media: admin-guide: add a generic building guide

Several of the existing documents under the media admin-guide
contain build procedures.

Add an specific chapter describing it. This document was
partially inspired on the modifications I made to the bttv.rst
file.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/building.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+Building support for a media device
+===================================
+
+The first step is to download the Kernel's source code, either via a
+distribution-specific source file or via the Kernel's main git tree\ [1]_.
+
+Please notice, however, that, if:
+
+- you're a braveheart and want to experiment with new stuff;
+- if you want to report a bug;
+- if you're developing new patches
+
+you should use the main media development tree ``master`` branch:
+
+    https://git.linuxtv.org/media_tree.git/
+
+In this case, you may find some useful information at the
+`LinuxTv wiki pages <https://linuxtv.org/wiki>`_:
+
+    https://linuxtv.org/wiki/index.php/How_to_Obtain,_Build_and_Install_V4L-DVB_Device_Drivers
+
+.. [1] The upstream Linux Kernel development tree is located at
+
+       https://git.kernel.org/pub/scm/li  nux/kernel/git/torvalds/linux.git/
+
+Configuring the Linux Kernel
+============================
+
+You can access a menu of Kernel building options with::
+
+    $ make menuconfig
+
+Then, select all desired options and exit it, saving the configuration.
+
+The changed configuration will be at the ``.config`` file. It would
+look like::
+
+    ...
+    # CONFIG_RC_CORE is not set
+    # CONFIG_CEC_CORE is not set
+    CONFIG_MEDIA_SUPPORT=m
+    CONFIG_MEDIA_SUPPORT_FILTER=y
+    ...
+
+The media subsystem is controlled by those menu configuration options::
+
+    Device Drivers --->
+	<M> Remote Controller support  --->
+	[ ] HDMI CEC RC integration
+	[ ] Enable CEC error injection support
+	[*] HDMI CEC drivers  --->
+	<*> Multimedia support  --->
+
+The ``Remote Controller support`` option enables the core support for
+remote controllers\ [2]_.
+
+The ``HDMI CEC RC integration`` option enables integration of HDMI CEC
+with Linux, allowing to receive data via HDMI CEC as if it were produced
+by a remote controller directly connected to the machine.
+
+The ``HDMI CEC drivers`` option allow selecting platform and USB drivers
+that receives and/or transmits CEC codes via HDMI interfaces\ [3]_.
+
+The last option (``Multimedia support``) enables support for cameras,
+audio/video grabbers and TV.
+
+The media subsystem support can either be built together with the main
+Kernel or as a module. For most use cases, it is preferred to have it
+built as modules.
+
+.. note::
+
+   Instead of using a menu, the Kernel provides a script with allows
+   enabling configuration options directly. To enable media support
+   and remote controller support using Kernel modules, you could use::
+
+	$ scripts/config -m RC_CORE
+	$ scripts/config -m MEDIA_SUPPORT
+
+.. [2] ``Remote Controller support`` should also be enabled if you
+       want to use some TV card drivers that may depend on the remote
+       controller core support.
+
+.. [3] Please notice that the DRM subsystem also have drivers for GPUs
+       that use the media HDMI CEC support.
+
+       Those GPU-specific drivers are selected via the ``Graphics support``
+       menu, under ``Device Drivers``.
+
+       When a GPU driver supports supports HDMI CEC, it will automatically
+       enable the CEC core support at the media subsystem.
+
+Media dependencies
+------------------
+
+It should be noticed that enabling the above from a clean config is
+usually not enough. The media subsystem depends on several other Linux
+core support in order to work.
+
+For example, most media devices use a serial communication bus in
+order to talk with some peripherals. Such bus is called I²C
+(Inter-Integrated Circuit). In order to be able to build support
+for such hardware, the I²C bus support should be enabled, either via
+menu or with::
+
+    ./scripts/config -m I2C
+
+Another example: the remote controller core requires support for
+input devices, with can be enabled with::
+
+    ./scripts/config -m INPUT
+
+Other core functionality may also be needed (like PCI and/or USB support),
+depending on the specific driver(s) you would like to enable.
