media: docs: move uAPI book to userspace-api/media

Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee3862 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

1 files changed, 171 insertions, 0 deletions

diff --git a/Documentation/userspace-api/media/v4l/vidioc-dbg-g-register.rst b/Documentation/userspace-api/media/v4l/vidioc-dbg-g-register.rst
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+++ b/Documentation/userspace-api/media/v4l/vidioc-dbg-g-register.rst
@@ -0,0 +1,171 @@
+.. Permission is granted to copy, distribute and/or modify this
+.. document under the terms of the GNU Free Documentation License,
+.. Version 1.1 or any later version published by the Free Software
+.. Foundation, with no Invariant Sections, no Front-Cover Texts
+.. and no Back-Cover Texts. A copy of the license is included at
+.. Documentation/userspace-api/media/fdl-appendix.rst.
+..
+.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _VIDIOC_DBG_G_REGISTER:
+
+**************************************************
+ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER
+**************************************************
+
+Name
+====
+
+VIDIOC_DBG_G_REGISTER - VIDIOC_DBG_S_REGISTER - Read or write hardware registers
+
+
+Synopsis
+========
+
+.. c:function:: int ioctl( int fd, VIDIOC_DBG_G_REGISTER, struct v4l2_dbg_register *argp )
+    :name: VIDIOC_DBG_G_REGISTER
+
+.. c:function:: int ioctl( int fd, VIDIOC_DBG_S_REGISTER, const struct v4l2_dbg_register *argp )
+    :name: VIDIOC_DBG_S_REGISTER
+
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :ref:`open() <func-open>`.
+
+``argp``
+    Pointer to struct :c:type:`v4l2_dbg_register`.
+
+
+Description
+===========
+
+.. note::
+
+    This is an :ref:`experimental` interface and may
+    change in the future.
+
+For driver debugging purposes these ioctls allow test applications to
+access hardware registers directly. Regular applications must not use
+them.
+
+Since writing or even reading registers can jeopardize the system
+security, its stability and damage the hardware, both ioctls require
+superuser privileges. Additionally the Linux kernel must be compiled
+with the ``CONFIG_VIDEO_ADV_DEBUG`` option to enable these ioctls.
+
+To write a register applications must initialize all fields of a struct
+:c:type:`v4l2_dbg_register` except for ``size`` and
+call ``VIDIOC_DBG_S_REGISTER`` with a pointer to this structure. The
+``match.type`` and ``match.addr`` or ``match.name`` fields select a chip
+on the TV card, the ``reg`` field specifies a register number and the
+``val`` field the value to be written into the register.
+
+To read a register applications must initialize the ``match.type``,
+``match.addr`` or ``match.name`` and ``reg`` fields, and call
+``VIDIOC_DBG_G_REGISTER`` with a pointer to this structure. On success
+the driver stores the register value in the ``val`` field and the size
+(in bytes) of the value in ``size``.
+
+When ``match.type`` is ``V4L2_CHIP_MATCH_BRIDGE``, ``match.addr``
+selects the nth non-sub-device chip on the TV card. The number zero
+always selects the host chip, e. g. the chip connected to the PCI or USB
+bus. You can find out which chips are present with the
+:ref:`VIDIOC_DBG_G_CHIP_INFO` ioctl.
+
+When ``match.type`` is ``V4L2_CHIP_MATCH_SUBDEV``, ``match.addr``
+selects the nth sub-device.
+
+These ioctls are optional, not all drivers may support them. However
+when a driver supports these ioctls it must also support
+:ref:`VIDIOC_DBG_G_CHIP_INFO`. Conversely
+it may support ``VIDIOC_DBG_G_CHIP_INFO`` but not these ioctls.
+
+``VIDIOC_DBG_G_REGISTER`` and ``VIDIOC_DBG_S_REGISTER`` were introduced
+in Linux 2.6.21, but their API was changed to the one described here in
+kernel 2.6.29.
+
+We recommended the v4l2-dbg utility over calling these ioctls directly.
+It is available from the LinuxTV v4l-dvb repository; see
+`https://linuxtv.org/repo/ <https://linuxtv.org/repo/>`__ for access
+instructions.
+
+
+.. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{3.5cm}|p{7.0cm}|
+
+.. c:type:: v4l2_dbg_match
+
+.. flat-table:: struct v4l2_dbg_match
+    :header-rows:  0
+    :stub-columns: 0
+    :widths:       1 1 2
+
+    * - __u32
+      - ``type``
+      - See :ref:`chip-match-types` for a list of possible types.
+    * - union {
+      - (anonymous)
+    * - __u32
+      - ``addr``
+      - Match a chip by this number, interpreted according to the ``type``
+	field.
+    * - char
+      - ``name[32]``
+      - Match a chip by this name, interpreted according to the ``type``
+	field. Currently unused.
+    * - }
+      -
+
+
+
+.. c:type:: v4l2_dbg_register
+
+.. flat-table:: struct v4l2_dbg_register
+    :header-rows:  0
+    :stub-columns: 0
+
+    * - struct v4l2_dbg_match
+      - ``match``
+      - How to match the chip, see :c:type:`v4l2_dbg_match`.
+    * - __u32
+      - ``size``
+      - The register size in bytes.
+    * - __u64
+      - ``reg``
+      - A register number.
+    * - __u64
+      - ``val``
+      - The value read from, or to be written into the register.
+
+
+
+.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
+
+.. _chip-match-types:
+
+.. flat-table:: Chip Match Types
+    :header-rows:  0
+    :stub-columns: 0
+    :widths:       3 1 4
+
+    * - ``V4L2_CHIP_MATCH_BRIDGE``
+      - 0
+      - Match the nth chip on the card, zero for the bridge chip. Does not
+	match sub-devices.
+    * - ``V4L2_CHIP_MATCH_SUBDEV``
+      - 4
+      - Match the nth sub-device.
+
+
+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.
+
+EPERM
+    Insufficient permissions. Root privileges are required to execute
+    these ioctls.