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poky/documentation/ref-manual/ref-features.rst
Nicolas Dechesne 9bd69b1f1d sphinx: initial sphinx support 
This commit is autogenerated pandoc to generate an inital set
of reST files based on DocBook XML files.

A .rst file is generated for each .xml files in all manuals with this
command:

cd <manual>
for i in *.xml; do \
  pandoc -f docbook -t rst --shift-heading-level-by=-1 \
  $i -o $(basename $i .xml).rst \
done

The conversion was done with: pandoc 2.9.2.1-91 (Arch Linux).

Also created an initial top level index file for each document, and
added all 'books' to the top leve index.rst file.

The YP manuals layout is organized as:

Book
  Chapter
    Section
      Section
        Section

Sphinx uses section headers to create the document structure.
ReStructuredText defines sections headers like that:

   To break longer text up into sections, you use section headers. These
   are a single line of text (one or more words) with adornment: an
   underline alone, or an underline and an overline together, in dashes
   "-----", equals "======", tildes "~~~~~~" or any of the
   non-alphanumeric characters = - ` : ' " ~ ^ _ * + # < > that you feel
   comfortable with. An underline-only adornment is distinct from an
   overline-and-underline adornment using the same character. The
   underline/overline must be at least as long as the title text. Be
   consistent, since all sections marked with the same adornment style
   are deemed to be at the same level:

Let's define the following convention when converting from Docbook:

Book                => overline ===   (Title)
  Chapter           => overline ***   (1.)
    Section         => ====           (1.1)
      Section       => ----           (1.1.1)
        Section     => ~~~~           (1.1.1.1)
          Section   => ^^^^           (1.1.1.1.1)

During the conversion with pandoc, we used --shift-heading-level=-1 to
convert most of DocBook headings automatically. However with this
setting, the Chapter header was removed, so I added it back
manually. Without this setting all headings were off by one, which was
more difficult to manually fix.

At least with this change, we now have the same TOC with Sphinx and
DocBook.

(From yocto-docs rev: 3c73d64a476d4423ee4c6808c685fa94d88d7df8)

