media: Documentation: v4l: document request API

Document the request API for V4L2 devices, and amend the documentation
of system calls influenced by it.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

1 files changed, 67 insertions, 0 deletions

diff --git a/Documentation/media/uapi/mediactl/request-func-ioctl.rst b/Documentation/media/uapi/mediactl/request-func-ioctl.rst
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+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _request-func-ioctl:
+
+***************
+request ioctl()
+***************
+
+Name
+====
+
+request-ioctl - Control a request file descriptor
+
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <sys/ioctl.h>
+
+
+.. c:function:: int ioctl( int fd, int cmd, void *argp )
+    :name: req-ioctl
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :ref:`MEDIA_IOC_REQUEST_ALLOC`.
+
+``cmd``
+    The request ioctl command code as defined in the media.h header file, for
+    example :ref:`MEDIA_REQUEST_IOC_QUEUE`.
+
+``argp``
+    Pointer to a request-specific structure.
+
+
+Description
+===========
+
+The :ref:`ioctl() <request-func-ioctl>` function manipulates request
+parameters. The argument ``fd`` must be an open file descriptor.
+
+The ioctl ``cmd`` code specifies the request function to be called. It
+has encoded in it whether the argument is an input, output or read/write
+parameter, and the size of the argument ``argp`` in bytes.
+
+Macros and structures definitions specifying request ioctl commands and
+their parameters are located in the media.h header file. All request ioctl
+commands, their respective function and parameters are specified in
+:ref:`media-user-func`.
+
+
+Return Value
+============
+
+On success 0 is returned, on error -1 and the ``errno`` variable is set
+appropriately. The generic error codes are described at the
+:ref:`Generic Error Codes <gen-errors>` chapter.
+
+Command-specific error codes are listed in the individual command
+descriptions.
+
+When an ioctl that takes an output or read/write parameter fails, the
+parameter remains unmodified.