linux-kernel - Re: [PATCH 1/2] uapi: update includes for drm content when no kernel API exists

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Date:	Sat, 13 Oct 2012 10:00:42 -0700
From:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	devel@...verdev.osuosl.org, backports@...r.kernel.org, rob@...com,
	arnd@...db.de, davej@...hat.com, airlied@...ux.ie,
	bskeggs@...hat.com, alan@...rguk.ukuu.org.uk, dhowells@...hat.com,
	tglx@...utronix.de, daniel.vetter@...ll.ch,
	jbarnes@...tuousgeek.org, alexander.deucher@....com,
	paulmck@...ux.vnet.ibm.com, gregkh@...uxfoundation.org
Subject: Re: [PATCH 1/2] uapi: update includes for drm content when no kernel
 API exists

On Sat, Oct 13, 2012 at 3:33 AM, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
> Hi Luis,
>
> On Friday 12 October 2012 16:49:31 Luis R. Rodriguez wrote:
>> From: "Luis R. Rodriguez" <mcgrof@...not-panic.com>
>>
>> The UAPI changes split kernel API and userspace API
>> content onto two separate header files. The userspace
>> API drm content was moved to include/uapi/drm/ with the
>> same file name while kernel specific API content was
>> kept under include/drm/ with the same file name. When
>> one file was split into two files the kernel header
>> includes the uapi header and a UAPI prefix was added to
>> the uapi header for its header guard. When there was no
>> kernel API content found the uapi header file was the
>> only one that was kept and the original guard for the
>> header file was kept. In this particular case the
>> original users of this header file were not modified
>> and the uapi header file is expected to be picked up
>> by path.
>>
>> This may work well at compilation on the kernel but when
>> backporting this creates a few complexities.
>
> Could you please provide more details about those complexities ?

Sure, the backported DRM code is as-is code from linux-next. The
header search path includes a copy of a few select header files from
linux-next, the rest are part of the compat [0] project to help with
backporting and the last set are from your own kernel. In this
particular case the UAPI effort pushed into include/uapi/drm a few
files which are now no longer present in the old include/drm/ location
when there was no respective kernel-only APIs exposed. For DRM code
that did not use the new uapi/drm/ path for old header includes this
means that upon backporting the older kernel's header file would be
used obviously leading to a series of compilation failures. By being
explicit about the path, as was done with a few other header files, we
can suck out DRM code intact from the kernel to be compilable for
older kernels. As of the v3.7-rc1 the compat-drivers project [1] will
be providing DRM modules. The new UAPI changes broke compilation on
compat-drivers when compiling DRM code so to fix this we either patch
code taken from the Linux kernel as I have done, force inclusion of a
few specific headers files, or use include_next tricks. It turns out
that forcing inclusion of -I$(M)/include/uapi as a path search prior
to your own kernel's at compilation time did not help. The
include_next trick can work as well but that'd mean synching the UAPI
files regularly into compat. I'd much prefer to have code intact when
possible when backporting so the option I stuck with then was to patch
the code directly and then as part of compat-drivers to always copy
that day's linux-next UAPI headers into the current directory for
compilation. I see no other driver code using the uapi path explicitly
though, is that by design? Would it be wrong to be explicit about
using a UAPI header alone if no kernel API files exist?

[0] https://backports.wiki.kernel.org/index.php/Documentation/compat
[1] https://backports.wiki.kernel.org/

  Luis
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