.gitignore: add SPDX License Identifier

Add SPDX License Identifier to all .gitignore files.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drm/i915: reimplement header test feature

I implemented a small build rule in drivers/gpu/drm/i915/Makefile
without relying on the special header-test-y syntax that was removed in
commit fcbb8461fd23 ("kbuild: remove header compile test").

I excluded some headers from the test coverage. I hope somebody
intrested can take a closer look at them.

Dummy subdir Makefiles can be removed altogether as single target build
use case is now covered by commit 394053f4a4b3 ("kbuild: make single
targets work more correctly").

v2 by Jani:
- add selftests/i915_perf_selftests.h to no-header-test
- add .gitignore for *.hdrtest

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191219155652.2666-3-jani.nikula@intel.com

kbuild: add support for ensuring headers are self-contained

Sometimes it's useful to be able to explicitly ensure certain headers
remain self-contained, i.e. that they are compilable as standalone
units, by including and/or forward declaring everything they depend on.

Add special target header-test-y where individual Makefiles can add
headers to be tested if CONFIG_HEADER_TEST is enabled. This will
generate a dummy C file per header that gets built as part of extra-y.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

drm/i915: add Makefile magic for testing headers are self-contained

The below commits added dummy files to test that certain headers are
self-contained, i.e. compilable as standalone units:

39e2f501c1b4 ("drm/i915: Split struct intel_context definition to its own header")
3a891a626794 ("drm/i915: Move intel_engine_mask_t around for use by i915_request_types.h")
8b74594aa455 ("drm/i915: Split out i915_priolist_types into its own header")

The idea is fine, but the implementation is a bit tedious and
inflexible, and does not really scale well.

Implement the same in make using autogenerated dummy sources to include
the headers.

v2 by Chris:
- Use patsubst
- Add .gitignore
- Add clean-files for generated dummy sources

v3 by Jani:
- Fix make clean
- Add the tests to i915-y instead of extra-y

v4 by Jani:
- quiet_cmd whitespace fix

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190403135236.8398-1-jani.nikula@intel.com