> -----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 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@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 > > > >>>> 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. > > > > 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 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel