headers: Add README file

Since we're trying to standardise and make things more consistent in
the area, add a basic README which covers some of the more popular
topics.

v2:
 - Drop drm-misc (Daniel Vetter)
v3:
 - Elaborate on when and which headers to update
 - Add a list of headers and the respective "issues"
 - Add file to EXTRA_DIST

Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

2 files changed, 158 insertions, 0 deletions

diff --git a/Makefile.am b/Makefile.am
index 630edc4c..2e46bde8 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -133,6 +133,7 @@ if HAVE_VMWGFX
 klibdrminclude_HEADERS += $(LIBDRM_INCLUDE_VMWGFX_H_FILES)
 endif
 
+EXTRA_DIST = include/drm/README
 
 copy-headers :
 	cp -r $(kernel_source)/include/uapi/drm/*.h $(top_srcdir)/include/drm/
diff --git a/include/drm/README b/include/drm/README
new file mode 100644
index 00000000..c3292f31
--- /dev/null
+++ b/include/drm/README
@@ -0,0 +1,157 @@
+What are these headers ?
+------------------------
+This is the canonical source of drm headers that user space should use for
+communicating with the kernel DRM subsystem.
+
+They flow from the kernel, thus any changes must be merged there first.
+Do _not_ attempt to "fix" these by deviating from the kernel ones !
+
+
+Non-linux platforms - changes/patches
+-------------------------------------
+If your platform has local changes, please send them upstream for inclusion.
+Even if your patches don't get accepted in their current form, devs will
+give you feedback on how to address things properly.
+
+git send-email --subject-prefix="PATCH libdrm" your patches to dri-devel
+mailing list.
+
+Before doing so, please consider the following:
+ - Have the [libdrm vs kernel] headers on your platform deviated ?
+Consider unifying them first.
+
+ - Have you introduced additional ABI that's not available in Linux ?
+Propose it for [Linux kernel] upstream inclusion.
+If that doesn't work out (hopefully it never does), move it to another header
+and/or keep the change(s) local ?
+
+ - Are your changes DRI1/UMS specific ?
+There is virtually no interest/power in keeping those legacy interfaces. They
+are around due to the kernel "thou shalt not break existing user space" rule.
+
+Consider porting the driver to DRI2/KMS - all (almost?) sensible hardware is
+capable of supporting those.
+
+
+Which headers go where ?
+------------------------
+A snipped from the, now removed, Makefile.am used to state:
+
+  XXX airlied says, nothing besides *_drm.h and drm*.h should be necessary.
+  however, r300 and via need their reg headers installed in order to build.
+  better solutions are welcome.
+
+Obviously the r300 and via headers are no longer around ;-)
+
+Reason behind is that the drm headers can be used as a basic communications
+channel with the respective kernel modules. If more advanced functionality is
+required one can pull the specific libdrm_$driver which is free to pull
+additional files from the kernel.
+
+For example: nouveau has nouveau/nvif/*.h while vc4 has vc4/*.h
+
+If your driver is still in prototyping/staging state, consider moving the
+$driver_drm.h into $driver and _not_ installing it. An header providing opaque
+definitions and access [via $driver_drmif.h or similar] would be better fit.
+
+
+When and which headers to update
+--------------------------------
+Ideally all files will be synced (updated) with the latest released kernel on
+each libdrm release. Sadly that's not yet possible since quite a few headers
+differ significantly - see Outdated or Broken Headers section below.
+
+That said, it's up-to the individual developers to sync with newer version
+(from drm-next) as they see fit.
+
+
+When and how to update these files
+----------------------------------
+In order to update the files do the following:
+ - Switch to a Linux kernel tree/branch which is not rebased.
+For example: airlied/drm-next
+ - Install the headers via `make headers_install' to a separate location.
+ - Copy the drm header[s] + git add + git commit.
+ - Note: Your commit message must include:
+   a) Brief summary on the delta. If there's any change that looks like an
+API/ABI break one _must_ explicitly state why it's safe to do so.
+   b) "Generated using make headers_install."
+   c) "Generated from $tree/branch commit $sha"
+
+
+Outdated or Broken Headers
+--------------------------
+This section contains a list of headers and the respective "issues" they might
+have relative to their kernel equivalent.
+
+Nearly all headers:
+ - Missing extern C notation.
+Status: Trivial.
+
+Most UMS headers:
+ - Not using fixed size interers - compat ioctls are broken.
+Status: ?
+Promote to fixed size ints, which match the current (32bit) ones.
+
+
+amdgpu_drm.h
+ - Using the stdint.h uint*_t over the respective __u* ones
+Status: Trivial.
+
+drm_mode.h
+ - Missing DPI encode/connector pair.
+Status: Trivial.
+
+i915_drm.h
+ - Missing PARAMS - HAS_POOLED_EU, MIN_EU_IN_POOL CONTEXT_PARAM_NO_ERROR_CAPTURE
+Status: Trivial.
+
+mga_drm.h
+ - Typo fix, use struct over typedef.
+Status: Trivial.
+
+nouveau_drm.h
+ - Missing macros NOUVEAU_GETPARAM*, NOUVEAU_DRM_HEADER_PATCHLEVEL, structs,
+enums, using stdint.h over the __u* types.
+Status: ?
+
+qxl_drm.h
+ - Using the stdint.h uint*_t over the respective __u* ones
+Status: Trivial.
+
+r128_drm.h
+ - Broken compat ioctls.
+
+radeon_drm.h
+ - Missing RADEON_TILING_R600_NO_SCANOUT, CIK_TILE_MODE_*, broken UMS ioctls,
+using stdint types.
+ - Both kernel and libdrm: missing padding -
+drm_radeon_gem_{create,{g,s}et_tiling,set_domain} others ?
+Status: ?
+
+savage_drm.h
+ - Renamed ioctls - DRM_IOCTL_SAVAGE_{,BCI}_EVENT_EMIT, compat ioctls are broken.
+Status: ?
+
+sis_drm.h
+ - Borken ioctls + libdrm uses int vs kernel long
+Status: ?
+
+via_drm.h
+ - Borken ioctls - libdrm int vs kernel long
+Status: ?
+
+
+omap_drm.h (living in $TOP/omap)
+ - License mismatch, missing DRM_IOCTL_OMAP_GEM_NEW and related struct
+Status: ?
+
+msm_drm.h (located in $TOP/freedreno/msm/)
+ - License mismatch, missing MSM_PIPE_*, MSM_SUBMIT_*. Renamed
+drm_msm_gem_submit::flags, missing drm_msm_gem_submit::fence_fd.
+Status: ?
+
+exynos_drm.h (living in $TOP/exynos)
+ - License mismatch, now using fixed size ints (but not everywhere). Lots of
+new stuff.
+Status: ?