docs/overview.txt
author "Robert P. J. DAY" <rpjday@crashcourse.ca>
Wed Feb 20 08:01:12 2008 +0000 (2008-02-20)
changeset 436 ecbb620acaa0
parent 391 11172b754564
child 437 dbfc5a7353b0
permissions -rw-r--r--
Fix some obvious typoes in docs/overview.txt
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File.........: overview.txt
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Content......: Overview of how crosstool-NG works.
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Copyrigth....: (C) 2007 Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
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License......: Creative Commons Attribution Share Alike (CC-by-sa), v2.5
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________________
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               /
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Introduction  /
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_____________/
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crosstool-NG aims at building toolchains. Toolchains are an essential component
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in a software development project. It will compile, assemble and link the code
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that is being developed. Some pieces of the toolchain will eventually end up
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in the resulting binary/ies: static libraries are but an example.
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So, a toolchain is a very sensitive piece of software, as any bug in one of the
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components, or a poorly configured component, can lead to execution problems,
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ranging from poor performance, to applications ending unexpectedly, to
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mis-behaving software (which more than often is hard to detect), to hardware
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damage, or even to human risks (which is more than regrettable).
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Toolchains are made of different piece of software, each being quite complex
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and requiring specially crafted options to build and work seamlessly. This
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is usually not that easy, even in the not-so-trivial case of native toolchains.
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The work reaches a higher degree of complexity when it comes to cross-
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compilation, where it can become quite a nightmare...
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Some cross-toolchains exist on the internet, and can be used for general
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development, but they have a number of limitations:
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  - they can be general purpose, in that they are configured for the majority:
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    no optimisation for your specific target,
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  - they can be prepared for a specific target and thus are not easy to use,
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    nor optimised for, or even supporting your target,
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  - they often are using aging components (compiler, C library, etc...) not
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    supporting special features of your shiny new processor;
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On the other side, these toolchain offer some advantages:
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  - they are ready to use and quite easy to install and setup,
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  - they are proven if used by a wide community.
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But once you want to get all the juice out of your specific hardware, you will
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want to build your own toolchain. This is where crosstool-NG comes into play.
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There are also a number of tools that build toolchains for specific needs,
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which are not really scalable. Examples are:
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  - buildroot (buildroot.uclibc.org) whose main purpose is to build root file
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    systems, hence the name. But once you have your toolchain with buildroot,
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    part of it is installed in the root-to-be, so if you want to build a whole
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    new root, you either have to save the existing one as a template and
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    restore it later, or restart again from scratch. This is not convenient,
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  - ptxdist (www.pengutronix.de/software/ptxdist), whose purpose is very
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    similar to buildroot,
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  - other projects (openembedded.org for example), which is again used to
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    build root file systems.
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crosstool-NG is really targeted at building toolchains, and only toolchains.
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It is then up to you to use it the way you want.
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___________
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          /
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History  /
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________/
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crosstool was first 'conceived' by Dan Kegel, who offered it to the community
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as a set of scripts, a repository of patches, and some pre-configured, general
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purpose setup files to be used to configure crosstool. This is available at
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http://www.kegel.com/crosstool, and the subversion repository is hosted on
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google at http://code.google.com/p/crosstool/.
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At the time of writing, crosstool supports building with only one C library,
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namely glibc, and one C compiler, gcc; it is crippled with historical support
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for legacy components, and is some kind of a mess to upgrade. Also, submitted
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patches take a loooong time before they are integrated into mainline.
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I once managed to add support for uClibc-based toolchains, but it did not make
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into mainline, mostly because I don't have time to port the patch forward to
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the new versions, due in part to the big effort it was taking.
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So I decided to clean up crosstool in the state it was, re-order the things
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in place, and add appropriate support for what I needed, that is uClibc
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support. That was a disaster, as inclusion into mainline is slow as hell,
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and the changes were so numerous.
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The only option left to me was rewrite crosstool from scratch. I decided to go
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this way, and name the new implementation crosstool-NG, standing for crosstool
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Next Generation, as many other comunity projects do, and as a wink at the TV
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series "Star Trek: The Next Generation". ;-)
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___________________________
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                          /
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Installing crosstool-NG  /
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________________________/
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There are two ways you can use crosstool-NG:
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 - build and install it, then get rid of the sources like you'd do for most
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   programs,
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 - or only build it and run from the source directory.