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
2020-09-17 10:09:33 +01:00

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********
Features
********
This chapter provides a reference of shipped machine and distro features
you can include as part of your image, a reference on image features you
can select, and a reference on feature backfilling.
Features provide a mechanism for working out which packages should be
included in the generated images. Distributions can select which
features they want to support through the ``DISTRO_FEATURES`` variable,
which is set or appended to in a distribution's configuration file such
as ``poky.conf``, ``poky-tiny.conf``, ``poky-lsb.conf`` and so forth.
Machine features are set in the ``MACHINE_FEATURES`` variable, which is
set in the machine configuration file and specifies the hardware
features for a given machine.
These two variables combine to work out which kernel modules, utilities,
and other packages to include. A given distribution can support a
selected subset of features so some machine features might not be
included if the distribution itself does not support them.
One method you can use to determine which recipes are checking to see if
a particular feature is contained or not is to ``grep`` through the
`Metadata <#metadata>`__ for the feature. Here is an example that
discovers the recipes whose build is potentially changed based on a
given feature: $ cd poky $ git grep
'contains.*MACHINE_FEATURES.*feature'
.. _ref-features-machine:
Machine Features
================
The items below are features you can use with
```MACHINE_FEATURES`` <#var-MACHINE_FEATURES>`__. Features do not have a
one-to-one correspondence to packages, and they can go beyond simply
controlling the installation of a package or packages. Sometimes a
feature can influence how certain recipes are built. For example, a
feature might determine whether a particular configure option is
specified within the ```do_configure`` <#ref-tasks-configure>`__ task
for a particular recipe.
This feature list only represents features as shipped with the Yocto
Project metadata:
- *acpi:* Hardware has ACPI (x86/x86_64 only)
- *alsa:* Hardware has ALSA audio drivers
- *apm:* Hardware uses APM (or APM emulation)
- *bluetooth:* Hardware has integrated BT
- *efi:* Support for booting through EFI
- *ext2:* Hardware HDD or Microdrive
- *keyboard:* Hardware has a keyboard
- *pcbios:* Support for booting through BIOS
- *pci:* Hardware has a PCI bus
- *pcmcia:* Hardware has PCMCIA or CompactFlash sockets
- *phone:* Mobile phone (voice) support
- *qvga:* Machine has a QVGA (320x240) display
- *rtc:* Machine has a Real-Time Clock
- *screen:* Hardware has a screen
- *serial:* Hardware has serial support (usually RS232)
- *touchscreen:* Hardware has a touchscreen
- *usbgadget:* Hardware is USB gadget device capable
- *usbhost:* Hardware is USB Host capable
- *vfat:* FAT file system support
- *wifi:* Hardware has integrated WiFi
.. _ref-features-distro:
Distro Features
===============
The items below are features you can use with
```DISTRO_FEATURES`` <#var-DISTRO_FEATURES>`__ to enable features across
your distribution. Features do not have a one-to-one correspondence to
packages, and they can go beyond simply controlling the installation of
a package or packages. In most cases, the presence or absence of a
feature translates to the appropriate option supplied to the configure
script during the ```do_configure`` <#ref-tasks-configure>`__ task for
the recipes that optionally support the feature.
Some distro features are also machine features. These select features
make sense to be controlled both at the machine and distribution
configuration level. See the
```COMBINED_FEATURES`` <#var-COMBINED_FEATURES>`__ variable for more
information.
This list only represents features as shipped with the Yocto Project
metadata:
- *alsa:* Include ALSA support (OSS compatibility kernel modules
installed if available).
- *api-documentation:* Enables generation of API documentation during
recipe builds. The resulting documentation is added to SDK tarballs
when the ``bitbake -c populate_sdk`` command is used. See the
"`Adding API Documentation to the Standard
SDK <&YOCTO_DOCS_SDK_URL;#adding-api-documentation-to-the-standard-sdk>`__"
section in the Yocto Project Application Development and the
Extensible Software Development Kit (eSDK) manual.
- *bluetooth:* Include bluetooth support (integrated BT only).
- *cramfs:* Include CramFS support.
- *directfb:* Include DirectFB support.
- *ext2:* Include tools for supporting for devices with internal
HDD/Microdrive for storing files (instead of Flash only devices).
- *ipsec:* Include IPSec support.
- *ipv6:* Include IPv6 support.
- *keyboard:* Include keyboard support (e.g. keymaps will be loaded
during boot).
- *ldconfig:* Include support for ldconfig and ``ld.so.conf`` on the
target.
- *nfs:* Include NFS client support (for mounting NFS exports on
device).
- *opengl:* Include the Open Graphics Library, which is a
cross-language, multi-platform application programming interface used
for rendering two and three-dimensional graphics.
- *pci:* Include PCI bus support.
- *pcmcia:* Include PCMCIA/CompactFlash support.
- *ppp:* Include PPP dialup support.
- *ptest:* Enables building the package tests where supported by
individual recipes. For more information on package tests, see the
"`Testing Packages With
ptest <&YOCTO_DOCS_DEV_URL;#testing-packages-with-ptest>`__" section
in the Yocto Project Development Tasks Manual.
- *smbfs:* Include SMB networks client support (for mounting
Samba/Microsoft Windows shares on device).
- *systemd:* Include support for this ``init`` manager, which is a full
replacement of for ``init`` with parallel starting of services,
reduced shell overhead, and other features. This ``init`` manager is
used by many distributions.
- *usbgadget:* Include USB Gadget Device support (for USB
networking/serial/storage).
- *usbhost:* Include USB Host support (allows to connect external
keyboard, mouse, storage, network etc).
- *usrmerge:* Merges the ``/bin``, ``/sbin``, ``/lib``, and ``/lib64``
directories into their respective counterparts in the ``/usr``
directory to provide better package and application compatibility.
- *wayland:* Include the Wayland display server protocol and the
library that supports it.
- *wifi:* Include WiFi support (integrated only).
- *x11:* Include the X server and libraries.
.. _ref-features-image:
Image Features
==============
The contents of images generated by the OpenEmbedded build system can be
controlled by the ```IMAGE_FEATURES`` <#var-IMAGE_FEATURES>`__ and
```EXTRA_IMAGE_FEATURES`` <#var-EXTRA_IMAGE_FEATURES>`__ variables that
you typically configure in your image recipes. Through these variables,
you can add several different predefined packages such as development
utilities or packages with debug information needed to investigate
application problems or profile applications.
