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

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Date:   Mon, 8 Oct 2018 07:10:37 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "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>,
        "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.

Thanks,
Guenter