be influenced by the
+:c:type:`request.event_buffer_size<gpio_v2_line_request>`.
+The default size is 16 times the number of lines requested.
+
+The buffer may overflow if bursts of events occur quicker than they are read
+by userspace. If an overflow occurs then the oldest buffered event is
+discarded. Overflow can be detected from userspace by monitoring the event
+sequence numbers.
+
+To minimize the number of calls required to copy events from the kernel to
+userspace, `read()` supports copying multiple events. The number of events
+copied is the lower of the number available in the kernel buffer and the
+number that will fit in the userspace buffer (``buf``).
+
+Changing the edge detection flags using gpio-v2-line-set-config-ioctl.rst
+does not remove or modify the events already contained in the kernel event
+buffer.
+
+The `read()` will block if no event is available and the ``req_fd`` has not
+been set **O_NONBLOCK**.
+
+The presence of an event can be tested for by checking that the ``req_fd`` is
+readable using `poll()` or an equivalent.
+
+Return Value
+============
+
+On success the number of bytes read, which will be a multiple of the size of a
+:c:type:`gpio_v2_line_event` event.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/gpio-v2-line-get-values-ioctl.rst b/Documentation/userspace-api/gpio/gpio-v2-line-get-values-ioctl.rst
new file mode 100644
index 000000000000..e4e74a1926d8
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-v2-line-get-values-ioctl.rst
@@ -0,0 +1,51 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_V2_LINE_GET_VALUES_IOCTL:
+
+*****************************
+GPIO_V2_LINE_GET_VALUES_IOCTL
+*****************************
+
+Name
+====
+
+GPIO_V2_LINE_GET_VALUES_IOCTL - Get the values of requested lines.
+
+Synopsis
+========
+
+.. c:macro:: GPIO_V2_LINE_GET_VALUES_IOCTL
+
+``int ioctl(int req_fd, GPIO_V2_LINE_GET_VALUES_IOCTL, struct gpio_v2_line_values *values)``
+
+Arguments
+=========
+
+``req_fd``
+    The file descriptor of the GPIO character device, as returned in the
+    :c:type:`request.fd<gpio_v2_line_request>` by gpio-v2-get-line-ioctl.rst.
+
+``values``
+    The :c:type:`line_values<gpio_v2_line_values>` to get with the ``mask`` set
+    to indicate the subset of requested lines to get.
+
+Description
+===========
+
+Get the values of requested lines.
+
+The values of both input and output lines may be read.
+
+For output lines, the value returned is driver and configuration dependent and
+may be either the output buffer (the last requested value set) or the input
+buffer (the actual level of the line), and depending on the hardware and
+configuration these may differ.
+
+Return Value
+============
+
+On success 0 and the corresponding :c:type:`values.bits<gpio_v2_line_values>`
+contain the value read.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/gpio-v2-line-set-config-ioctl.rst b/Documentation/userspace-api/gpio/gpio-v2-line-set-config-ioctl.rst
new file mode 100644
index 000000000000..126c2626ba6b
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-v2-line-set-config-ioctl.rst
@@ -0,0 +1,57 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_V2_LINE_SET_CONFIG_IOCTL:
+
+*****************************
+GPIO_V2_LINE_SET_CONFIG_IOCTL
+*****************************
+
+Name
+====
+
+GPIO_V2_LINE_SET_CONFIG_IOCTL - Update the configuration of previously requested lines.
+
+Synopsis
+========
+
+.. c:macro:: GPIO_V2_LINE_SET_CONFIG_IOCTL
+
+``int ioctl(int req_fd, GPIO_V2_LINE_SET_CONFIG_IOCTL, struct gpio_v2_line_config *config)``
+
+Arguments
+=========
+
+``req_fd``
+    The file descriptor of the GPIO character device, as returned in the
+    :c:type:`request.fd<gpio_v2_line_request>` by gpio-v2-get-line-ioctl.rst.
+
+``config``
+    The new :c:type:`configuration<gpio_v2_line_config>` to apply to the
+    requested lines.
+
+Description
+===========
+
+Update the configuration of previously requested lines, without releasing the
+line or introducing potential glitches.
+
+The new configuration must specify the configuration of all requested lines.
+
+The same :ref:`gpio-v2-get-line-config-rules` that apply when requesting the lines
+also apply when updating the line configuration.
+
+The motivating use case for this command is changing direction of
+bi-directional lines between input and output, but it may also be used to
+dynamically control edge detection, or more generally move lines seamlessly
+from one configuration state to another.
+
+To only change the value of output lines, use
+gpio-v2-line-set-values-ioctl.rst.
