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:

39e2f501 ("drm/i915: Split struct intel_context definition to its own header")
3a891a62 ("drm/i915: Move intel_engine_mask_t around for use by i915_request_types.h")
8b74594a ("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

drivers/gpu/drm/i915/.gitignore

+header_test_*.c

drivers/gpu/drm/i915/Makefile

@@ -32,10 +32,13 @@ CFLAGS_intel_fbdev.o = $(call cc-disable-warning, override-init)
 subdir-ccflags-y += \
 	$(call as-instr,movntdqa (%eax)$(comma)%xmm0,-DCONFIG_AS_MOVNTDQA)
 
+# Extra header tests
+include $(src)/Makefile.header-test
+
 # Please keep these build lists sorted!
 
 # core driver code
-i915-y := i915_drv.o \
+i915-y += i915_drv.o \
 	  i915_irq.o \
 	  i915_memcpy.o \
 	  i915_mm.o \
@@ -57,17 +60,6 @@ i915-$(CONFIG_COMPAT)   += i915_ioc32.o
 i915-$(CONFIG_DEBUG_FS) += i915_debugfs.o intel_pipe_crc.o
 i915-$(CONFIG_PERF_EVENTS) += i915_pmu.o
 
-# Test the headers are compilable as standalone units
-i915-$(CONFIG_DRM_I915_WERROR) += \
-	test_i915_active_types_standalone.o \
-	test_i915_gem_context_types_standalone.o \
-	test_i915_priolist_types_standalone.o \
-	test_i915_scheduler_types_standalone.o \
-	test_i915_timeline_types_standalone.o \
-	test_intel_context_types_standalone.o \
-	test_intel_engine_types_standalone.o \
-	test_intel_workarounds_types_standalone.o
-
 # GEM code
 i915-y += \
 	  i915_active.o \

drivers/gpu/drm/i915/Makefile.header-test

+# SPDX-License-Identifier: MIT
+# Copyright © 2019 Intel Corporation
+
+# Test the headers are compilable as standalone units
+header_test := \
+	i915_active_types.h \
+	i915_gem_context_types.h \
+	i915_priolist_types.h \
+	i915_scheduler_types.h \
+	i915_timeline_types.h \
+	intel_context_types.h \
+	intel_engine_types.h \
+	intel_workarounds_types.h
+
+quiet_cmd_header_test = HDRTEST $@
+      cmd_header_test = echo "\#include \"$(<F)\"" > $@
+
+header_test_%.c: %.h
+	$(call cmd,header_test)
+
+i915-$(CONFIG_DRM_I915_WERROR) += $(foreach h,$(header_test),$(patsubst %.h,header_test_%.o,$(h)))
+
+clean-files += $(foreach h,$(header_test),$(patsubst %.h,header_test_%.c,$(h)))

drivers/gpu/drm/i915/test_i915_active_types_standalone.c

-/*
- * SPDX-License-Identifier: MIT
- *
- * Copyright © 2019 Intel Corporation
- */
-
-#include "i915_active_types.h"

drivers/gpu/drm/i915/test_i915_gem_context_types_standalone.c

-/*
- * SPDX-License-Identifier: MIT
- *
- * Copyright © 2019 Intel Corporation
- */
-
-#include "i915_gem_context_types.h"

drivers/gpu/drm/i915/test_i915_priolist_types_standalone.c

-/*
- * SPDX-License-Identifier: MIT
- *
- * Copyright © 2019 Intel Corporation
- */
-
-#include "i915_priolist_types.h"

drivers/gpu/drm/i915/test_i915_scheduler_types_standalone.c

-/*
- * SPDX-License-Identifier: MIT
- *
- * Copyright © 2019 Intel Corporation
- */
-
-#include "i915_scheduler_types.h"

drivers/gpu/drm/i915/test_i915_timeline_types_standalone.c

-/*
- * SPDX-License-Identifier: MIT
- *
- * Copyright © 2019 Intel Corporation
- */
-
-#include "i915_timeline_types.h"

drivers/gpu/drm/i915/test_intel_context_types_standalone.c

-/*
- * SPDX-License-Identifier: MIT
- *
- * Copyright © 2019 Intel Corporation
- */
-
-#include "intel_context_types.h"

drivers/gpu/drm/i915/test_intel_engine_types_standalone.c

-/*
- * SPDX-License-Identifier: MIT
- *
- * Copyright © 2019 Intel Corporation
- */
-
-#include "intel_engine_types.h"

drivers/gpu/drm/i915/test_intel_workarounds_types_standalone.c

-/*
- * SPDX-License-Identifier: MIT
- *
- * Copyright © 2019 Intel Corporation
- */
-
-#include "intel_workarounds_types.h"