[Buildroot] Using a precompiled busybox toolchain as an external toolchain

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Dear Willy Lambert,

On Tue, 29 Jan 2013 16:57:56 +0100, Willy Lambert wrote:

> > As I have seen some chat about buildroot toolchain and these problem
> > of relocability, a question arise :
> >
> > Is buildroot toolchain supposed to be maintained in the long run ? Or
> > do you expect people switching more and more on external toolchains ?

Let's make a status of the available toolchain backends:

 * Buildroot internal toolchain backend. This is the historical
   backend. Everything is done by Makefile directly inside Buildroot:
   building binutils, gcc, uClibc, etc. It is limited to uClibc. There
   are discussions since a long time on phasing out this backend in
   favor of the Crosstool-NG backend, but this hasn't happened yet, as
   there are remaining issues to solve with the Crosstool-NG backend. I
   am not sure when/if the internal backend will really go away one day.

 * External toolchain backend. It allows to re-use existing, pre-built,
   toolchains. Those toolchains could have been generated by
   Crosstool-NG, Buildroot, manually, or be provided by a hardware/SoC
   manufacturer. Buildroot really doesn't care here, it simply uses the
   pre-built toolchain as is, and it doesn't build the toolchain.

 * Crosstool-NG backend. In this case, Buildroot builds the toolchain,
   but instead of doing it with internal Makefiles (as done by the
   internal backend described above), it downloads Crosstool-NG,
   installs it, generates a configuration file for it, and triggers the
   build. Once the Crosstool-NG build is done, we are back to the normal
   case where a toolchain is now available, and Buildroot can continue
   its work.

> I will try crosstool-ng. Is this toolchain relocable  ? so I can build
> it first and then take it out of buildroot ?

If you use Crosstool-NG outside of Buildroot, then the toolchain
produced by Crosstool-NG itself is relocatable. This is a raw
toolchain: it has only the C library, C library headers and kernel
headers. No additional libraries.

Then, you can use this pre-build toolchain as an external toolchain in
Buildroot. Buildroot will build additional libraries, and will install
them and their headers in a copy of the toolchain sysroot. In some
sense, the $(O)/host directory of Buildroot becomes a new toolchain,
which contains your pre-built toolchain, and additional libraries and
headers. And this new toolchain (in $(O)/host) is *NOT* relocatable at
the moment, due to various issues: hardcoded paths in .la files, in
<foobar>-config scripts, in the RPATH of the binaries installed in
$(O)/host/usr/bin/, etc.

This is something we intend to fix, but that hasn't happened yet.

Does that clarify the situation?

Thomas
-- 
Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com