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The former should be used if you got crosstool-NG from a packaged tarball, see
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"Install method", below, while the latter is most useful for developpers that
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checked the code out from SVN, and want to submit patches, see "The Hacker's
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way", below.
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Install method |
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---------------+
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If you go for the install, then you just follow the classical, but yet easy
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./configure way:
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  ./configure --prefix=/some/place
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  make
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  make install
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  export PATH="${PATH}:/some/place/bin"
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You can then get rid of crosstool-NG source. Next create a directory to serve
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as a working place, cd in there and run:
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  ct-ng help
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See below for complete usage.
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The Hacker's way |
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-----------------+
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If you go the hacker's way, then the usage is a bit different, although very
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simple:
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  ./configure --local
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  make
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Now, *do not* remove crosstool-NG sources. They are needed to run crosstool-NG!
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Stay in the directory holding the sources, and run:
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  ./ct-ng help
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See below for complete usage.
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Now, provided you checked-out the code, you can send me your interesting changes
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by running:
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  svn diff
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and mailing me the result! :-P
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____________________________
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                           /
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Configuring crosstool-NG  /
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_________________________/
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crosstool-NG is configured by a configurator presenting a menu-stuctured set of
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options. These options let you specify the way you want your toolchain built,
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where you want it installed, what architecture and specific processor it
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will support, the version of the components you want to use, etc... The
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value for those options are then stored in a configuration file.
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The configurator works the same way you configure your Linux kernel.It is
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assumed you now how to handle this.
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To enter the menu, type:
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  ct-ng menuconfig
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Almost every config item has a help entry. Read them carefully.
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String and number options can refer to environment variables. In such a case,
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you must use the shell syntax: ${VAR}. You shall neither single- nor double-
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quote the string/number options.
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There are three environment variables that are computed by crosstool-NG, and
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that you can use:
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CT_TARGET:
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  It represents the target tuple you are building for. You can use it for
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  example in the installation/prefix directory, such as:
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    /opt/x-tools/${CT_TARGET}
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CT_TOP_DIR:
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  The top directory where crosstool-NG is running. You shouldn't need it in
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  most cases. There is one case where you may need it: if you have local
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  patches and you store them in your running directory, you can refer to them
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  by using CT_TOP_DIR, such as:
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    ${CT_TOP_DIR}/patches.myproject
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CT_VERSION:
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  The version of crosstool-NG you are using. Not much use for you, but it's
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  there if you need it.
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Interesting config options |
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---------------------------*
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CT_LOCAL_TARBALLS_DIR:
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  If you already have some tarballs in a direcotry, enter it here. That will
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  speed up the retrieving phase, where crosstool-NG would otherwise download
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  those tarballs.
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CT_PREFIX_DIR:
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  This is where the toolchain will be installed in (and for now, where it
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  will run from). Common use it to add the target tuple in the directory
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  path, such as (see above):
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    /opt/x-tools/${CT_TARGET}
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CT_TARGET_VENDOR:
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  An identifier for your toolchain, will take place in the vendor part of the
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  target tuple. It shall *not* contain spaces or dashes. Usually, keep it
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  to a one-word string, or use underscores to separate words if you need.
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  Avoid dots, commas, and special characters.
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CT_TARGET_ALIAS:
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  An alias for the toolchian. It will be used as a prefix to the toolchain
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  tools. For example, you will have ${CT_TARGET_ALIAS}-gcc
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Also, if you think you don't see enough versions, you can try to enable one of
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those:
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CT_OBSOLETE:
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  Show obsolete versions or tools. Most of the time, you don't want to base
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  your toolchain on too old a version (of gcc, for example). But at times, it
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  can come handy to use such an old version for regression tests. Those old
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  versions are hidden behind CT_OBSOLETE.
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CT_EXPERIMENTAL:
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  Show experimental versions or tools. Again, you might not want to base your
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  toolchain on too recent tools (eg. gcc) for production. But if you need a
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  feature present only in a recent version, or a new tool, you can find them
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  hidden behind CT_EXPERIMENTAL.
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CT_BROKEN:
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  Show broken versions or tools. Some usefull tools are currently broken: they
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  won't compile, run, or worse, cause defects when running. But if you are
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  brave enough, you can try and debug them. They are hidden behind CT_BROKEN,
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  which itself is hidden behind EXPERIMENTAL.
