linux-kernel - Re: [PATCH v3 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for CC=clang LLVM

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Message-ID: <CAKwvOdm0xs4ikb0K0_b8Az0T=Kxu_-6AHjWHOhjsKZb3hTrH2A@mail.gmail.com>
Date:   Thu, 29 Jul 2021 17:19:03 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...nel.org>, linux-kernel@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for
 CC=clang LLVM_IAS=1

On Thu, Jul 29, 2021 at 2:00 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> While I understand that the LLVM=1 LLVM_IAS=1 case works perfectly fine
> with this series, I am of the belief that making it work for CC=clang
> LLVM_IAS=1 is a mistake because there is no way for that configuration
> to work for cross compiling without CROSS_COMPILE.

So with v3 of this change, rather than:

$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang -j72

If you wanted to omit CROSS_COMPILE, you'd need:

$ ARCH=arm64 make CC=clang LLVM_IAS=1 LD=ld.lld OBJCOPY=llvm-objcopy
STRIP=llvm-strip

or

$ ARCH=arm64 make CC=clang LLVM_IAS=1 LD=aarch64-linux-gnu-ld
OBJCOPY=aarch64-linux-gnu-objcopy STRIP=aarch64-linux-gnu-strip

That's straight up worse IMO and defeats the purpose of "shortening
the command line," which should be the goal.  Not "making CC=clang
maximally flexible."  We don't want folks generally using CC=clang;
preferably they'd use LLVM=1.  I need to rewrite our docs to make that
more explicit and straightforward.  And if folks would prefer to use
CC=clang for whatever reason, let them explicitly state CROSS_COMPILE
then.

So I agree with Nathan, and hope Masahiro will reconsider that perhaps
the v2 variant that required LLVM=1 maybe makes more sense.

Either way, I need to fix the comment in the new script, commit
message, and docs, so v4 is necessary.

I'm tempted to add a rewrite of our docs to say "just use LLVM=1"
front and center, then get into finer grain details below, moving this
second patch to be the third in a series.  Let's see what Masahiro's
thoughts are though first. (I do appreciate them, even when I
disagree).
-- 
Thanks,
~Nick Desaulniers