[Buildroot] [PATCH 1/2] core/pkg-kconfig: add an option to use olddefconfig

[Romain Izard <romain.izard.pro@gmail.com>]

Using 'yes "" | make oldconfig' to regenerate a .config file from an
existing defconfig does not work reliably in all cases. Specifically, it
does not work well with tristate choice entries.

The correct way to do it is to use 'make olddefconfig', but this target
is not supported in projects that use an old version of kconfig. As
most projects do not use tristate entries, there is no need for them to
upgrade.

As a result, add support in the pkg-kconfig infrastructure for both
cases, with a configuration entry to select which mode to use.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
---
 docs/manual/adding-packages-kconfig.txt |  5 +++++
 package/pkg-kconfig.mk                  | 10 ++++++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/docs/manual/adding-packages-kconfig.txt b/docs/manual/adding-packages-kconfig.txt
index 3290024..50c4ecb 100644
--- a/docs/manual/adding-packages-kconfig.txt
+++ b/docs/manual/adding-packages-kconfig.txt
@@ -53,6 +53,11 @@ It is mandatory to provide either, but not both:
 * +FOO_KCONFIG_FILE+ specifies the path to a defconfig or full-config file
   to be used to configure the package.
 
+* +FOO_KCONFIG_OLDDEFCONFIG+ when set specifies that the 'make' rule to
+  call with defconfig files is 'olddefconfig'. Only some projects support
+  it, but the default rule called 'oldconfig' can misbehave when the
+  config file changes tristate 'choice' entries.
+
 * +FOO_KCONFIG_DEFCONFIG+ specifies the defconfig 'make' rule to call to
   configure the package.
 
diff --git a/package/pkg-kconfig.mk b/package/pkg-kconfig.mk
index 90f3f9f..71c6865 100644
--- a/package/pkg-kconfig.mk
+++ b/package/pkg-kconfig.mk
@@ -60,8 +60,7 @@ $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_FRAGMENT_FILES): | $(1)-patch
 # The specified source configuration file and any additional configuration file
 # fragments are merged together to .config, after the package has been patched.
 # Since the file could be a defconfig file it needs to be expanded to a
-# full .config first. We use 'make oldconfig' because this can be safely
-# done even when the package does not support defconfigs.
+# full .config first.
 $$($(2)_DIR)/.config: $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_FRAGMENT_FILES)
 	$$(if $$($(2)_KCONFIG_DEFCONFIG), \
 		$$($(2)_MAKE_ENV) $$(MAKE) -C $$($(2)_DIR) \
@@ -69,8 +68,11 @@ $$($(2)_DIR)/.config: $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_FRAGMENT_FILES)
 		cp $$($(2)_KCONFIG_FILE) $$(@))
 	support/kconfig/merge_config.sh -m -O $$(@D) \
 		$$(@) $$($(2)_KCONFIG_FRAGMENT_FILES)
-	$$(Q)yes "" | $$($(2)_MAKE_ENV) $$(MAKE) -C $$($(2)_DIR) \
-		$$($(2)_KCONFIG_OPTS) oldconfig
+	$$(if $$($(2)_KCONFIG_OLDDEFCONFIG), \
+		$$($2_MAKE_ENV) $$(MAKE) -C $$($(2)_DIR) \
+			$$($(2)_KCONFIG_OPTS) olddefconfig, \
+		$$(Q)yes "" | $$($(2)_MAKE_ENV) $$(MAKE) -C $$($(2)_DIR) \
+			$$($(2)_KCONFIG_OPTS) oldconfig)
 
 # If _KCONFIG_FILE or _KCONFIG_FRAGMENT_FILES exists, this dependency is
 # already implied, but if we only have a _KCONFIG_DEFCONFIG we have to add
-- 
2.5.0