+
+Return Value
+============
+
+On success 0.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/gpio-v2-line-set-values-ioctl.rst b/Documentation/userspace-api/gpio/gpio-v2-line-set-values-ioctl.rst
new file mode 100644
index 000000000000..6d2d1886950b
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-v2-line-set-values-ioctl.rst
@@ -0,0 +1,47 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_V2_LINE_SET_VALUES_IOCTL:
+
+*****************************
+GPIO_V2_LINE_SET_VALUES_IOCTL
+*****************************
+
+Name
+====
+
+GPIO_V2_LINE_SET_VALUES_IOCTL - Set the values of requested output lines.
+
+Synopsis
+========
+
+.. c:macro:: GPIO_V2_LINE_SET_VALUES_IOCTL
+
+``int ioctl(int req_fd, GPIO_V2_LINE_SET_VALUES_IOCTL, struct gpio_v2_line_values *values)``
+
+Arguments
+=========
+
+``req_fd``
+    The file descriptor of the GPIO character device, as returned in the
+    :c:type:`request.fd<gpio_v2_line_request>` by gpio-v2-get-line-ioctl.rst.
+
+``values``
+    The :c:type:`line_values<gpio_v2_line_values>` to set with the ``mask`` set
+    to indicate the subset of requested lines to set and ``bits`` set to
+    indicate the new value.
+
+Description
+===========
+
+Set the values of requested output lines.
+
+Only the values of output lines may be set.
+Attempting to set the value of an input line is an error (**EPERM**).
+
+Return Value
+============
+
+On success 0.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/gpio-v2-lineinfo-changed-read.rst b/Documentation/userspace-api/gpio/gpio-v2-lineinfo-changed-read.rst
new file mode 100644
index 000000000000..24ad325e7253
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-v2-lineinfo-changed-read.rst
@@ -0,0 +1,81 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_V2_LINEINFO_CHANGED_READ:
+
+*****************************
+GPIO_V2_LINEINFO_CHANGED_READ
+*****************************
+
+Name
+====
+
+GPIO_V2_LINEINFO_CHANGED_READ - Read line info changed events for watched
+lines from the chip.
+
+Synopsis
+========
+
+``int read(int chip_fd, void *buf, size_t count)``
+
+Arguments
+=========
+
+``chip_fd``
+    The file descriptor of the GPIO character device returned by `open()`.
+
+``buf``
+    The buffer to contain the :c:type:`events<gpio_v2_line_info_changed>`.
+
+``count``
+    The number of bytes available in ``buf``, which must be at least the size
+    of a :c:type:`gpio_v2_line_info_changed` event.
+
+Description
+===========
+
+Read line info changed events for watched lines from the chip.
+
+.. note::
+    Monitoring line info changes is not generally required, and would typically
+    only be performed by a system monitoring component.
+
+    These events relate to changes in a line's request state or configuration,
+    not its value. Use gpio-v2-line-event-read.rst to receive events when a
+    line changes value.
+
+A line must be watched using gpio-v2-get-lineinfo-watch-ioctl.rst to generate
+info changed events.  Subsequently, a request, release, or reconfiguration
+of the line will generate an info changed event.
+
+The kernel timestamps events when they occur and stores them in a buffer
+from where they can be read by userspace at its convenience using `read()`.
+
+The size of the kernel event buffer is fixed at 32 events per ``chip_fd``.
+
+The buffer may overflow if bursts of events occur quicker than they are read
+by userspace. If an overflow occurs then the most recent event is discarded.
+Overflow cannot be detected from userspace.
+
+Events read from the buffer are always in the same order that they were
+detected by the kernel, including when multiple lines are being monitored by
+the one ``chip_fd``.
+
+To minimize the number of calls required to copy events from the kernel to
+userspace, `read()` supports copying multiple events. The number of events
+copied is the lower of the number available in the kernel buffer and the
+number that will fit in the userspace buffer (``buf``).
+
+A `read()` will block if no event is available and the ``chip_fd`` has not
+been set **O_NONBLOCK**.
+
+The presence of an event can be tested for by checking that the ``chip_fd`` is
+readable using `poll()` or an equivalent.
+
+Return Value
+============
+
+On success the number of bytes read, which will be a multiple of the size
+of a :c:type:`gpio_v2_line_info_changed` event.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.
diff --git a/Documentation/userspace-api/gpio/index.rst b/Documentation/userspace-api/gpio/index.rst
new file mode 100644
index 000000000000..072b9fa18aea
--- /dev/null
+++ b/Documentation/userspace-api/gpio/index.rst
@@ -0,0 +1,17 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====
+GPIO
+====
+
+.. toctree::
+    :maxdepth: 1
+
+    Character Device Userspace API <chardev>
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
index 031df47a7c19..8e174a605f69 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -25,6 +25,7 @@ place where this information is gathered.
    dma-buf-alloc-exchange
    ebpf/index
    ELF
+   gpio/index
    ioctl/index
    iommu
    iommufd
-- 
2.39.2