Re: [PATCH 0/2] infer CROSS_COMPILE from ARCH for LLVM=1 LLVM_IAS=1

[Arnd Bergmann <arnd@kernel.org>]

On Thu, Jul 8, 2021 at 8:04 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:

> > /usr/bin/powerpc64-linux-gnu-gcc-5.2.0
> > /usr/bin/powerpc64-linux-gnu-gcc -> powerpc64-linux-gnu-gcc-5.2.0
> > /usr/local/bin/ppc64le-linux-gcc-9
> > ~/bin/powerpc/powerpc-linux-unknown-gcc-12.0.20210708.experimental
> >
> > all of these should be able to cross-build any powerpc kernel, but
> > there is no obvious first choice (highest version, first in path,
> > ordered list of target triples, ...). I tried coming up with a heuristic
> > to pick a reasonable toolchain, but at some point gave up because
> > I failed to express that in a readable bash or Makefile syntax.
>
> Right; foremost in my mind was arm-linux-gnueabi-gcc vs
> arm-linux-gnueabihf-gcc.  That's not even to mention the versioned
> suffixes.
>
> In terms of multiversion support; this series doesn't regress doing
> things the hard/verbose way.  But I think for most users we can have a
> simpler common case; folks can play with their $PATH or focus on more
> hermetic builds if they want this new feature (CROSS_COMPILE
> inference) AND support for multiple versions of the same toolchain.

Fair enough. So how something like this:

powerpc-targets := powerpc32 powerpc64 powerpc32le \
        powerpc32be powerpc64le powerpc64be ppc64le ppc64be
arm-targets := arm-linux-gnueabi arm-linux-gnueabihf
x86-targets := x86_64 i386 i686
x86_64-targets := x86
i386-targets := i686 x86 x86_64
parisc-targets := hppa64 hppa
...

CROSS_COMPILE ?= `find-toolchain $(ARCH) $($(ARCH)-targets)`

where find-toolchain just finds the first working toolchain based, looking
for $(target)-linux-gcc $(target)-gcc $(target)-unknown-linux-gcc etc
in $(PATH) but ignoring the versions?

What I had actually planned was a set of helpers that allow you to
do this in multiple steps:

- if $(objtree)/scripts/cross/bin/gcc (or something else we pick)
  exists and CROSS_COMPILE is not set, set CROSS_COMPILE
  to $(objtree)/scripts/cross/bin/ in the Makefile
- add script to enumerate the installed toolchains
- add a second script to symlink one of those toolchains to
  $(objtree)/scripts/cross/bin
- add a third script to download a cross-toolchain from kernel.org
  for $(ARCH) and install it to one of the locations that the first
  script looks for (/opt/cross/, $(HOME)/cross/, $(objtree)scripts/cross/)

        Arnd