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Date: Fri, 11 Nov 2016 15:00:47 +0100
From: Jiri Olsa <jolsa@redhat.com>
To: Jiri Slaby <jslaby@suse.cz>
Cc: Michal Marek <MMarek@suse.com>, linux-kbuild@vger.kernel.org,
        Linux kernel mailing list <linux-kernel@vger.kernel.org>
Subject: Re: Kbuild: CC= handling broken for tools/
Message-ID: <20161111140047.GA13610@krava>
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On Fri, Nov 11, 2016 at 12:23:25PM +0100, Jiri Slaby wrote:
> Hi,
> 
> I am trying a new gcc with new warnings enabled:
> 
> make O=../a/gcc7/ CC='gcc-7' V=1 kernel/exit.o
> EXTRA_CFLAGS='-Wimplicit-fallthrough=3'
> 
> But the build fails when building under tools/:
> ...
> make -f /home/latest/linux/tools/build/Makefile.build dir=. obj=fixdep
>   gcc
> -Wp,-MD,/home/latest/a/gcc7/tools/objtool/.exec-cmd.o.d,-MT,/home/latest/a/gcc7/tools/objtool/exec-cmd.o
> -Wbad-function-cast -Wdeclaration-after-statement -Wformat-security
> -Wformat-y2k -Winit-self -Wmissing-declarations -Wmissing-prototypes
> -Wnested-externs -Wno-system-headers -Wold-style-definition -Wpacked
> -Wredundant-decls -Wshadow -Wstrict-aliasing=3 -Wstrict-prototypes
> -Wswitch-default -Wswitch-enum -Wundef -Wwrite-strings -Wformat
> -Wimplicit-fallthrough=3 -ggdb3 -Wall -Wextra -std=gnu99 -O6
> -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fPIC -Werror
> -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE
> -I/home/latest/linux/tools/include/ -I/home/latest/linux/include/uapi
> -I/home/latest/linux/include -D"BUILD_STR(s)=#s"   -c -o
> /home/latest/a/gcc7/tools/objtool/exec-cmd.o exec-cmd.c
> gcc: error: unrecognized command line option ‘-Wimplicit-fallthrough=3’;
> did you mean ‘-Wno-fallthrough’?
> 
> 
> Apparently, CC is not respected for tools/ dir.
> 
> Well, it is used (tools/build/Makefile.build):
> # Compile command
> quiet_cmd_cc_o_c = CC       $@
>       cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $<
> 
> 
> But I don't know why CFLAGS propagate from Makefile to tools/, but CC
> doesn't (both are exported)? Any ideas?

so the root Makefile assign CC directly:
  CC              = $(CROSS_COMPILE)gcc

we are trying to be a little bit smarter in perf
and set it only if it's not set by user

# Allow setting CC and AR and LD, or setting CROSS_COMPILE as a prefix.
$(call allow-override,CC,$(CROSS_COMPILE)gcc)
$(call allow-override,AR,$(CROSS_COMPILE)ar)
$(call allow-override,LD,$(CROSS_COMPILE)ld)

I did not check the other tools..

thanks,
jirka