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

From: Rob Clark
Date: Sun Nov 06 2016 - 08:03:58 EST

On Sun, Nov 6, 2016 at 4:47 AM, Christian KÃnig
<christian.koenig@xxxxxxx> wrote:
> Am 05.11.2016 um 17:49 schrieb Rob Clark:
>> On Sat, Nov 5, 2016 at 12:38 PM, Eric Engestrom <eric@xxxxxxxxxxxx> wrote:
>>> On Saturday, 2016-11-05 13:11:36 +0100, Christian KÃnig 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.

if I'm not mistaken, that is what we are doing ;-)

>> 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.

hmm, actually the struct is a nice idea then if the compiler wouldn't
catch the wrong-size-array

> 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?

Two things trouble me about the kasprintf approach.. (ignoring the
fact that atm it is not GFP_ATOMIC)
1) you can't do DRM_DEBUG("format: %s\n", drm_get_format_name(..)) so
it pulls the memory allocation and sprintf outside of the drm_debug
2) seems awfully easy to forget the kfree... I wouldn't have even
known that now you need to free the result (with some patches I'm
working on) if it weren't for the fact that lockdep alerted me to the
GFP_KERNEL allocation in atomic ctx ;-)


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