On Thu, Jul 8, 2021 at 12:27 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote: > > 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. I agree. > > 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 Yes. There is a seldom used Kbuild macro for this: cc-cross-prefix. It's hard coded to check for prefixed versions of gcc, but it will handle the enumeration. Perhaps it could be used more widely, once we know whether we're building with gcc or clang. But as you note, enumeration is kind of a waste of time for clang as these target triples are known ahead of time. I guess we could check that your build of clang was configured with support for that target ARCH (since you can disable backends when building clang), but I consider disabling backends an antipattern, and I think the user will figure out pretty quickly when they're trying to build a target that LLVM doesn't support (whether due to configured-off or unimplemented target). > /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. -- Thanks, ~Nick Desaulniers
