kbuild: fix endless syncconfig in case arch Makefile sets CROSS_COMPILE

Commit 21c54b77 ("kconfig: show compiler version text in the top
comment") was intended to detect the compiler upgrade, but Geert
reported a breakage on the m68k build.

The compiler upgrade is detected by the change of the environment
variable, CC_VERSION_TEXT, which contains the first line of the output
from $(CC) --version.  Currently, this works well when CROSS_COMPILE
is given via the environment variable or the Make command line.

However, some architectures such as m68k can specify CROSS_COMPILE
from arch/$(SRCARCH)/Makefile as well.  In this case, "make ARCH=m68k"
ends up with endless syncconfig loop.

  $ make ARCH=m68k defconfig
  *** Default configuration is based on 'multi_defconfig'
  #
  # configuration written to .config
  #
  $ make ARCH=m68k
  scripts/kconfig/conf  --syncconfig Kconfig
  scripts/kconfig/conf  --syncconfig Kconfig
  scripts/kconfig/conf  --syncconfig Kconfig
  scripts/kconfig/conf  --syncconfig Kconfig

Things are happening like this:

Because arch/$(SRCARCH)/Makefile is included after CC_VERSION_TEXT
is set, it contains the host compiler version in the defconfig phase.

To create or update auto.conf, the following line is triggered:

include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
        $(Q)$(MAKE) -f $(srctree)/Makefile syncconfig

This recurses the top Makefile after arch/$(SRCARCH)/Makefile is
included.  CROSS_COMPILE is set to a m68k toolchain prefix and
exported to the recursed Make.  Then, syncconfig is invoked with
the target compiler version in CC_VERSION_TEXT.

The Make will restart because auto.conf and auto.conf.cmd have been
updated.  At this point, CROSS_COMPILE is reset, so CC_VERSION_TEXT
is set to the host compiler version again.  Then, syncconfig is
triggered due to the change of CC_VERSION_TEXT.  This loop continues
eternally.

To fix this problem, $(CC_VERSION_TEXT) must be evaluated only after
arch/$(SRCARCH)/Makefile.  Setting it earlier is OK as long as it is
defined by using the '=' operator instead of ':='.

For the defconfig phase, $(CC_VERSION_TEXT) is evaluated when Kbuild
descends into scripts/kconfig/, so it contains the target compiler
version correctly.

include/config/auto.conf.cmd references $(CC_VERSION_TEXT) as well,
so it must be included after arch/$(SRCARCH)/Makefile.

Fixes: 21c54b77 ("kconfig: show compiler version text in the top comment")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>

Makefile

@@ -442,8 +442,6 @@ export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE
 export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
 export KBUILD_ARFLAGS
 
-export CC_VERSION_TEXT := $(shell $(CC) --version | head -n 1)
-
 # When compiling out-of-tree modules, put MODVERDIR in the module
 # tree rather than in the kernel tree. The kernel tree might
 # even be read-only.
@@ -514,6 +512,12 @@ ifeq ($(shell $(CONFIG_SHELL) $(srctree)/scripts/cc-can-link.sh $(CC)), y)
   export CC_CAN_LINK
 endif
 
+# The expansion should be delayed until arch/$(SRCARCH)/Makefile is included.
+# Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
+# CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
+# and from include/config/auto.conf.cmd to detect the compiler upgrade.
+CC_VERSION_TEXT = $(shell $(CC) --version | head -n 1)
+
 ifeq ($(config-targets),1)
 # ===========================================================================
 # *config targets only - make sure prerequisites are updated, and descend
@@ -523,7 +527,7 @@ ifeq ($(config-targets),1)
 # KBUILD_DEFCONFIG may point out an alternative default configuration
 # used for 'make defconfig'
 include arch/$(SRCARCH)/Makefile
-export KBUILD_DEFCONFIG KBUILD_KCONFIG
+export KBUILD_DEFCONFIG KBUILD_KCONFIG CC_VERSION_TEXT
 
 config: scripts_basic outputmakefile FORCE
 	$(Q)$(MAKE) $(build)=scripts/kconfig $@
@@ -585,12 +589,32 @@ virt-y		:= virt/
 endif # KBUILD_EXTMOD
 
 ifeq ($(dot-config),1)
-# Read in config
 -include include/config/auto.conf
+endif
+
+# The all: target is the default when no target is given on the
+# command line.
+# This allow a user to issue only 'make' to build a kernel including modules
+# Defaults to vmlinux, but the arch makefile usually adds further targets
+all: vmlinux
+
+CFLAGS_GCOV	:= -fprofile-arcs -ftest-coverage \
+	$(call cc-option,-fno-tree-loop-im) \
+	$(call cc-disable-warning,maybe-uninitialized,)
+export CFLAGS_GCOV CFLAGS_KCOV
+
+# The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
+# values of the respective KBUILD_* variables
+ARCH_CPPFLAGS :=
+ARCH_AFLAGS :=
+ARCH_CFLAGS :=
+include arch/$(SRCARCH)/Makefile
 
+ifeq ($(dot-config),1)
 ifeq ($(KBUILD_EXTMOD),)
-# Read in dependencies to all Kconfig* files, make sure to run
-# oldconfig if changes are detected.
+# Read in dependencies to all Kconfig* files, make sure to run syncconfig if
+# changes are detected. This should be included after arch/$(SRCARCH)/Makefile
+# because some architectures define CROSS_COMPILE there.
 -include include/config/auto.conf.cmd
 
 # To avoid any implicit rule to kick in, define an empty command
@@ -622,24 +646,6 @@ else
 include/config/auto.conf: ;
 endif # $(dot-config)
 
-# The all: target is the default when no target is given on the
-# command line.
-# This allow a user to issue only 'make' to build a kernel including modules
-# Defaults to vmlinux, but the arch makefile usually adds further targets
-all: vmlinux
-
-CFLAGS_GCOV	:= -fprofile-arcs -ftest-coverage \
-	$(call cc-option,-fno-tree-loop-im) \
-	$(call cc-disable-warning,maybe-uninitialized,)
-export CFLAGS_GCOV CFLAGS_KCOV
-
-# The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
-# values of the respective KBUILD_* variables
-ARCH_CPPFLAGS :=
-ARCH_AFLAGS :=
-ARCH_CFLAGS :=
-include arch/$(SRCARCH)/Makefile
-
 KBUILD_CFLAGS	+= $(call cc-option,-fno-delete-null-pointer-checks,)
 KBUILD_CFLAGS	+= $(call cc-disable-warning,frame-address,)
 KBUILD_CFLAGS	+= $(call cc-disable-warning, format-truncation)