RE: [PATCH] amdgpu/gmc : fix compile warning

[Deucher, Alexander]

> -----Original Message-----
> From: Guenter Roeck <groeck7@xxxxxxxxx> On Behalf Of Guenter Roeck
> Sent: Monday, October 8, 2018 1:41 PM
> To: Deucher, Alexander <Alexander.Deucher@xxxxxxx>
> Cc: Koenig, Christian <Christian.Koenig@xxxxxxx>; Peng Hao
> <peng.hao2@xxxxxxxxxx>; airlied@xxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> dri-devel@xxxxxxxxxxxxxxxxxxxxx; amd-gfx@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH] amdgpu/gmc : fix compile warning
> 
> On Mon, Oct 08, 2018 at 03:57:07PM +0000, Deucher, Alexander wrote:
> > > -----Original Message-----
> > > From: Guenter Roeck <groeck7@xxxxxxxxx> On Behalf Of Guenter
> Roeck
> > > Sent: Monday, October 8, 2018 10:11 AM
> > > To: Koenig, Christian <Christian.Koenig@xxxxxxx>; Peng Hao
> > > <peng.hao2@xxxxxxxxxx>
> > > Cc: airlied@xxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; dri-
> > > devel@xxxxxxxxxxxxxxxxxxxxx; amd-gfx@xxxxxxxxxxxxxxxxxxxxx; Deucher,
> > > Alexander <Alexander.Deucher@xxxxxxx>
> > > 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 KÃnig 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:
> > > >>>>>>>  ÂÂÂÂÂ In function âgmc_v8_0_process_interruptâ:
> > > >>>>>>> drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c:1447:10:
> > > >>>>>>>  ÂÂÂÂÂ warning: missing braces around initializer
> > > >>>>>>> [-Wmissing-braces]
> > > >>>>>>>
> > > >>>>>>> Signed-off-by: Peng Hao <peng.hao2@xxxxxxxxxx>
> > > >>>>>> 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
> > > >>>>  ÂÂÂ 144ÂÂÂ 1180ÂÂ 11802
> > > >>>> $ git grep "{ *{ *0 *} *}" drivers/gpu/drm/amd/ | wc
> > > >>>>  ÂÂÂÂ 50ÂÂÂÂ 459ÂÂÂ 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.
> >
> 
> I think this is factually incorrect. What you might want to try to say is that
> padding may not be initialized when using anything but memset().
> But that is a different problem.
>

I just meant that there are a number of places were warning fix patches got applied that did the same thing this patch attempted to do ( replace { 0 } with { { 0 } }) which may have introduced subtle issues.

Alex