On Fri, Jul 30, 2021 at 6:00 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote: > > I realized that the title of this commit is not really right. We are not > inferring CROSS_COMPILE, we are inferring '--target='. > > On 7/29/2021 9:50 AM, 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 SRCARCH. SRCARCH itself is derived from ARCH > > I feel like the beginning of this needs a little work. > > 1. "...invocation of make is too long when compiling with LLVM" would be > a little more accurate. > > 2. "it's mostly redundant for a given SRCARCH" is not quite true in my > eyes. For example, you could have aarch64-linux-, aarch64-elf-, or > aarch64-linux-gnu-, and to my knowledge, all of these can compile a > working Linux kernel. Again, saying "with LLVM", even mentioning its > multitargeted nature, might make it a little more accurate to the casual > passerby. > > > (normalized for a few different targets). > > > > If CROSS_COMPILE is not set, simply set --target= for CLANG_FLAGS, > > KBUILD_CFLAGS, and KBUILD_AFLAGS based on $SRCARCH. > > > > Previously, we'd cross compile via: > > $ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make LLVM=1 LLVM_IAS=1 > > Now: > > $ ARCH=arm64 make LLVM=1 LLVM_IAS=1 > > > > For native builds (not involving cross compilation) we now explicitly > > specify a target triple rather than rely on the implicit host triple. > > > > Link: https://github.com/ClangBuiltLinux/linux/issues/1399 > > Suggested-by: Arnd Bergmann <arnd@xxxxxxxxxx> > > Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx> > > Suggested-by: Masahiro Yamada <masahiroy@xxxxxxxxxx> > > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > > Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx> > Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx> > > > --- > > Changes v2 -> v3: > > * Drop check/requirement for LLVM=1, as per Masahiro. > > * Change oneliner from LLVM=1 LLVM_IAS=1 to CC=clang LLVM_IAS=1. > > * Don't carry forward Nathan's RB/TB tags. :( Sorry Nathan, but thank > > you for testing+reviewing v2. > > * Update wording of docs slightly. > > > > Changes v1 -> v2: > > * Fix typos in commit message as per Geert and Masahiro. > > * Use SRCARCH instead of ARCH, simplifying x86 handling, as per > > Masahiro. Add his sugguested by tag. > > * change commit oneline from 'drop' to 'infer.' > > * Add detail about explicit host --target and relationship of ARCH to > > SRCARCH, as per Masahiro. > > > > Changes RFC -> v1: > > * Rebase onto linux-kbuild/for-next > > * Keep full target triples since missing the gnueabi suffix messes up > > 32b ARM. Drop Fangrui's sugguested by tag. Update commit message to > > drop references to arm64. > > * Flush out TODOS. > > * Add note about -EL/-EB, -m32/-m64. > > * Add note to Documentation/. > > > > Documentation/kbuild/llvm.rst | 6 ++++++ > > scripts/Makefile.clang | 32 ++++++++++++++++++++++++++++++-- > > 2 files changed, 36 insertions(+), 2 deletions(-) > > > > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst > > index b18401d2ba82..aef1587fc09b 100644 > > --- a/Documentation/kbuild/llvm.rst > > +++ b/Documentation/kbuild/llvm.rst > > @@ -46,6 +46,12 @@ example: :: > > > > clang --target=aarch64-linux-gnu foo.c > > > > +When both ``CC=clang`` (set via ``LLVM=1``) and ``LLVM_IAS=1`` are used, > > +``CROSS_COMPILE`` becomes unnecessary and can be inferred from ``ARCH``. > > I am not a fan of this sentence because it implies that something like > 'make ARCH=arm64 CC=clang LLVM_IAS=1' will work fine, which is not true. > We still need CROSS_COMPILE for binutils in this configuration. Agree. That sentence is misleading, and moreover, it is wrong. > > CROSS_COMPILE provides the value for '--target=' and the prefix for the > GNU tools such as ld, objcopy, and readelf. I think this direction is a > regression because we are just talking about the first use of > CROSS_COMPILE rather than the second at the same time. > > With LLVM=1 LLVM_IAS=1, we KNOW that the user will be using all LLVM > tools. Sure, they are free to override LD, OBJCOPY, READELF, etc with > the GNU variants but they have to provide the prefix because LLVM=1 > overrides the $(CROSS_COMPILE)<tool> assignments so it is irrelevant to > us. As Masahiro mentioned, the user is free to individually specify all > the tools by their individual variables such as LD=ld.lld BUT at that > point, the user should be aware of what they are doing and specify > CROSS_COMPILE. > > 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. LLVM=1 is a too strong requirement. LLVM=1 switches not only target tools (CC=clang, LD=ld.lld, AR=llvm-ar...) but also host tools (HOSTCC=clang, HOSTCXX=g++...). Obviously host-tools are don't-care here. Specifying the target tools individually, as in make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \ OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf ... is a perfectly correct command that makes CROSS_COMPILE unnecessary. There is no reason to stop inferring --target for this case. The problem is NOT removing the LLVM=1 check but the wrong documentation. Let's write a precise document. For example, the following document exactly explains what is happening in the code, and is still clear. diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst index b18401d2ba82..a0d862bd73ac 100644 --- a/Documentation/kbuild/llvm.rst +++ b/Documentation/kbuild/llvm.rst @@ -63,6 +63,26 @@ They can be enabled individually. The full list of the parameters: :: Currently, the integrated assembler is disabled by default. You can pass ``LLVM_IAS=1`` to enable it. + +Omitting CROSS_COMPILE +---------------------- + +As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``. + +Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive +``--prefix=<path>`` to search for the back-end GNU assembler. + +If CROSS_COMPILE is not specified, the ``--target=<triple>`` is inferred from +``ARCH``. + +It means, if you use only LLVM tools, `CROSS_COMPILE`` becomes unnecessary. + +For example, to cross-compile the arm64 kernel:: + + ARCH=arm64 make LLVM=1 LLVM_IAS=1 + + Supported Architectures ----------------------- BTW, I noticed LLVM_IAS=1 check is also unneeded for the same reason. So, it should go away. -- Best Regards Masahiro Yamada
