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

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Message-ID: <c43669b7-1074-714c-fb22-81685826b3ad@amd.com>
Date:   Mon, 8 Oct 2018 13:47:55 +0000
From:   "Koenig, Christian" <Christian.Koenig@....com>
To:     Guenter Roeck <linux@...ck-us.net>, 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

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().

>
> 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 {}.

Regards,
Christian.

>
> Thanks,
> Guenter