linux-kernel - Re: [PATCH kernel] vfio-pci/nvlink2: Fix ancient gcc warnings

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Message-ID: <878szbx98q.fsf@concordia.ellerman.id.au>
Date:   Thu, 24 Jan 2019 10:08:37 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     Alexey Kardashevskiy <aik@...abs.ru>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        KVM list <kvm@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH kernel] vfio-pci/nvlink2: Fix ancient gcc warnings

Geert Uytterhoeven <geert@...ux-m68k.org> writes:
> Hi Alex,
>
> On Wed, Jan 23, 2019 at 5:30 AM Alex Williamson
> <alex.williamson@...hat.com> wrote:
>> The below patch comes about from the build regressions and improvements
>> list you've sent out, but something doesn't add up that we'd be testing
>> with an old compiler where initialization with { 0 } generates a
>> "missing braces around initialization" warning.  Is this really the
>> case or are we missing something here?  There's no harm that I can see
>> with Alexey's fix, but are these really just false positives from a
>> compiler bug that we should selectively ignore if the "fix" is less
>> clean?  Thanks,
>
> Yes, they are false positives, AFAIK.
>
>> On Wed, 23 Jan 2019 15:07:11 +1100
>> Alexey Kardashevskiy <aik@...abs.ru> wrote:
>>
>> > Using the {0} construct as a generic initializer is perfectly fine in C,
>> > however due to a bug in old gcc there is a warning:
>> >
>> >   + /kisskb/src/drivers/vfio/pci/vfio_pci_nvlink2.c: warning: (near
>> > initialization for 'cap.header') [-Wmissing-braces]:  => 181:9
>
> These all seem to come from an old gcc 4.6, which is the oldest still
> supported version for compiling Linux
> http://kisskb.ellerman.id.au/kisskb/buildresult/13663641/
>
> Note that kisskb is also using gcc 4.6.3 for s390x and mips, which are the only
> other builds showing missing braces warnings.

As documented here:

  https://www.kernel.org/doc/html/latest/process/changes.html#current-minimal-requirements

x86 has effectively dropped support for 4.6 because it doesn't support
retpoline and CONFIG_RETPOLINE is default y.

So it might be time to stop supporting 4.6, but I'd rather that happened
by someone sending a patch to change the requirements doc above, and
then kisskb can stop doing the builds with 4.6.

cheers