[Buildroot] [PATCH v2 09/23] linux: build out of tree

From: Florian Fainelli via buildroot <buildroot@buildroot.org>
To: buildroot@buildroot.org
Cc: "Eric Le Bihan" <eric.le.bihan.dev@free.fr>,
	"Gary Bisson" <bisson.gary@gmail.com>,
	"Jérôme Pouiller" <jezz@sysmic.org>,
	"Florian Fainelli" <florian.fainelli@broadcom.com>,
	"Bernd Kuhls" <bernd@kuhls.net>,
	"Thomas Perale" <thomas.perale@mind.be>,
	"Maxime Chevallier" <maxime.chevallier@bootlin.com>,
	"Refik Tuzakli" <tuzakli.refik@gmail.com>,
	"Francois Perrad" <francois.perrad.86@gmail.com>,
	"Thomas Petazzoni" <thomas.petazzoni@bootlin.com>,
	"Arnout Vandecappelle" <arnout@rnout.be>,
	"Petr Vorel" <petr.vorel@gmail.com>,
	"Spenser Gilliland" <spenser@gillilanding.com>,
	"Giulio Benetti" <giulio.benetti@benettiengineering.com>,
	"Pascal de Bruijn" <p.debruijn@unilogic.nl>,
	"Luca Ceresoli" <luca.ceresoli@bootlin.com>,
	"Romain Naour" <romain.naour@gmail.com>,
	"Mahyar Koshkouei" <mahyar.koshkouei@gmail.com>
Subject: [Buildroot] [PATCH v2 09/23] linux: build out of tree
Date: Wed, 28 Oct 2026 14:37:30 -0700	[thread overview]
Message-ID: <20261028213744.511429-10-florian.fainelli@broadcom.com> (raw)
In-Reply-To: <20261028213744.511429-1-florian.fainelli@broadcom.com>

Introduce <pkg>_SUPPORTS_OUT_OF_TREE. A package that sets it is always
built out of tree:

 - <pkg>_SRCDIR is $(SRC_DIR)/<basename>, i.e. output/source/..., where
   the sources are extracted and patched, while <pkg>_BUILDDIR remains
   in $(BUILD_DIR) and only contains generated files. Other packages
   keep extracting in $(BUILD_DIR), so they are not affected;
 - when <pkg>_OVERRIDE_SRCDIR is set, <pkg>_SRCDIR is the overridden
   directory, used in place: nothing is rsynced into $(BUILD_DIR), only
   the pre/post rsync hooks still run, and the source-related stamp
   files (rsynced, patched, ...) live in the build directory so that
   nothing is created in the user's tree;
 - kconfig packages start from the source directory with O=<builddir>,
   which makes kbuild generate its wrapper Makefile in the build
   directory, so that all the existing -C $(LINUX_DIR) invocations keep
   working unmodified;
 - <pkg>-dirclean and 'make clean' also remove the extracted sources.

Enable it for the Linux kernel. When the sources are overridden, the
source tree must be clean (no in-tree .config, include/config,
arch/*/include/generated).

Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
---
 Makefile               |  4 ++--
 linux/linux.mk         |  5 +++++
 package/pkg-generic.mk | 46 ++++++++++++++++++++++++++++++------------
 package/pkg-kconfig.mk |  6 +++++-
 4 files changed, 45 insertions(+), 16 deletions(-)

diff --git a/Makefile b/Makefile
index c1f12a4bfc38..7aa43126540c 100644
--- a/Makefile
+++ b/Makefile
@@ -211,7 +211,7 @@ endif
 # Avoids doing the $(or...) everytime
 BR_GRAPH_OUT := $(or $(BR2_GRAPH_OUT),pdf)
 
-SRC_DIR := $(BASE_DIR)/build
+SRC_DIR := $(BASE_DIR)/source
 BUILD_DIR := $(BASE_DIR)/build
 BINARIES_DIR := $(BASE_DIR)/images
 BASE_TARGET_DIR := $(BASE_DIR)/target
@@ -1133,7 +1133,7 @@ show-vars:
 .PHONY: clean
 clean:
 	rm -rf $(BASE_TARGET_DIR) $(BINARIES_DIR) $(HOST_DIR) $(HOST_DIR_SYMLINK) \
-		$(BUILD_DIR) $(BASE_DIR)/staging \
+		$(BUILD_DIR) $(SRC_DIR) $(BASE_DIR)/staging \
 		$(LEGAL_INFO_DIR) $(GRAPHS_DIR) $(PER_PACKAGE_DIR) $(O)/pkg-stats.*
 
