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

[Nick Desaulniers <ndesaulniers@google.com>]

On Fri, Jul 9, 2021 at 1:07 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> 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?

Sure, debian doesn't even package different versions of the cross GCC
packages AFAIK; no idea about other distros.  Though the user may have
built from source, or have multiple versions fetched from tarballs.

I think we can simplify the common case of "I just wan't to cross
compile, I don't necessarily care about an older compiler version
co-installed with a newer one." ("and if I did, I could still use
CROSS_COMPILE the verbose way").

> 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

(and check the symlink isn't broken should the user uninstall a
toolchain, or have their distro update their toolchain version)

> - 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/)

Would the user be prompted for the download? So during
`defconfig`/configuration we could prompt and say "it looks like
you're cross compiling without setting CROSS_COMPILE, would you like
me to fetch a cross compiler for you?"

Seems reasonable, when cross compiling with GCC.
-- 
Thanks,
~Nick Desaulniers