Re: [PATCH] amdgpu/gmc : fix compile warning

From: "Koenig, Christian" <Christian.Koenig@amd.com>
To: "Deucher, Alexander" <Alexander.Deucher@amd.com>,
	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>
Subject: Re: [PATCH] amdgpu/gmc : fix compile warning
Date: Mon, 8 Oct 2018 17:22:24 +0000	[thread overview]
Message-ID: <c6880c5b-e259-4923-8d5a-80c620563398@amd.com> (raw)
In-Reply-To: <BN6PR12MB1809E1A07CD1B5B0F41273EBF7E60@BN6PR12MB1809.namprd12.prod.outlook.com>

Am 08.10.2018 um 17:57 schrieb Deucher, Alexander:
>>>> 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.

Well then you would need to blacklist basically all gcc variants of the 
last decade or so.

Initializing only known members of structures is a perfectly valid 
optimization and well known issue when you then compare the structure 
with memcpy() or use the bytes for hashing or something similar.

>>>> 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.

See here for a good example of how people try to avoid issues: 
https://cgit.freedesktop.org/mesa/mesa/commit/?id=9422999e4041d4e984acbd2f44813d5928d20f18

> 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.

Yes, agree.

Christian.

>
> Alex
>