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

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Date:	Tue, 16 Oct 2012 14:34:39 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	"Luis R. Rodriguez" <mcgrof@...not-panic.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

Hi Luis,

On Saturday 13 October 2012 10:00:42 Luis R. Rodriguez wrote:
> On Sat, Oct 13, 2012 at 3:33 AM, Laurent Pinchart wrote:
> > 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.

Shouldn't that be added as a search path *after* include/linux/ instead of 
before ?

> 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?

As far as I understand that's by design, yes. Kernel code isn't expected to 
reference uapi/ headers directly.

> Would it be wrong to be explicit about using a UAPI header alone if no
> kernel API files exist?

I personally don't really like including such "features" in a mainline just 
for backporting, especially when they go against the spirit of the 
architecture (the uapi/ split design principles in this case). The way we're 
dealing with this in the V4L backport tree 
(http://git.linuxtv.org/media_build.git) is to play with Makefiles, include 
compatibility headers and, if nothing else can work, maintain backport patches 
for mainline code.

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

-- 
Regards,

Laurent Pinchart

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