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Re-building an existing toolchain |
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----------------------------------+
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If you have an existing toolchain, you can re-use the options used to build it
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to create a new toolchain. That needs a very little bit of effort on your side
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but is quite easy. The options to build a toolchain are saved in the build log
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file that is saved within the toolchain. crosstool-NG can extract those options
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to recreate a new configuration:
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  ct-ng extractconfig </path/to/your/build.log
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will extract those options, prompt you for the new ones, which you can later
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edit with menuconfig.
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Of course, if your build log was compressed, you'd have to use something like:
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  bzcat /path/to/your/build.log.bz2 |ct-ng extractconfig
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________________________
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                       /
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Running crosstool-NG  /
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_____________________/
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To build the toolchain, simply type:
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  ct-ng build
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This will use the above configuration to retrieve, extract and patch the
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components, build, install and eventually test your newly built toolchain.
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You are then free to add the toolchain /bin directory in your PATH to use
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it at will.
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In any case, you can get some terse help. Just type:
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  ct-ng help
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or:
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  man 1 ct-ng
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Stopping and restarting a build |
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-------------------------------*
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If you want to stop the build after a step you are debugging, you can pass the
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variable STOP to make:
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  ct-ng STOP=some_step
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Conversely, if you want to restart a build at a specific step you are
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debugging, you can pass the RESTART variable to make:
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  ct-ng RESTART=some_step
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Alternatively, you can call make with the name of a step to just do that step:
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  ct-ng libc_headers
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is equivalent to:
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  ct-ng RESTART=libs_headers STOP=libc_headers
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The shortcuts +step_name and step_name+ allow to respectively stop or restart
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at that step. Thus:
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  ct-ng +libc_headers        and:    ct-ng libc_headers+
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are equivalent to:
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  ct-ng STOP=libc_headers    and:    ct-ng RESTART=libc_headers
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To obtain the list of acceptable steps, please call:
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  ct-ng liststeps
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Note that in order to restart a build, you'll have to say 'Y' to the config
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option CT_DEBUG_CT_SAVE_STEPS, and that the previous build effectively went
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that far.
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Testing all toolchains at once |
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-------------------------------*
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You can test-build all samples; simply call:
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  ct-ng regtest
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Overriding the number of // jobs |
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---------------------------------*
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If you want to override the number of jobs to run in // (the -j option to
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make), you can either re-enter the menuconfig, or simply add it on the command
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line, as such:
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  ct-ng build.4
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which tells crosstool-NG to override the number of // jobs to 4.
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You can see the actions that support overriding the number of // jobs in
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the help menu. Those are the ones with [.#] after them (eg. build[.#] or
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regtest[.#], and so on...).
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_______________________
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                      /
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Using the toolchain  /
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____________________/
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Using the toolchain is as simple as adding the toolchain's bin directory in
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your PATH, such as:
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  export PATH="${PATH}:/your/toolchain/path/bin"
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and then using the target tuple to tell the build systems to use your
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toolchain:
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  ./configure --target=your-target-tuple
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or
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  make CC=your-target-tuple-gcc
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or
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  make CROSS_COMPILE=your-target-tuple-
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and so on...
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When your root directory is ready, it is still missing some important bits: the
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toolchain's libraries. To populate your root directory with those libs, just
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run:
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  your-target-tuple-populate -s /your/root -d /your/root-populated
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This will copy /your/root into /your/root-populated, and put the needed and only
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the needed libraries there. Thus you don't polute /your/root with any cruft that
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would no longer be needed should you have to remove stuff. /your/root always
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contains only those things you install in it.
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You can then use /your/root-populated to build up your file system image, a
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tarball, or to NFS-mount it from your target, or whatever you need.
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populate accepts the following options:
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 -s [src_dir]
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    Use 'src_dir' as the 'source', un-populated root directory
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 -d [dst_dir]
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    Put the 'destination', populated root directory in 'dst_dir'
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 -f
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    Remove 'dst_dir' if it previously existed
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 -v
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    Be verbose, and tell what's going on (you can see exactly where libs are
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    coming from).
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 -h
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    Print the help
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___________________
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                  /
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Toolchain types  /
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________________/
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There are four kinds of toolchains you could encounter.
