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Date: Tue, 18 Apr 2017 13:25:19 -0700
From: Andi Kleen <andi@firstfloor.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>,
        kbuild test robot <fengguang.wu@intel.com>, kbuild-all@01.org,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: arch/x86//kernel/ftrace.c:35:3: error: #error The following
 combination is not supported: ((compiler missing -mfentry) || (CONFIG_X86_32
 and !CONFIG_DYNAMIC_FTRACE)) && CONFIG_FUNCTION_GRAPH_TRACER &&
 CONFIG_CC_OPTIMIZE_FOR_SIZE
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On Tue, Apr 18, 2017 at 03:19:42PM -0500, Josh Poimboeuf wrote:
> On Tue, Apr 18, 2017 at 11:52:41AM -0700, Andi Kleen wrote:
> > Josh Poimboeuf <jpoimboe@redhat.com> writes:
> > >
> > > The error is working as designed.  gcc < 4.6.0 doesn't have -mfentry, so
> > > it fails the above check on x86.  Can you add a skip rule?  It should
> > > skip building the following case:
> > >
> > >   x86 && ((gcc < 4.6.0) || (CONFIG_X86_32 and !CONFIG_DYNAMIC_FTRACE))
> > > && CONFIG_FUNCTION_GRAPH_TRACER && CONFIG_CC_OPTIMIZE_FOR_SIZE
> > 
> > This will also break everyone's randconfig builds. The better way
> > would be to check this in the Makefile and disable one of the options
> 
> I don't know how to disable a config option from the Makefile like that.

You just don't do something in the Makefile, e.g. don't set the -Os 

For an example see the LTO Makefile here

https://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc.git/tree/scripts/Makefile.lto?h=lto-411-1


-Andi