Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for LLVM=1 LLVM_IAS=1

[Arnd Bergmann]

On Tue, Jul 20, 2021 at 7:43 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Jul 20, 2021 at 1:05 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>
> We do most of the other heavy lifting in this area in Kconfig anyway,
> why not add that compiler choice?
>
> Obviously it would be gated by the tests to see which compilers are
> _installed_ (and that they are valid versions), so that it doesn't ask
> stupid things ("do you want gcc or clang" when only one of them is
> installed and/or viable).

I don't see a good way of making Kconfig options both select the
compiler and defining variables based on the compiler, since that
would mean teaching Kconfig about re-evaluating all compiler
dependent settings whenever the first option changes.

I do have another idea that I think would work though.

> Hmm? So then any "LLVM=1" thing would be about the "make config"
> stage, not the actual build stage.
>
> (It has annoyed me for years that if you want to cross-compile, you
> first have to do "make ARCH=xyz config" and then remember to do "make
> ARCH=xyz" for the build too, but I cross-compile so seldom that I've
> never really cared).

The best thing that I have come up with is a pre-configure step, where
an object tree gets seeded with a makefile fragment that gets included
for any 'make' invocation. This would set 'ARCH=', 'CROSS_COMPILE',
'CC=' and possibly any other option that gets passed to 'make' as
a variable and has to exist before calling 'make *config'.

This is probably the easiest part here, and it lets you set up multiple
object directories in which you can then do

make O=obj-x86 defconfig vmlinux modules -skj32
make O=obj-x86-clang
make O=obj-arm64 randconfig
...

without ever having to type the additional CC/CROSS_COMPILE/LLVM
variables again.

One step further is the script that automatically finds a working toolchain
for a given architecture and sets up that object directory accordingly,
or even downloads a working cross-toolchain from kernel.org when
asked to do that.

        Arnd