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First off, you must understand the following: when it comes to compilers there
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are up to four machines involved:
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  1) the machine configuring the toolchain components: the config machine
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  2) the machine building the toolchain components:    the build machine
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  3) the machine running the toolchain:                the host machine
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  4) the machine the toolchain is generating code for: the target machine
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We can most of the time assume that the config machine and the build machine
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are the same. Most of the time, this will be true. The only time it isn't
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is if you're using distributed compilation (such as distcc). Let's forget
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this for the sake of simplicity.
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So we're left with three machines:
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 - build
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 - host
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 - target
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Any toolchain will involve those three machines. You can be as pretty sure of
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this as "2 and 2 are 4". Here is how they come into play:
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1) build == host == target
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    This is a plain native toolchain, targetting the exact same machine as the
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    one it is built on, and running again on this exact same machine. You have
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    to build such a toolchain when you want to use an updated component, such
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    as a newer gcc for example.
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    crosstool-NG calls it "native".
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2) build == host != target
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    This is a classic cross-toolchain, which is expected to be run on the same
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    machine it is compiled on, and generate code to run on a second machine,
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    the target.
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    crosstool-NG calls it "cross".
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3) build != host == target
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    Such a toolchain is also a native toolchain, as it targets the same machine
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    as it runs on. But it is build on another machine. You want such a
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    toolchain when porting to a new architecture, or if the build machine is
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    much faster than the host machine.
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    crosstool-NG calls it "cross-native".
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4) build != host != target
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    This one is called a canadian-toolchain (*), and is tricky. The three
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    machines in play are different. You might want such a toolchain if you
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    have a fast build machine, but the users will use it on another machine,
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    and will produce code to run on a third machine.
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    crosstool-NG calls it "canadian".
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crosstool-NG can build all these kinds of toolchains (or is aiming at it,
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anyway!)
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(*) The term Canadian Cross came about because at the time that these issues
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    were all being hashed out, Canada had three national political parties.
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    http://en.wikipedia.org/wiki/Cross_compiler
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_____________
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            /
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Internals  /
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__________/
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Internally, crosstool-NG is script-based. To ease usage, the frontend is
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Makefile-based.
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Makefile front-end |
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-------------------*
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The entry point to crosstool-NG is the Makefile script "ct-ng". Calling this
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script with an action will act exactly as if the Makefile was in the current
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working directory and make was called with the action as rule. Thus:
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  ct-ng menuconfig
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is equivalent to having the Makefile in CWD, and calling:
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  make menuconfig
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Having ct-ng as it is avoids copying the Makefile everywhere, and acts as a
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traditional command.
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ct-ng loads sub- Makefiles from the library directory $(CT_LIB_DIR), as set up
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at configuration time with ./configure.
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ct-ng also search for config files, sub-tools, samples, scripts and patches in
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that library directory.
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Because of a stupid make behavior/bug I was unable to track down, implicit make
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rules are disabled: installing with --local would triger those rules, and mconf
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was unbuildable.
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Kconfig parser |
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---------------*
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The kconfig language is a hacked version, vampirised from the toybox project
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by Rob LANDLEY (http://www.landley.net/code/toybox/), itself coming from the
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Linux kernel (http://www.kernel.org/), and (heavily) adapted to my needs.
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The kconfig parsers (conf and mconf) are not installed pre-built, but as
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source files. Thus you can have the directory where crosstool-NG is installed,
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exported (via NFS or whatever) and have clients with different architectures
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use the same crosstool-NG installation, and most notably, the same set of
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patches.
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Architecture-specific |
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----------------------*
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An architecture is defined by:
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 - a human-readable name, in lower case letters, with numbers as appropriate.
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   The underscore is allowed. Eg.: arm, x86_64
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 - a boolean kconfig option named after the architecture (in capital letters
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   if possible) prefixed with "ARCH_". Eg.: ARCH_ARM, ARCH_x86_64
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 - a directory in "arch/" named after the architecture, with the same letters
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   as above. Eg.: arch/arm, arch/x86_64
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   This directory contains:
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   - a configuration file in kconfig syntax, named "config.in", which may be
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     empty. Eg.: arch/arm/config.in
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   - a function script in bash-3.0 syntax, named "functions", which shall
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     follow the API defined below. Eg.: arch/arm/functions
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The "functions" file API:
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 > the function "CT_DoArchValues"
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   + parameters: none
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   + environment:
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      - all variables from the ".config" file,
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      - the two variables "target_endian_eb" and "target_endian_el" which are
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        the endianness suffixes
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   + return value: 0 upon success, !0 upon failure
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   + provides:
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     - mandatory
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     - the environment variable CT_TARGET_ARCH
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     - contains:
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       the architecture part of the target tuple.
