32cec3be97
by using this standard extension `adoc`, these files are rendered on gitlab & github Signed-off-by: Francois Perrad <francois.perrad@gadz.org> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
145 lines
4.5 KiB
Plaintext
145 lines
4.5 KiB
Plaintext
// -*- mode:doc; -*-
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// vim: set syntax=asciidoc:
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=== Infrastructure for virtual packages
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[[virtual-package-tutorial]]
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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' platforms. So +libgles+ will be a virtual
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package and +sunxi-mali-utgard+ and +ti-gfx+ will be the providers.
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==== +virtual-package+ tutorial
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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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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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The +.mk+ for the virtual package should just evaluate the +virtual-package+ macro:
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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: $(eval $(virtual-package))
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---------------------------
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The ability to have target and host packages is also available, with the
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+host-virtual-package+ macro.
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==== Provider's +Config.in+ file
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When adding a package as a provider, only the +Config.in+ file requires some
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modifications.
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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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==== Provider's +.mk+ file
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The +.mk+ file should also declare an additional variable
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+SOME_PROVIDER_PROVIDES+ to contain the names of all the virtual
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packages it is an implementation of:
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---------------------------
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01: SOME_PROVIDER_PROVIDES = something-virtual
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---------------------------
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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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==== Notes on depending on a virtual package
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When adding a package that requires a certain +FEATURE+ provided by a virtual
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package, you have to use +depends on BR2_PACKAGE_HAS_FEATURE+, like so:
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---------------------------
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config BR2_PACKAGE_HAS_FEATURE
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bool
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config BR2_PACKAGE_FOO
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bool "foo"
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depends on BR2_PACKAGE_HAS_FEATURE
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---------------------------
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==== Notes on depending on a specific provider
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If your package really requires a specific provider, then you'll have to
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make your package +depends on+ this provider; you can _not_ +select+ a
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provider.
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Let's take an example with two providers for a +FEATURE+:
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---------------------------
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config BR2_PACKAGE_HAS_FEATURE
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bool
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config BR2_PACKAGE_FOO
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bool "foo"
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select BR2_PACKAGE_HAS_FEATURE
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config BR2_PACKAGE_BAR
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bool "bar"
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select BR2_PACKAGE_HAS_FEATURE
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---------------------------
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And you are adding a package that needs +FEATURE+ as provided by +foo+,
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but not as provided by +bar+.
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If you were to use +select BR2_PACKAGE_FOO+, then the user would still
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be able to select +BR2_PACKAGE_BAR+ in the menuconfig. This would create
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a configuration inconsistency, whereby two providers of the same +FEATURE+
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would be enabled at once, one explicitly set by the user, the other
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implicitly by your +select+.
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Instead, you have to use +depends on BR2_PACKAGE_FOO+, which avoids any
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implicit configuration inconsistency.
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