The following image features are available for all images:
- *allow-empty-password:* Allows Dropbear and OpenSSH to accept root
logins and logins from accounts having an empty password string.
- *dbg-pkgs:* Installs debug symbol packages for all packages installed
in a given image.
- *debug-tweaks:* Makes an image suitable for development (e.g. allows
root logins without passwords and enables post-installation logging).
See the 'allow-empty-password', 'empty-root-password', and
'post-install-logging' features in this list for additional
information.
- *dev-pkgs:* Installs development packages (headers and extra library
links) for all packages installed in a given image.
- *doc-pkgs:* Installs documentation packages for all packages
installed in a given image.
- *empty-root-password:* Sets the root password to an empty string,
which allows logins with a blank password.
- *package-management:* Installs package management tools and preserves
the package manager database.
- *post-install-logging:* Enables logging postinstall script runs to
the ``/var/log/postinstall.log`` file on first boot of the image on
the target system.
.. note::
To make the
/var/log
directory on the target persistent, use the
VOLATILE_LOG_DIR
variable by setting it to "no".
- *ptest-pkgs:* Installs ptest packages for all ptest-enabled recipes.
- *read-only-rootfs:* Creates an image whose root filesystem is
read-only. See the "`Creating a Read-Only Root
Filesystem <&YOCTO_DOCS_DEV_URL;#creating-a-read-only-root-filesystem>`__"
section in the Yocto Project Development Tasks Manual for more
information.
- *splash:* Enables showing a splash screen during boot. By default,
this screen is provided by ``psplash``, which does allow
customization. If you prefer to use an alternative splash screen
package, you can do so by setting the ``SPLASH`` variable to a
different package name (or names) within the image recipe or at the
distro configuration level.
- *staticdev-pkgs:* Installs static development packages, which are
static libraries (i.e. ``*.a`` files), for all packages installed in
a given image.
Some image features are available only when you inherit the
```core-image`` <#ref-classes-core-image>`__ class. The current list of
these valid features is as follows:
- *hwcodecs:* Installs hardware acceleration codecs.
- *nfs-server:* Installs an NFS server.
- *perf:* Installs profiling tools such as ``perf``, ``systemtap``, and
``LTTng``. For general information on user-space tools, see the
`Yocto Project Application Development and the Extensible Software
Development Kit (eSDK) <&YOCTO_DOCS_SDK_URL;>`__ manual.
- *ssh-server-dropbear:* Installs the Dropbear minimal SSH server.
- *ssh-server-openssh:* Installs the OpenSSH SSH server, which is more
full-featured than Dropbear. Note that if both the OpenSSH SSH server
and the Dropbear minimal SSH server are present in
``IMAGE_FEATURES``, then OpenSSH will take precedence and Dropbear
will not be installed.
- *tools-debug:* Installs debugging tools such as ``strace`` and
``gdb``. For information on GDB, see the "`Debugging With the GNU
Project Debugger (GDB)
Remotely <&YOCTO_DOCS_DEV_URL;#platdev-gdb-remotedebug>`__" section
in the Yocto Project Development Tasks Manual. For information on
tracing and profiling, see the `Yocto Project Profiling and Tracing
Manual <&YOCTO_DOCS_PROF_URL;>`__.
- *tools-sdk:* Installs a full SDK that runs on the device.
- *tools-testapps:* Installs device testing tools (e.g. touchscreen
debugging).
- *x11:* Installs the X server.
- *x11-base:* Installs the X server with a minimal environment.
- *x11-sato:* Installs the OpenedHand Sato environment.
.. _ref-features-backfill:
Feature Backfilling
===================
Sometimes it is necessary in the OpenEmbedded build system to extend
```MACHINE_FEATURES`` <#var-MACHINE_FEATURES>`__ or
```DISTRO_FEATURES`` <#var-DISTRO_FEATURES>`__ to control functionality
that was previously enabled and not able to be disabled. For these
cases, we need to add an additional feature item to appear in one of
these variables, but we do not want to force developers who have
existing values of the variables in their configuration to add the new
feature in order to retain the same overall level of functionality.
Thus, the OpenEmbedded build system has a mechanism to automatically
"backfill" these added features into existing distro or machine
configurations. You can see the list of features for which this is done
by finding the
```DISTRO_FEATURES_BACKFILL`` <#var-DISTRO_FEATURES_BACKFILL>`__ and
```MACHINE_FEATURES_BACKFILL`` <#var-MACHINE_FEATURES_BACKFILL>`__
variables in the ``meta/conf/bitbake.conf`` file.
Because such features are backfilled by default into all configurations
as described in the previous paragraph, developers who wish to disable
the new features need to be able to selectively prevent the backfilling
from occurring. They can do this by adding the undesired feature or
features to the
```DISTRO_FEATURES_BACKFILL_CONSIDERED`` <#var-DISTRO_FEATURES_BACKFILL_CONSIDERED>`__
or
```MACHINE_FEATURES_BACKFILL_CONSIDERED`` <#var-MACHINE_FEATURES_BACKFILL_CONSIDERED>`__
variables for distro features and machine features respectively.
Here are two examples to help illustrate feature backfilling:
- *The "pulseaudio" distro feature option*: Previously, PulseAudio
support was enabled within the Qt and GStreamer frameworks. Because
of this, the feature is backfilled and thus enabled for all distros
through the ``DISTRO_FEATURES_BACKFILL`` variable in the
``meta/conf/bitbake.conf`` file. However, your distro needs to
disable the feature. You can disable the feature without affecting
other existing distro configurations that need PulseAudio support by
adding "pulseaudio" to ``DISTRO_FEATURES_BACKFILL_CONSIDERED`` in
your distro's ``.conf`` file. Adding the feature to this variable
when it also exists in the ``DISTRO_FEATURES_BACKFILL`` variable
prevents the build system from adding the feature to your
configuration's ``DISTRO_FEATURES``, effectively disabling the
feature for that particular distro.
- *The "rtc" machine feature option*: Previously, real time clock (RTC)
support was enabled for all target devices. Because of this, the
feature is backfilled and thus enabled for all machines through the
``MACHINE_FEATURES_BACKFILL`` variable in the
``meta/conf/bitbake.conf`` file. However, your target device does not
have this capability. You can disable RTC support for your device
without affecting other machines that need RTC support by adding the
feature to your machine's ``MACHINE_FEATURES_BACKFILL_CONSIDERED``
list in the machine's ``.conf`` file. Adding the feature to this
variable when it also exists in the ``MACHINE_FEATURES_BACKFILL``
variable prevents the build system from adding the feature to your
configuration's ``MACHINE_FEATURES``, effectively disabling RTC
support for that particular machine.
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