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       Eg.: "armeb" for big endian ARM
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            "i386" for an i386
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   + provides:
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     - optional
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     - the environment variable CT_TARGET_SYS
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     - contain:
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       the sytem part of the target tuple.
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       Eg.: "gnu" for glibc on most architectures
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            "gnueabi" for glibc on an ARM EABI
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     - defaults to:
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       - for glibc-based toolchain: "gnu"
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       - for uClibc-based toolchain: "uclibc"
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   + provides:
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     - optional
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     - the environment variable CT_KERNEL_ARCH
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     - contains:
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       the architecture name as understandable by the Linux kernel build
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       system.
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       Eg.: "arm" for an ARM
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            "powerpc" for a PowerPC
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            "i386" for an x86
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     - defaults to:
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       ${CT_ARCH}
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   + provides:
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     - optional
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     - the environment variables to configure the cross-gcc
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       - CT_ARCH_WITH_ARCH
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       - CT_ARCH_WITH_ABI
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       - CT_ARCH_WITH_CPU
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       - CT_ARCH_WITH_TUNE
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       - CT_ARCH_WITH_FPU
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       - CT_ARCH_WITH_FLOAT
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     - contain (defaults):
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       - CT_ARCH_WITH_ARCH    : the gcc ./configure switch to select architecture level         ( "--with-arch=${CT_ARCH_ARCH}"       )
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       - CT_ARCH_WITH_ABI     : the gcc ./configure switch to select ABI level                  ( "--with-abi=${CT_ARCH_ARCH}"        )
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       - CT_ARCH_WITH_CPU     : the gcc ./configure switch to select CPU instruction set        ( "--with-cpu=${CT_ARCH_ARCH}"        )
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       - CT_ARCH_WITH_TUNE    : the gcc ./configure switch to select scheduling                 ( "--with-tune=${CT_ARCH_ARCH}"       )
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       - CT_ARCH_WITH_FPU     : the gcc ./configure switch to select FPU type                   ( "--with-fpu=${CT_ARCH_ARCH}"        )
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       - CT_ARCH_WITH_FLOAT   : the gcc ./configure switch to select floating point arithmetics ( "--with-float=soft" or /empty/      )
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   + provides:
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     - optional
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     - the environment variables to pass to the cross-gcc to build target binaries
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       - CT_ARCH_ARCH_CFLAG
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       - CT_ARCH_ABI_CFLAG
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       - CT_ARCH_CPU_CFLAG
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       - CT_ARCH_TUNE_CFLAG
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       - CT_ARCH_FPU_CFLAG
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       - CT_ARCH_FLOAT_CFLAG
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       - CT_ARCH_ENDIAN_CFLAG
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     - contain (defaults):
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       - CT_ARCH_ARCH_CFLAG   : the gcc switch to select architecture level                     ( "-march=${CT_ARCH_ARCH}"            )
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       - CT_ARCH_ABI_CFLAG    : the gcc switch to select ABI level                              ( "-mabi=${CT_ARCH_AABI}"             )
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       - CT_ARCH_CPU_CFLAG    : the gcc switch to select CPU instruction set                    ( "-mcpu=${CT_ARCH_CPU}"              )
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       - CT_ARCH_TUNE_CFLAG   : the gcc switch to select scheduling                             ( "-mtune=${CT_ARCH_TUNE}"            )
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       - CT_ARCH_FPU_CFLAG    : the gcc switch to select FPU type                               ( "-mfpu=${CT_ARCH_FPU}"              )
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       - CT_ARCH_FLOAT_CFLAG  : the gcc switch to choose floating point arithmetics             ( "-msoft-float" or /empty/           )
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       - CT_ARCH_ENDIAN_CFLAG : the gcc switch to choose big or little endian                   ( "-mbig-endian" or "-mlittle-endian" )
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     - default to:
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       see above.
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Build scripts |
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--------------*
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To Be Written later...