linux-kernel - RE: [PATCH] amdgpu/gmc : fix compile warning

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Message-ID: <BN6PR12MB1809E1A07CD1B5B0F41273EBF7E60@BN6PR12MB1809.namprd12.prod.outlook.com>
Date:   Mon, 8 Oct 2018 15:57:07 +0000
From:   "Deucher, Alexander" <Alexander.Deucher@....com>
To:     Guenter Roeck <linux@...ck-us.net>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        Peng Hao <peng.hao2@....com.cn>
CC:     "airlied@...ux.ie" <airlied@...ux.ie>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "amd-gfx@...ts.freedesktop.org" <amd-gfx@...ts.freedesktop.org>
Subject: RE: [PATCH] amdgpu/gmc : fix compile warning

> -----Original Message-----
> From: Guenter Roeck <groeck7@...il.com> On Behalf Of Guenter Roeck
> Sent: Monday, October 8, 2018 10:11 AM
> To: Koenig, Christian <Christian.Koenig@....com>; Peng Hao
> <peng.hao2@....com.cn>
> Cc: airlied@...ux.ie; linux-kernel@...r.kernel.org; dri-
> devel@...ts.freedesktop.org; amd-gfx@...ts.freedesktop.org; Deucher,
> Alexander <Alexander.Deucher@....com>
> Subject: Re: [PATCH] amdgpu/gmc : fix compile warning
> 
> On 10/08/2018 06:47 AM, Koenig, Christian wrote:
> > Am 08.10.2018 um 15:33 schrieb Guenter Roeck:
> >> On 10/08/2018 01:00 AM, Christian König wrote:
> >>> Am 05.10.2018 um 10:38 schrieb Guenter Roeck:
> >>>> On 10/05/2018 01:14 AM, Koenig, Christian wrote:
> >>>>> Am 04.10.2018 um 20:52 schrieb Guenter Roeck:
> >>>>>> Hi,
> >>>>>>
> >>>>>> On Fri, Sep 14, 2018 at 06:05:52PM +0800, Peng Hao wrote:
> >>>>>>> drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c:
> >>>>>>>        In function ‘gmc_v8_0_process_interrupt’:
> >>>>>>> drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c:1447:10:
> >>>>>>>        warning: missing braces around initializer
> >>>>>>> [-Wmissing-braces]
> >>>>>>>
> >>>>>>> Signed-off-by: Peng Hao <peng.hao2@....com.cn>
> >>>>>> Was there any feedback on this patch ? The problem does affect
> >>>>>> us, and we'll need a fix.
> >>>>>
> >>>>> Well as discussed using "{ { 0 } }" is as wrong as using "{ 0 }".
> >>>>>
> >>>>
> >>>> Ah, sorry, I must have missed the discussion.
> >>>>
> >>>> It is for sure not the best solution, but at least it compiles, and
> >>>> it seems to be proliferating.
> >>>
> >>> Yeah, and exactly that's the problem. As the discussion showed "{ {
> >>> 0 } }" is buggy because it tells the compiler to only initialize the
> >>> first member of the structure, but not all of it.
> >>>
> >>> That is incorrect and rather dangerous cause it can lead to
> >>> unforeseen results and should probably trigger a warning.
> >>>
> >>>>
> >>>> $ git grep "{ *{ *0 *} *}" | wc
> >>>>      144    1180   11802
> >>>> $ git grep "{ *{ *0 *} *}" drivers/gpu/drm/amd/ | wc
> >>>>       50     459    5239
> >>>>
> >>>>> We should either use only "{ }" or even better make nails with
> >>>>> heads and use memset().
> >>>>
> >>>> I'd rather leave it up to the compiler to decide what is most
> >>>> efficient.
> >>>
> >>> And I would rather prefer to have a working driver :)
> >>>
> >>
> >> So { } isn't correct either ?
> >
> > Yes, initializing structures with { } is known to be problematic as well.
> >
> > It doesn't necessary initialize all bytes when you have padding
> > causing random failures when structures are memcmp().
> >
> >>
> >> One thing I found missing in the discussion was the reference to the
> >> C standard.
> >> The C99 standard states in section 6.7.8 (Initialization) clause 19:
> >> "... all
> >> subobjects that are not initialized explicitly shall be initialized
> >> implicitly the same as objects that have static storage duration".
> >> Clause 21 makes further reference to partial initialization,
> >> suggesting the same. Various online resources, including the gcc
> >> documentation, all state the same. I don't find any reference to a
> >> partial initialization which would leave members of a structure
> >> undefined. It would be interesting for me to understand how and why
> >> this does not apply here.
> >>
> >> In this context, it is interesting that the other 48 instances of the
> >> { { 0 } } initialization in the same driver don't raise similar
> >> concerns, nor seemed to have caused any operational problems.
> >
> > Feel free to provide patches to replace those with memset().
> >
> 
> Not me. As I see it, the problem, if it exists, would be a violation of the C
> standard. I don't believe hacking around bad C compilers. I would rather
> blacklist such compilers.
> 
> >>
> >> Anyway, I fixed up the code in our tree (with { }), so I'll leave it
> >> up to you folks to decide what if anything to do about it.
> >
> > Well considering the known problems with {} initialization I'm
> > certainly rejecting all patches which turns memset() into {}.
> >
> 
> Please point me to specific instances of this problem.

I think there are a number of places in DC (drivers/gpu/drm/amd/display) where we applied the original proposed solution before realizing that it would only initialize the first element.  It would be nice to get them fixed up.

Alex