From: "Koenig, Christian" <Christian.Koenig@amd.com>
To: Guenter Roeck <linux@roeck-us.net>, Peng Hao <peng.hao2@zte.com.cn>
Cc: "airlied@linux.ie" <airlied@linux.ie>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
"amd-gfx@lists.freedesktop.org" <amd-gfx@lists.freedesktop.org>,
"Deucher, Alexander" <Alexander.Deucher@amd.com>
Subject: Re: [PATCH] amdgpu/gmc : fix compile warning
Date: Mon, 8 Oct 2018 13:47:55 +0000 [thread overview]
Message-ID: <c43669b7-1074-714c-fb22-81685826b3ad@amd.com> (raw)
In-Reply-To: <c72a23ea-efea-4c0e-d397-623d6ac16ebb@roeck-us.net>
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().
>
> 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
next prev parent reply other threads:[~2018-10-08 13:48 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-14 10:05 [PATCH] amdgpu/gmc : fix compile warning Peng Hao
2018-10-04 18:52 ` Guenter Roeck
2018-10-05 8:14 ` Koenig, Christian
2018-10-05 8:38 ` Guenter Roeck
2018-10-08 8:00 ` Christian König
2018-10-08 13:33 ` Guenter Roeck
2018-10-08 13:47 ` Koenig, Christian [this message]
2018-10-08 14:10 ` Guenter Roeck
2018-10-08 15:57 ` Deucher, Alexander
2018-10-08 17:22 ` Koenig, Christian
2018-10-08 17:46 ` Guenter Roeck
2018-10-08 18:13 ` Koenig, Christian
2018-10-19 8:53 ` Daniel Vetter
2018-10-19 8:56 ` Daniel Vetter
2018-10-19 13:08 ` Guenter Roeck
2018-10-19 15:30 ` Alex Deucher
2018-10-08 17:41 ` Guenter Roeck
2018-10-08 18:24 ` Deucher, Alexander
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