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

[Guenter Roeck]

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.

Thanks,
Guenter