Messages in this thread | | | Subject | Re: [PATCH] amdgpu/gmc : fix compile warning | From | Guenter Roeck <> | Date | Mon, 8 Oct 2018 07:10:37 -0700 |
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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@zte.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.
Thanks, Guenter
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