linux-kernel - Re: [PATCH] Kbuild: use cc-disable-warning consistently for maybe-uninitialized

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Date:   Wed, 15 Mar 2017 01:17:21 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Michal Marek <mmarek@...e.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Fengguang Wu <fengguang.wu@...el.com>, kbuild-all@...org,
        stable@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Kbuild: use cc-disable-warning consistently for maybe-uninitialized

Hi Arnd,

2017-01-14 0:40 GMT+09:00 Arnd Bergmann <arnd@...db.de>:
> In commit a76bcf557ef4 ("Kbuild: enable -Wmaybe-uninitialized warning
> for "make W=1""), I reverted another change that happened to fix a problem
> with old compilers, and now we get this report again with old compilers
> (prior to gcc-4.8) and GCOV enabled:
>
>    cc1: warnings being treated as errors
>    drivers/gpu/drm/i915/intel_ringbuffer.c: In function 'intel_ring_setup_status_page':
>    drivers/gpu/drm/i915/intel_ringbuffer.c:438: error: 'mmio.reg' may be used uninitialized in this function
>    At top level:
>>> cc1: error: unrecognized command line option "-Wno-maybe-uninitialized"
>
> The problem is that we turn off the warning conditionally in a number
> of places as we should, but one of them does it unconditionally.
> Instead, change it to call cc-disable-warning as we do elsewhere.
>
> The original patch that caused it was merged into linux-4.7, then
> 4.8 removed the change and 4.9 brought it back, so we probably want
> a backport to 4.9 once this is merged.
>
> Cc: stable@...r.kernel.org
> Fixes: a76bcf557ef4 ("Kbuild: enable -Wmaybe-uninitialized warning for "make W=1"")
> Fixes: e72e2dfe7c16 ("gcov: disable -Wmaybe-uninitialized warning")
> Reported-by: kbuild test robot <fengguang.wu@...el.com>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>


Applied to linux-kbuild/fixes.


But, please let me ask one question.

When I was checking your commit a76bcf557ef4,
I noticed the following code:


KBUILD_CFLAGS += $(call cc-ifversion, -lt, 0409, \
                       $(call cc-disable-warning,maybe-uninitialized,))



It was not clear to me where this comes from
because it is not the revert of Linus' commit.


My best guess is:
maybe-uninitialized was introduced by gcc 4.7,
but only reliable for gcc 4.9 or later.
So it should be disabled for gcc < 4.9


Is this correct?


Thanks!

-- 
Best Regards
Masahiro Yamada