Re: [PATCH] arm64: drop CROSS_COMPILE for LLVM=1 LLVM_IAS=1

[Nathan Chancellor]

On 7/7/2021 12:04 PM, Nick Desaulniers wrote:
> On Sat, Jul 3, 2021 at 5:47 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
> > On Fri, Jul 02, 2021 at 11:29:31AM -0700, Nick Desaulniers wrote:
> > > On Fri, Jul 2, 2021 at 4:59 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> > > > On Fri, Jul 2, 2021 at 1:55 AM 'Nick Desaulniers' via Clang Built
> > > > Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> > > > > +ifneq ($(LLVM),)
> > > > > +ifneq ($(LLVM_IAS),)
> > > > > +ifeq ($(CROSS_COMPILE),)
> > > > > +CLANG_TARGET   :=--target=aarch64-linux
> > > > > +CLANG_FLAGS    += $(CLANG_TARGET)
> > > > > +KBUILD_CFLAGS  += $(CLANG_TARGET)
> > > > > +KBUILD_AFLAGS  += $(CLANG_TARGET)
> > > > > +endif
> > > > > +endif
> > > > > +endif
> > > >
> > > > I think only the "CLANG_TARGET   :=--target=aarch64-linux" line should
> > > > go into the
> > > > per-architecture Makefile. It doesn't hurt to just set that
> > > > unconditionally here,
> > > > and then change the CLANG_FLAGS logic in the top-level Makefile to use this
> > > > in place of $(notdir $(CROSS_COMPILE:%-=%)).
> > >
> > > I don't think we can do that. Based on the order the arch/ specific
> > > Makefiles are included, if we don't eagerly add --target to the
> > > KBUILD_{C|A}FLAGS, then cc-option, as-option, and as-instr macros
> > > (defined in scripts/Makefile.compiler) checks in per arch/ Makefiles
> > > may fail erroneously because --target was not set for
> > > KBUILD_{C|A}FLAGS yet.
> > >
> > > Another issue is the order of operations between the top level
> > > Makefile and the per arch/ Makefiles.  The `notdir` block you
> > > reference occurs earlier than the per-arch includes:
> > >
> > >   609 TENTATIVE_CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
> > > ...
> > >   648 include $(srctree)/arch/$(SRCARCH)/Makefile
> > >
> > > We would need the opposite order to do what you describe. Reordering
> > > these would effectively be a revert of
> > > commit ae6b289a3789 ("kbuild: Set KBUILD_CFLAGS before incl. arch Makefile")
> > > which I'm not sure we want to do.  But maybe there's another way I'm
> > > not seeing yet?
> >
> > Is there any reason we cannot just add this sort of logic to the main
> > Makefile?
> >
> > Such as (indentation to emphasis diff):
> >
> > ifeq ($(CROSS_COMPILE),)
> > ifneq ($(LLVM),)
> > ifeq ($(LLVM_IAS),1)
> >          ifeq ($(ARCH),arm64)
> >                  TENTATIVE_CLANG_FLAGS   += --target=aarch64-linux
> >          else ifeq ($(ARCH),s390)
> >                  TENTATIVE_CLANG_FLAGS   += --target=s390x-linux
> >          else ifeq ($(ARCH),x86_64)
> >                  TENTATIVE_CLANG_FLAGS   += --target=x86_64-linux
> >          else
> >                  $(error Specify CROSS_COMPILE or add '--target=' option to Makefile)
> >          endif
> > endif
> > endif
> > else
> > TENTATIVE_CLANG_FLAGS   += --target=$(notdir $(CROSS_COMPILE:%-=%))
> > ifeq ($(LLVM_IAS),1)
> > TENTATIVE_CLANG_FLAGS   += -integrated-as
> > else
> > TENTATIVE_CLANG_FLAGS   += -no-integrated-as
> > GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
> > TENTATIVE_CLANG_FLAGS   += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
> > endif
> > endif
> >
> > I know this looks a little cumbersome but it does help us avoid
> > duplication across architecture Makefiles and ordering dependencies.
>
> Yeah, ok.
>
> I like the use of `include` to compartmentalize the top level Makefile
> further.  We can move this whole block of LLVM related flag handling
> into something under scripts, then add this block and it doesn't look
> too bad IMO.  Masahiro, are you ok with that?  If so, I'd break this
> into 2 patches:
> 1. moving this block of existing code into a new file.
> 2. adding the CROSS_COMPILE functionality.
>
> See https://groups.google.com/g/clang-built-linux/c/s-voh6WQFxM for
> the gist of what I was thinking (though not broken into 2 patches yet,
> just testing that it works; it does).

Yeah, I think that looks okay. Not sure how I feel about the name since 
it is handling more than just the target triple but that is a bikeshed 
for another time :)

> This approach will collide with Miguel's series in -next.  Should I
> base the patches on mainline, or linux-kbuild, then have Miguel rebase
> his patches on that or what?

Yes, the patches should be based on mainline or linux-kbuild then Miguel 
will have to solve the conflicts and let Stephen Rothwell know about 
them so that -next keeps working.

Cheers,
Nathan