linux-kernel - 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

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Date:   Tue, 18 Apr 2017 15:19:42 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     kbuild test robot <fengguang.wu@...el.com>, kbuild-all@...org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.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

On Tue, Apr 18, 2017 at 11:52:41AM -0700, Andi Kleen wrote:
> Josh Poimboeuf <jpoimboe@...hat.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.
Any ideas?

BTW, I think it will only break randconfigs for old versions of gcc,
before 4.6.0.  The error message isn't quite right.  I just noticed the
FUNCTION_GRAPH_TRACER config has:

  depends on !X86_32 || !CC_OPTIMIZE_FOR_SIZE

which means that X86_32 and CC_OPTIMIZE_FOR_SIZE can never be set with
FUNCTION_GRAPH_TRACER.  So the error message could be simplified to:

  #error The following combination is not supported: (compiler missing -mfentry) && CONFIG_FUNCTION_GRAPH_TRACER && CONFIG_CC_OPTIMIZE_FOR_SIZE

-- 
Josh