Re: [PATCH] drm: move allocation out of drm_get_format_name()

From: "Christian König" <christian.koenig@amd.com>
To: Rob Clark <robdclark@gmail.com>, Eric Engestrom <eric@engestrom.ch>
Cc: "dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"Wei Yongjun" <yongjun_wei@trendmicro.com.cn>,
	"Daniel Vetter" <daniel.vetter@intel.com>,
	"Flora Cui" <Flora.Cui@amd.com>,
	"Gustavo Padovan" <gustavo.padovan@collabora.co.uk>,
	"Tom St Denis" <tom.stdenis@amd.com>,
	"Thomas Hellstrom" <thellstrom@vmware.com>,
	"Laurent Pinchart" <laurent.pinchart+renesas@ideasonboard.com>,
	"Xinliang Liu" <z.liuxinliang@hisilicon.com>,
	"VMware Graphics" <linux-graphics-maintainer@vmware.com>,
	"Vitaly Prosyak" <vitaly.prosyak@amd.com>,
	"Alexandre Demers" <alexandre.f.demers@gmail.com>,
	"Intel Graphics Development" <intel-gfx@lists.freedesktop.org>,
	"Emily Deng" <Emily.Deng@amd.com>,
	"Ken Wang" <Qingqing.Wang@amd.com>,
	"Junwei Zhang" <Jerry.Zhang@amd.com>,
	"Michel Dänzer" <michel.daenzer@amd.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Alex Deucher" <alexander.deucher@amd.com>,
	"Colin Ian King" <colin.king@canonical.com>
Subject: Re: [PATCH] drm: move allocation out of drm_get_format_name()
Date: Sun, 6 Nov 2016 10:47:25 +0100	[thread overview]
Message-ID: <8af873a2-31a9-1146-289d-35d2e48edffa@amd.com> (raw)
In-Reply-To: <CAF6AEGuEB82fjZ_YF61MAAqU=8ahXuMo06MN0_ZuWY5fkyHnoQ@mail.gmail.com>

Am 05.11.2016 um 17:49 schrieb Rob Clark:
> On Sat, Nov 5, 2016 at 12:38 PM, Eric Engestrom <eric@engestrom.ch> wrote:
>> On Saturday, 2016-11-05 13:11:36 +0100, Christian König wrote:
>>> Am 05.11.2016 um 02:33 schrieb Eric Engestrom:
>>>> +typedef char drm_format_name_buf[32];
>>> Please don't use a typedef for this, just define the maximum size of
>>> characters the function might write somewhere.
>>>
>>> See the kernel coding style as well:
>>>> In general, a pointer, or a struct that has elements that can reasonably
>>>> be directly accessed should **never** be a typedef.
>> I would normally agree as I tend to hate typedefs ($DAYJOB {ab,mis}uses
>> them way too much), and your way was what I wrote at first, but Rob Clark's
>> typedef idea makes it much harder for someone to allocate a buffer of
>> the wrong size, which IMO is good thing here.
> IMHO I would make a small test program to verify this actually helps
> the compiler catch problems.  And if it does, I would stick with it.
> The coding-style should be guidelines, not something that supersedes
> common sense / practicality.

Well completely agree that we should be able to question the coding 
style rules, but when we do it we discuss this on a the mailing list 
first and then start to use it in code. Not the other way around.

>
> That is my $0.02 anyways.. if others vehemently disagree and want to
> dogmatically stick to the coding-style guidelines, ok then.  OTOH, if
> this approach doesn't help the compiler catch issues, then it isn't
> worth it.

Yeah, exactly that's the point. If I'm not completely mistaken the 
compiler won't issue a warning here if you pass an array with the wrong 
size.

I think you need something like "struct drm_format_name_buf { char 
str[32]; };" to trigger this.

Apart from that is this function really called so often that using 
kasprintf() is a problem here? Or is there another motivation behind the 
change?

Regards,
Christian.

>
> BR,
> -R
>
>> I can rewrite the typedef out if you think it's better.
>>
>> Cheers,
>>    Eric
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