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Subject: Re: [PATCH] arm64: drop CROSS_COMPILE for LLVM=1 LLVM_IAS=1
To:     Nick Desaulniers <ndesaulniers@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Miguel Ojeda <ojeda@kernel.org>
Cc:     Arnd Bergmann <arnd@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Fangrui Song <maskray@google.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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From:   Nathan Chancellor <nathan@kernel.org>
Message-ID: <546f3d11-8580-7f23-875d-ef856913cb6d@kernel.org>
Date:   Wed, 7 Jul 2021 12:08:24 -0700
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On 7/7/2021 12:04 PM, Nick Desaulniers wrote:
> On Sat, Jul 3, 2021 at 5:47 PM Nathan Chancellor <nathan@kernel.org> 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@kernel.org> wrote:
>>>>
>>>> On Fri, Jul 2, 2021 at 1:55 AM 'Nick Desaulniers' via Clang Built
>>>> Linux <clang-built-linux@googlegroups.com> 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