 .PHONY: distclean
diff --git a/linux/linux.mk b/linux/linux.mk
index a90e09afe906..5734d78aa9a2 100644
--- a/linux/linux.mk
+++ b/linux/linux.mk
@@ -6,6 +6,11 @@
 
 LINUX_VERSION = $(call qstrip,$(BR2_LINUX_KERNEL_VERSION))
 LINUX_LICENSE = GPL-2.0
+
+# The kernel is always built out of tree: the sources are in $(LINUX_SRCDIR)
+# and the kernel is built in $(LINUX_BUILDDIR) with O=. When
+# LINUX_OVERRIDE_SRCDIR is set, the sources are not rsynced.
+LINUX_SUPPORTS_OUT_OF_TREE = YES
 LINUX_LICENSE_FILES = $(call qstrip,$(BR2_LINUX_KERNEL_LICENSE_FILES))
 
 LINUX_CPE_ID_VENDOR = linux
diff --git a/package/pkg-generic.mk b/package/pkg-generic.mk
index 138fe9ccef4f..0f4bf73fa064 100644
--- a/package/pkg-generic.mk
+++ b/package/pkg-generic.mk
@@ -171,7 +171,7 @@ endef
 ################################################################################
 
 # Retrieve the archive
-$(SRC_DIR)/%/.stamp_downloaded:
+$(BUILD_DIR)/%/.stamp_downloaded $(SRC_DIR)/%/.stamp_downloaded:
 	@$(call step_start,download)
 	$(call prepare-per-package-directory,$($(PKG)_FINAL_DOWNLOAD_DEPENDENCIES))
 	$(foreach hook,$($(PKG)_PRE_DOWNLOAD_HOOKS),$(call $(hook))$(sep))
@@ -196,7 +196,7 @@ $(SRC_DIR)/%/.stamp_downloaded:
 	$(Q)touch $@
 
 # Retrieve actual source archive, e.g. for prebuilt external toolchains
-$(SRC_DIR)/%/.stamp_actual_downloaded:
+$(BUILD_DIR)/%/.stamp_actual_downloaded $(SRC_DIR)/%/.stamp_actual_downloaded:
 	@$(call step_start,actual-download)
 	$(call DOWNLOAD,$($(PKG)_ACTUAL_SOURCE_SITE)/$($(PKG)_ACTUAL_SOURCE_TARBALL))
 	$(Q)mkdir -p $(@D)
@@ -204,7 +204,7 @@ $(SRC_DIR)/%/.stamp_actual_downloaded:
 	$(Q)touch $@
 
 # Unpack the archive
-$(SRC_DIR)/%/.stamp_extracted:
+$(BUILD_DIR)/%/.stamp_extracted $(SRC_DIR)/%/.stamp_extracted:
 	@$(call step_start,extract)
 	@$(call MESSAGE,"Extracting")
 	$(call prepare-per-package-directory,$($(PKG)_FINAL_EXTRACT_DEPENDENCIES))
@@ -219,19 +219,19 @@ $(SRC_DIR)/%/.stamp_extracted:
 
 # Rsync the source directory if the <pkg>_OVERRIDE_SRCDIR feature is
 # used.
-$(SRC_DIR)/%/.stamp_rsynced:
+$(BUILD_DIR)/%/.stamp_rsynced $(SRC_DIR)/%/.stamp_rsynced:
 	@$(call step_start,rsync)
-	@$(call MESSAGE,"Syncing from source dir $(SRCDIR)")
+	@$(call MESSAGE,"$(if $($(PKG)_OUT_OF_TREE),Using out-of-tree source dir,Syncing from source dir) $(SRCDIR)")
 	@mkdir -p $(@D)
 	$(foreach hook,$($(PKG)_PRE_RSYNC_HOOKS),$(call $(hook))$(sep))
 	@test -d $(SRCDIR) || (echo "ERROR: $(SRCDIR) does not exist" ; exit 1)
-	rsync -au --chmod=u=rwX,go=rX $($(PKG)_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS) $(RSYNC_VCS_EXCLUSIONS) $(call qstrip,$(SRCDIR))/ $(@D)
+	$(if $($(PKG)_OUT_OF_TREE),,rsync -au --chmod=u=rwX,go=rX $($(PKG)_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS) $(RSYNC_VCS_EXCLUSIONS) $(call qstrip,$(SRCDIR))/ $(@D))
 	$(foreach hook,$($(PKG)_POST_RSYNC_HOOKS),$(call $(hook))$(sep))
 	@$(call step_end,rsync)
 	$(Q)touch $@
 
