linux-kernel - Re: Kbuild: CC= handling broken for tools/

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Date:   Fri, 11 Nov 2016 15:04:35 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Michal Marek <MMarek@...e.com>, linux-kbuild@...r.kernel.org,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: Kbuild: CC= handling broken for tools/

On 11/11/2016, 03:00 PM, Jiri Olsa wrote:
> 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..

Note that I don't care about tools at all. objtool (see the dump above)
is apparently needed to build the kernel proper, so it is built as a
part of kernel.

BTW shouldn't it use HOSTCC anyway?

thanks,
-- 
js
suse labs