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 at do-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