+
+Enabling Remote Controller Support
+----------------------------------
+
+The remote controller menu allows selecting drivers for specific devices.
+It's menu looks like this::
+
+         --- Remote Controller support
+         <M>   Compile Remote Controller keymap modules
+         [*]   LIRC user interface
+         [*]     Support for eBPF programs attached to lirc devices
+         [*]   Remote controller decoders  --->
+         [*]   Remote Controller devices  --->
+
+The ``Compile Remote Controller keymap modules`` option creates key maps for
+several popular remote controllers.
+
+The ``LIRC user interface`` option adds enhanced functionality when using the
+``lirc`` program, by enabling an API that allows userspace to receive raw data
+from remote controllers.
+
+The ``Support for eBPF programs attached to lirc devices`` option allows
+the usage of special programs (called eBPF) that would allow aplications
+to add extra remote controller decoding functionality to the Linux Kernel.
+
+The ``Remote controller decoders`` option allows selecting the
+protocols that will be recognized by the Linux Kernel. Except if you
+want to disable some specific decoder, it is suggested to keep all
+sub-options enabled.
+
+The ``Remote Controller devices`` allows you to select the drivers
+that would be needed to support your device.
+
+The same configuration can also be set via the ``script/config``
+script. So, for instance, in order to support the ITE remote controller
+driver (found on Intel NUCs and on some ASUS x86 desktops), you could do::
+
+	$ scripts/config -e INPUT
+	$ scripts/config -e ACPI
+	$ scripts/config -e MODULES
+	$ scripts/config -m RC_CORE
+	$ scripts/config -e RC_DEVICES
+	$ scripts/config -e RC_DECODERS
+	$ scripts/config -m IR_RC5_DECODER
+	$ scripts/config -m IR_ITE_CIR
+
+Enabling HDMI CEC Support
+-------------------------
+
+The HDMI CEC support is set automatically when a driver requires it. So,
+all you need to do is to enable support either for a graphics card
+that needs it or by one of the existing HDMI drivers.
+
+The HDMI-specific drivers are available at the ``HDMI CEC drivers``
+menu\ [4]_::
+
+	--- HDMI CEC drivers
+	< >   ChromeOS EC CEC driver
+	< >   Amlogic Meson AO CEC driver
+	< >   Amlogic Meson G12A AO CEC driver
+	< >   Generic GPIO-based CEC driver
+	< >   Samsung S5P CEC driver
+	< >   STMicroelectronics STiH4xx HDMI CEC driver
+	< >   STMicroelectronics STM32 HDMI CEC driver
+	< >   Tegra HDMI CEC driver
+	< >   SECO Boards HDMI CEC driver
+	[ ]     SECO Boards IR RC5 support
+	< >   Pulse Eight HDMI CEC
+	< >   RainShadow Tech HDMI CEC
+
+.. [4] The above contents is just an example. The actual options for
+       HDMI devices depends on the system's architecture and may vary
+       on new Kernels.
+
+Enabling Media Support
+----------------------
+
+The Media menu has a lot more options than the remote controller menu.
+Once selected, you should see the following options::
+
+	--- Media support
+	[ ] Filter media drivers
+	[*] Autoselect ancillary drivers
+	    Media device types --->
+	    Media core support --->
+	    Video4Linux options --->
+	    Media controller options --->
+	    Digital TV options --->
+	    HDMI CEC options --->
+	    Media drivers --->
+	    Media ancillary drivers --->
+
+Except if you know exactly what you're doing, or if you want to build
+a driver for a SoC platform, it is strongly recommended to keep the
+``Autoselect ancillary drivers`` option turned on, as it will auto-select
+the needed I²C ancillary drivers.
+
+There are now two ways to select media device drivers, as described
+below.
+
+``Filter media drivers`` menu
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This menu is meant to easy setup for PC and Laptop hardware. It works
+by letting the user to specify what kind of media drivers are desired,
+with those options::
+
+	[ ] Cameras and video grabbers
+	[ ] Analog TV
+	[ ] Digital TV
+	[ ] AM/FM radio receivers/transmitters
+	[ ] Software defined radio
+	[ ] Platform-specific devices
+	[ ] Test drivers
+
+So, if you want to add support to a camera or video grabber only,
+select just the first option. Multiple options are allowed.
+
+Once the options on this menu are selected, the building system will
+auto-select the needed core drivers in order to support the selected
