kselftest: arm64: extend toplevel skeleton Makefile

Modify KSFT arm64 toplevel Makefile to maintain arm64 kselftests organized
by subsystem, keeping them into distinct subdirectories under arm64 custom
KSFT directory: tools/testing/selftests/arm64/

Add to such toplevel Makefile a mechanism to guess the effective location
of Kernel headers as installed by KSFT framework.

Fit existing arm64 tags kselftest into this new schema moving them into
their own subdirectory (arm64/tags).

Reviewed-by: Dave Martin <[e-mail]>
Signed-off-by: Cristian Marussi <[e-mail]>
Signed-off-by: Catalin Marinas <[e-mail]>

tools/testing/selftests/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 TARGETS = android
+TARGETS += arm64
 TARGETS += bpf
 TARGETS += breakpoints
 TARGETS += capabilities

tools/testing/selftests/arm64/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 
-# ARCH can be overridden by the user for cross compiling
+# When ARCH not overridden for crosscompiling, lookup machine
 ARCH ?= $(shell uname -m 2>/dev/null || echo not)
 
 ifneq (,$(filter $(ARCH),aarch64 arm64))
-CFLAGS += -I../../../../usr/include/
-TEST_GEN_PROGS := tags_test
-TEST_PROGS := run_tags_test.sh
+ARM64_SUBTARGETS ?= tags
+else
+ARM64_SUBTARGETS :=
 endif
 
-include ../lib.mk
+CFLAGS := -Wall -O2 -g
+
+# A proper top_srcdir is needed by KSFT(lib.mk)
+top_srcdir = $(realpath ../../../../)
+
+# Additional include paths needed by kselftest.h and local headers
+CFLAGS += -I$(top_srcdir)/tools/testing/selftests/
+
+# Guessing where the Kernel headers could have been installed
+# depending on ENV config
+ifeq ($(KBUILD_OUTPUT),)
+khdr_dir = $(top_srcdir)/usr/include
+else
+# the KSFT preferred location when KBUILD_OUTPUT is set
+khdr_dir = $(KBUILD_OUTPUT)/kselftest/usr/include
+endif
+
+CFLAGS += -I$(khdr_dir)
+
+export CFLAGS
+export top_srcdir
+
+all:
+	@for DIR in $(ARM64_SUBTARGETS); do				\
+		BUILD_TARGET=$(OUTPUT)/$$DIR;			\
+		mkdir -p $$BUILD_TARGET;			\
+		make OUTPUT=$$BUILD_TARGET -C $$DIR $@;		\
+	done
+
+install: all
+	@for DIR in $(ARM64_SUBTARGETS); do				\
+		BUILD_TARGET=$(OUTPUT)/$$DIR;			\
+		make OUTPUT=$$BUILD_TARGET -C $$DIR $@;		\
+	done
+
+run_tests: all
+	@for DIR in $(ARM64_SUBTARGETS); do				\
+		BUILD_TARGET=$(OUTPUT)/$$DIR;			\
+		make OUTPUT=$$BUILD_TARGET -C $$DIR $@;		\
+	done
+
+# Avoid any output on non arm64 on emit_tests
+emit_tests: all
+	@for DIR in $(ARM64_SUBTARGETS); do				\
+		BUILD_TARGET=$(OUTPUT)/$$DIR;			\
+		make OUTPUT=$$BUILD_TARGET -C $$DIR $@;		\
+	done
+
+clean:
+	@for DIR in $(ARM64_SUBTARGETS); do				\
+		BUILD_TARGET=$(OUTPUT)/$$DIR;			\
+		make OUTPUT=$$BUILD_TARGET -C $$DIR $@;		\
+	done
+
+.PHONY: all clean install run_tests emit_tests

tools/testing/selftests/arm64/README

···
+KSelfTest ARM64
+===============
+
+- These tests are arm64 specific and so not built or run but just skipped
+  completely when env-variable ARCH is found to be different than 'arm64'
+  and `uname -m` reports other than 'aarch64'.
+
+- Holding true the above, ARM64 KSFT tests can be run within the KSelfTest
+  framework using standard Linux top-level-makefile targets:
+
+      $ make TARGETS=arm64 kselftest-clean
+      $ make TARGETS=arm64 kselftest
+
+      or
+
+      $ make -C tools/testing/selftests TARGETS=arm64 \
+		INSTALL_PATH=<your-installation-path> install
+
+      or, alternatively, only specific arm64/ subtargets can be picked:
+
+      $ make -C tools/testing/selftests TARGETS=arm64 ARM64_SUBTARGETS="tags signal" \
+		INSTALL_PATH=<your-installation-path> install
+
+   Further details on building and running KFST can be found in:
+     Documentation/dev-tools/kselftest.rst

tools/testing/selftests/arm64/tags/Makefile

···
+# SPDX-License-Identifier: GPL-2.0
+
+CFLAGS += -I../../../../../usr/include/
+TEST_GEN_PROGS := tags_test
+TEST_PROGS := run_tags_test.sh
+
+include ../../lib.mk