manual: add virtual package tutorial
The manual now features a new section with instructions about how to add a virtual package. Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr> [yann.morin.1998@free.fr: move down statement about provider's .mk] Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr> Reviewed-by: Samuel Martin <s.martin49@gmail.com> Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
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docs/manual/adding-packages-virtual.txt
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docs/manual/adding-packages-virtual.txt
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// -*- mode:doc; -*-
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// vim: set syntax=asciidoc:
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[[virtual-package-tutorial]]
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Virtual package tutorial
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~~~~~~~~~~~~~~~~~~~~~~~~
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In Buildroot, a virtual package is a package whose functionalities are
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provided by one or more packages, referred to as 'providers'. The virtual
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package management is an extensible mechanism allowing the user to choose
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the provider used in the rootfs.
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For example, 'OpenGL ES' is an API for 2D and 3D graphics on embedded systems.
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The implementation of this API is different for the 'Allwinner Tech Sunxi' and
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the 'Texas Instruments OMAP35xx' plaftorms. So +libgles+ will be a virtual
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package and +sunxi-mali+ and +ti-gfx+ will be the providers.
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In the following example, we will explain how to add a new virtual package
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('something-virtual') and a provider for it ('some-provider').
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First, let's create the virtual package.
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Virtual package's +Config.in+ file
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The +Config.in+ file of virtual package 'something-virtual' should contain:
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---------------------------
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01: config BR2_PACKAGE_HAS_SOMETHING_VIRTUAL
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02: bool
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03:
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04: config BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL
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05: depends on BR2_PACKAGE_HAS_SOMETHING_VIRTUAL
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06: string
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---------------------------
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In this file, we declare two options, +BR2_PACKAGE_HAS_SOMETHING_VIRTUAL+ and
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+BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL+, whose values will be used by the
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providers.
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Virtual package's +*.mk+ file
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The Makefile +package/something-virtual/something-virtual.mk+ should contain:
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---------------------------
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01: ################################################################################
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02: #
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03: # something-virtual
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04: #
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05: ################################################################################
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06:
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07: SOMETHING_VIRTUAL_SOURCE =
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08: SOMETHING_VIRTUAL_DEPENDENCIES = $(call qstrip,$(BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL))
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09:
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10: ifeq ($(BR2_PACKAGE_HAS_SOMETHING_VIRTUAL),y)
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11: ifeq ($(SOMETHING_VIRTUAL_DEPENDENCIES),)
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12: $(error No something-virtual implementation selected. Configuration error.)
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13: endif
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14: endif
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15:
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16: $(eval $(generic-package))
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---------------------------
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The Makefile is quite small as it will only check if a provider for the
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virtual package has been selected.
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Provider's +Config.in+ file
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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When adding a package as a provider, only the +Config.in+ file requires some
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modifications. The +*.mk+ file should follow the Buildroot infrastructure with
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no change at all.
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The +Config.in+ file of the package 'some-provider', which provides the
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functionalities of 'something-virtual', should contain:
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---------------------------
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01: config BR2_PACKAGE_SOME_PROVIDER
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02: bool "some-provider"
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03: select BR2_PACKAGE_HAS_SOMETHING_VIRTUAL
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04: help
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05: This is a comment that explains what some-provider is.
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06:
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07: http://foosoftware.org/some-provider/
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08:
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09: if BR2_PACKAGE_SOME_PROVIDER
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10: config BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL
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11: default "some-provider"
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12: endif
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---------------------------
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On line 3, we select +BR2_PACKAGE_HAS_SOMETHING_VIRTUAL+, and on line 11, we
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set the value of +BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL+ to the name of the
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provider, but only if it is selected.
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Of course, do not forget to add the proper build and runtime dependencies for
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this package!
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@ -24,6 +24,8 @@ include::adding-packages-luarocks.txt[]
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include::adding-packages-perl.txt[]
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include::adding-packages-perl.txt[]
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include::adding-packages-virtual.txt[]
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include::adding-packages-hooks.txt[]
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include::adding-packages-hooks.txt[]
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include::adding-packages-gettext.txt[]
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include::adding-packages-gettext.txt[]
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