linux-kernel - Re: [PATCH v4 04/79] drm_mode.h: use __u32 and _

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Date:	Wed, 21 Oct 2015 12:27:27 -0400
From:	Alex Deucher <alexdeucher@...il.com>
To:	Emil Velikov <emil.l.velikov@...il.com>
Cc:	Mikko Rapeli <mikko.rapeli@....fi>,
	"open list:ABI/API" <linux-api@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Maling list - DRI developers 
	<dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 04/79] drm_mode.h: use __u32 and __u64 from linux/types.h

On Wed, Oct 21, 2015 at 12:21 PM, Emil Velikov <emil.l.velikov@...il.com> wrote:
> On 21 October 2015 at 16:18, Alex Deucher <alexdeucher@...il.com> wrote:
>> On Wed, Oct 21, 2015 at 11:09 AM, Emil Velikov <emil.l.velikov@...il.com> wrote:
>>> Hi Alex,
>>>
>>> On 15 October 2015 at 14:48, Mikko Rapeli <mikko.rapeli@....fi> wrote:
>>>> On Thu, Oct 15, 2015 at 09:32:10AM -0400, Alex Deucher wrote:
>>>>> On Thu, Oct 15, 2015 at 1:55 AM, Mikko Rapeli <mikko.rapeli@....fi> wrote:
>>>>> > Fixes userspace compilation error:
>>>>> >
>>>>> > drm/drm_mode.h:472:2: error: unknown type name ‘uint32_t’
>>>>> >
>>>>> > Signed-off-by: Mikko Rapeli <mikko.rapeli@....fi>
>>>>>
>>>>> NACK on all these type conversions.  This has not been a problem for
>>>>> years and years and the result looks terrible.
>>>>
>>>> Documentation/CodingStyle, section 5
>>>>
>>>>  (e) Types safe for use in userspace.
>>>>
>>>>      In certain structures which are visible to userspace, we cannot
>>>>      require C99 types and cannot use the 'u32' form above. Thus, we
>>>>      use __u32 and similar types in all structures which are shared
>>>>      with userspace.
>>>>
>>>> I have only been looking at kernel headers from userspace occationally in
>>>> the past 10 years and had a several cases where the provided headers did
>>>> not compile when included into trivial programs trying to use the structs
>>>> for an ioctl() for example. This long lasting problem triggered me to write
>>>> a test for this and provide these fixes too. In previous reviews usage
>>>> of <stdint.h> and its types in kernel headers was already NACK'ed
>>>> so I changed several places from uint32_t's to __u32.
>>>>
>>>> With these changes it is btw trivial now to add a grep test the there
>>>> are no uint32_t's in include/uapi/ anymore, thus enforcing that coding style
>>>> rule.
>>>>
>>> Based of the reply from Mikko, can you please elaborate your concern ?
>>> Are you thinking about some corner case where this may cause breakage,
>>> or it's solely on stylistic point of view ?
>>
>> Style.
>>
> In that case shouldn't the kernel coding style (not to mention the
> issues with stdint.h types Linus has brought a while back) be
> considered/prevail ?

I don't really have a strong opinion one way or another at this point.
The types just look ugly.

>
>>>
>>> Over the last few years we've been doing some ad-hoc 'synchronisation'
>>> with the headers in libdrm, and this will get us one step closer to
>>> doing things properly.
>>
>> How does this affect libdrm one way or another?
>>
> In a strange way. Some (most?) distributions do not ship the uapi
> headers from the kernel, but rely on the ones in libdrm. Due to
> various issues (as addressed in the series) we have not synchronised
> the headers, thus users such as the gallium radeon winsys has ended up
> redefining/duplicating internally.

The problem is that you generally need the latest headers to support
the latest features in the user mode stacks, so if the user tries to
build against an older kernel, they will be missing definitions.

Alex
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