kbuild: Show marked Kconfig fragments in "help"

Currently the Kconfig fragments in kernel/configs and arch/*/configs
that aren't used internally aren't discoverable through "make help",
which consists of hard-coded lists of config fragments. Instead, list
all the fragment targets that have a "# Help: " comment prefix so the
targets can be generated dynamically.

Add logic to the Makefile to search for and display the fragment and
comment. Add comments to fragments that are intended to be direct targets.

Signed-off-by: Kees Cook <keescook@chromium.org>
Co-developed-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1552,7 +1552,6 @@ help:
 	@echo  '  mrproper	  - Remove all generated files + config + various backup files'
 	@echo  '  distclean	  - mrproper + remove editor backup and patch files'
 	@echo  ''
-	@echo  'Configuration targets:'
 	@$(MAKE) -f $(srctree)/scripts/kconfig/Makefile help
 	@echo  ''
 	@echo  'Other generic targets:'

arch/arm/configs/dram_0x00000000.config

@@ -1 +1,2 @@
+# Help: DRAM base at 0x00000000
 CONFIG_DRAM_BASE=0x00000000

arch/arm/configs/dram_0xc0000000.config

@@ -1 +1,2 @@
+# Help: DRAM base at 0xc0000000
 CONFIG_DRAM_BASE=0xc0000000

arch/arm/configs/dram_0xd0000000.config

@@ -1 +1,2 @@
+# Help: DRAM base at 0xd0000000
 CONFIG_DRAM_BASE=0xd0000000

arch/arm/configs/lpae.config

@@ -1,2 +1,3 @@
+# Help: Enable Large Physical Address Extension mode
 CONFIG_ARM_LPAE=y
 CONFIG_VMSPLIT_2G=y

arch/arm64/configs/virt.config

@@ -1,3 +1,4 @@
+# Help: Virtualization guest
 #
 # Base options for platforms
 #

arch/powerpc/configs/disable-werror.config

@@ -1 +1,2 @@
+# Help: Disable -Werror
 CONFIG_PPC_DISABLE_WERROR=y

arch/powerpc/configs/security.config

@@ -1,3 +1,5 @@
+# Help: Common security options for PowerPC builds
+
 # This is the equivalent of booting with lockdown=integrity
 CONFIG_SECURITY=y
 CONFIG_SECURITYFS=y

arch/riscv/configs/32-bit.config

@@ -1,3 +1,4 @@
+# Help: Build a 32-bit image
 CONFIG_ARCH_RV32I=y
 CONFIG_32BIT=y
 # CONFIG_PORTABLE is not set

arch/riscv/configs/64-bit.config

@@ -1,2 +1,3 @@
+# Help: Build a 64-bit image
 CONFIG_ARCH_RV64I=y
 CONFIG_64BIT=y

arch/s390/configs/btf.config

@@ -1 +1,2 @@
+# Help: Enable BTF debug info
 CONFIG_DEBUG_INFO_BTF=y

arch/s390/configs/kasan.config

@@ -1,3 +1,4 @@
+# Help: Enable KASan for debugging
 CONFIG_KASAN=y
 CONFIG_KASAN_INLINE=y
 CONFIG_KASAN_VMALLOC=y

arch/x86/Makefile

@@ -335,9 +335,5 @@ define archhelp
   echo  '			  bzdisk/fdimage*/hdimage/isoimage also accept:'
   echo  '			  FDARGS="..."  arguments for the booted kernel'
   echo  '			  FDINITRD=file initrd for the booted kernel'
-  echo  ''
-  echo  '  kvm_guest.config	- Enable Kconfig items for running this kernel as a KVM guest'
-  echo  '  xen.config		- Enable Kconfig items for running this kernel as a Xen guest'
-  echo  '  x86_debug.config	- Enable tip tree debugging options for testing'
 
 endef

kernel/configs/debug.config

@@ -1,3 +1,5 @@
+# Help: Debugging for CI systems and finding regressions
+#
 # The config is based on running daily CI for enterprise Linux distros to
 # seek regressions on linux-next builds on different bare-metal and virtual
 # platforms. It can be used for example,

kernel/configs/kvm_guest.config

@@ -1,3 +1,4 @@
+# Help: Bootable as a KVM guest
 CONFIG_NET=y
 CONFIG_NET_CORE=y
 CONFIG_NETDEVICES=y

kernel/configs/nopm.config

@@ -1,3 +1,5 @@
+# Help: Disable Power Management
+
 CONFIG_PM=n
 CONFIG_SUSPEND=n
 CONFIG_HIBERNATION=n

kernel/configs/rust.config

@@ -1 +1,2 @@
+# Help: Enable Rust
 CONFIG_RUST=y

kernel/configs/x86_debug.config

@@ -1,3 +1,4 @@
+# Help: Debugging options for tip tree testing
 CONFIG_X86_DEBUG_FPU=y
 CONFIG_LOCK_STAT=y
 CONFIG_DEBUG_VM=y

kernel/configs/xen.config

@@ -1,3 +1,5 @@
+# Help: Bootable as a Xen guest
+#
 # global stuff - these enable us to allow some
 # of the not so generic stuff below for xen
 CONFIG_PARAVIRT=y

scripts/kconfig/Makefile

@@ -93,11 +93,13 @@ endif
 %_defconfig: $(obj)/conf
 	$(Q)$< $(silent) --defconfig=arch/$(SRCARCH)/configs/$@ $(Kconfig)
 
-configfiles=$(wildcard $(srctree)/kernel/configs/$@ $(srctree)/arch/$(SRCARCH)/configs/$@)
+configfiles = $(wildcard $(srctree)/kernel/configs/$(1) $(srctree)/arch/$(SRCARCH)/configs/$(1))
+all-config-fragments = $(call configfiles,*.config)
+config-fragments = $(call configfiles,$@)
 
 %.config: $(obj)/conf
-	$(if $(call configfiles),, $(error No configuration exists for this target on this architecture))
-	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh -m .config $(configfiles)
+	$(if $(config-fragments),, $(error $@ fragment does not exists on this architecture))
+	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh -m .config $(config-fragments)
 	$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig
 
 PHONY += tinyconfig
@@ -115,6 +117,7 @@ clean-files += tests/.cache
 
 # Help text used by make help
 help:
+	@echo  'Configuration targets:'
 	@echo  '  config	  - Update current config utilising a line-oriented program'
 	@echo  '  nconfig         - Update current config utilising a ncurses menu based program'
 	@echo  '  menuconfig	  - Update current config utilising a menu based program'
@@ -141,6 +144,12 @@ help:
 	@echo  '                    default value without prompting'
 	@echo  '  tinyconfig	  - Configure the tiniest possible kernel'
 	@echo  '  testconfig	  - Run Kconfig unit tests (requires python3 and pytest)'
+	@echo  ''
+	@echo  'Configuration topic targets:'
+	@$(foreach f, $(all-config-fragments), \
+		if help=$$(grep -m1 '^# Help: ' $(f)); then \
+			printf '  %-25s - %s\n' '$(notdir $(f))' "$${help#*: }"; \
+		fi;)
 
 # ===========================================================================
 # object files used by all kconfig flavours