+functionality.
+
+.. note::
+
+   Most TV cards are hybrid: they support both Analog TV and Digital TV.
+
+   If you have an hybrid card, you may need to enable both ``Analog TV``
+   and ``Digital TV`` at the menu.
+
+When using this option, the defaults for the the media support core
+functionality are usually good enough to provide the basic functionality
+for the driver. Yet, you could manually enable some desired extra (optional)
+functionality using the settings under each of the following
+``Media support`` sub-menus::
+
+	    Media core support --->
+	    Video4Linux options --->
+	    Media controller options --->
+	    Digital TV options --->
+	    HDMI CEC options --->
+
+Once you select the desired filters, the drivers that matches the filtering
+criteria will be available at the ``Media support->Media drivers`` sub-menu.
+
+``Media Core Support`` menu without filtering
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you disable the ``Filter media drivers`` menu, all drivers available
+for your system whose dependencies are met should be shown at the
+``Media drivers`` menu.
+
+Please notice, however, that you should first ensure that the
+``Media Core Support`` menu has all the core functionality your drivers
+would need, as otherwise the corresponding device drivers won't be shown.
+
+Example
+-------
+
+In order to enable modular support for one of the boards listed on
+:doc:`this table <cx231xx-cardlist>`, with modular media core modules, the
+``.config`` file should contain those lines::
+
+    CONFIG_MODULES=y
+    CONFIG_USB=y
+    CONFIG_I2C=y
+    CONFIG_INPUT=y
+    CONFIG_RC_CORE=m
+    CONFIG_MEDIA_SUPPORT=m
+    CONFIG_MEDIA_SUPPORT_FILTER=y
+    CONFIG_MEDIA_ANALOG_TV_SUPPORT=y
+    CONFIG_MEDIA_DIGITAL_TV_SUPPORT=y
+    CONFIG_MEDIA_USB_SUPPORT=y
+    CONFIG_VIDEO_CX231XX=y
+    CONFIG_VIDEO_CX231XX_DVB=y
+
+Building and installing a new Kernel
+====================================
+
+Once the ``.config`` file has everything needed, all it takes to build
+is to run the ``make`` command::
+
+    $ make
+
+And then install the new Kernel and its modules::
+
+    $ sudo make modules_install
+    $ sudo make install
+
+Building just the new media drivers and core
+============================================
+
+Running a new development Kernel from the development tree is usually risky,
+because it may have experimental changes that may have bugs. So, there are
+some ways to build just the new drivers, using alternative trees.
+
+There is the `Linux Kernel backports project
+<https://backports.wiki.kernel.org/index.php/Main_Page>`_, with contains
+newer drivers meant to be compiled against stable Kernels.
+
+The LinuxTV developers, with are responsible for maintaining the media
+subsystem also maintains a backport tree, with just the media drivers
+daily updated from the newest kernel. Such tree is available at:
+
+https://git.linuxtv.org/media_build.git/
+
+It should be noticed that, while it should be relatively safe to use the
+``media_build`` tree for testing purposes, there are not warranties that
+it would work (or even build) on a random Kernel. This tree is maintained
+using a "best-efforts" principle, as time permits us to fix issues there.
+
+If you notice anything wrong on it, feel free to submit patches at the
+Linux media subsystem's mailing list: media@vger.kernel.org. Please
+add ``[PATCH media-build]`` at the e-mail's subject if you submit a new
+patch for the media-build.
+
+Before using it, you should run::
+
+    $ ./build
+
+.. note::
+
+    1) you may need to run it twice if the ``media-build`` tree gets
+       updated;
+    2) you may need to do a ``make distclean`` if you had built it
+       in the past for a different Kernel version than the one you're
+       currently using;
+    3) by default, it will use the same config options for media as
+       the ones defined on the Kernel you're running.
+
+In order to select different drivers or different config options,
+use::
+
+    $ make menuconfig
+
+Then, you can build and install the new drivers::
+
+    $ make && sudo make install
+
+This will override the previous media drivers that your Kernel were
+using.

Documentation/admin-guide/media/index.rst

@@ -35,6 +35,7 @@ Video4Linux (V4L)  driver-specific documentation
 	:numbered:
 
 	intro
+	building
 
 	cardlist