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

From: Arnd Bergmann
Date: Thu Jul 08 2021 - 03:27:57 EST

On Thu, Jul 8, 2021 at 7:49 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> On Wed, Jul 07, 2021 at 03:43:08PM -0700, Nick Desaulniers wrote:
> > We get constant feedback that the command line invocation of make is too
> > long. CROSS_COMPILE is helpful when a toolchain has a prefix of the
> > target triple, or is an absolute path outside of $PATH, but it's mostly
> > redundant for a given ARCH.
> >
> > Instead, let's infer it from ARCH, and move some flag handling into a
> > new file included from the top level Makefile.
> Why only for LLVM? I really hate the mess we currently have with
> ARCH and CROSS_COMPILE. Being able to set both in .config (and maybe
> even inferring CROSS_COMPILE where possible) would make my life so
> much easier.

I agree this would be best, but solving the problem for llvm is
trivial with a 1:1
mapping between ARCH= and --target= strings. Doing the same for gcc
requires enumerating all possible target triples, and possibly deciding between
different versions, e.g. when your path contains

/usr/bin/powerpc64-linux-gnu-gcc -> powerpc64-linux-gnu-gcc-5.2.0

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.