 # Patch
-$(SRC_DIR)/%/.stamp_patched:
+$(BUILD_DIR)/%/.stamp_patched $(SRC_DIR)/%/.stamp_patched:
 	@$(call step_start,patch)
 	@$(call MESSAGE,"Patching")
 	$(foreach hook,$($(PKG)_PRE_PATCH_HOOKS),$(call $(hook))$(sep))
@@ -390,6 +390,7 @@ $(BUILD_DIR)/%/.stamp_installed:
 $(BUILD_DIR)/%/.stamp_dircleaned:
 	$(if $(BR2_PER_PACKAGE_DIRECTORIES),rm -Rf $(PER_PACKAGE_DIR)/$(NAME))
 	rm -Rf $(@D)
+	$(if $(filter $(SRC_DIR)/%,$($(PKG)_SRCDIR)),rm -Rf $($(PKG)_SRCDIR))
 
 ################################################################################
 # virt-provides-single -- check that provider-pkg is the declared provider for
@@ -559,13 +560,32 @@ ifndef $(2)_STRIP_COMPONENTS
 endif
 
 $(2)_SRCDIR = $$($(2)_DIR)
-$(2)_BUILDDIR = $$($(2)_DIR)
+$(2)_BUILDDIR ?= $$($(2)_DIR)
 $(2)_SRCSUBDIR = $$($(2)_SRCDIR)/$$($(2)_SUBDIR)
 
 ifneq ($$($(2)_OVERRIDE_SRCDIR),)
 $(2)_VERSION = custom
 endif
 
+# Packages that set <pkg>_SUPPORTS_OUT_OF_TREE = YES are always built out of
+# tree: the sources live in $(SRC_DIR), and only the build directory lives in
+# $(BUILD_DIR). When <pkg>_OVERRIDE_SRCDIR is set, the sources are used
+# where they are (no rsync). The source-related stamp files must then not be
+# created in the (user-owned) source directory, so they live in the build
+# directory.
+# Out-of-tree builds are disabled by default. A package infrastructure may
+# set a different default before calling this macro.
+$(3)_SUPPORTS_OUT_OF_TREE ?= NO
+$(2)_STAMP_SRCDIR = $$($(2)_SRCDIR)
+ifeq ($$($(3)_SUPPORTS_OUT_OF_TREE),YES)
+$(2)_OUT_OF_TREE = YES
+$(2)_SRCDIR = $$(SRC_DIR)/$$($(2)_BASENAME)
+ifneq ($$($(2)_OVERRIDE_SRCDIR),)
+$(2)_SRCDIR = $$(abspath $$(call qstrip,$$($(2)_OVERRIDE_SRCDIR)))
+$(2)_STAMP_SRCDIR = $$($(2)_BUILDDIR)
+endif
+endif
+
 ifndef $(2)_SOURCE
  ifdef $(3)_SOURCE
   $(2)_SOURCE = $$($(3)_SOURCE)
@@ -844,11 +864,11 @@ $(2)_TARGET_INSTALL_IMAGES =	$$($(2)_BUILDDIR)/.stamp_images_installed
 $(2)_TARGET_INSTALL_HOST =	$$($(2)_BUILDDIR)/.stamp_host_installed
 $(2)_TARGET_BUILD =		$$($(2)_BUILDDIR)/.stamp_built
 $(2)_TARGET_CONFIGURE =		$$($(2)_BUILDDIR)/.stamp_configured
-$(2)_TARGET_RSYNC =		$$($(2)_SRCDIR)/.stamp_rsynced
-$(2)_TARGET_PATCH =		$$($(2)_SRCDIR)/.stamp_patched
-$(2)_TARGET_EXTRACT =		$$($(2)_SRCDIR)/.stamp_extracted
-$(2)_TARGET_SOURCE =		$$($(2)_SRCDIR)/.stamp_downloaded
-$(2)_TARGET_ACTUAL_SOURCE =	$$($(2)_SRCDIR)/.stamp_actual_downloaded
+$(2)_TARGET_RSYNC =		$$($(2)_STAMP_SRCDIR)/.stamp_rsynced
+$(2)_TARGET_PATCH =		$$($(2)_STAMP_SRCDIR)/.stamp_patched
+$(2)_TARGET_EXTRACT =		$$($(2)_STAMP_SRCDIR)/.stamp_extracted
+$(2)_TARGET_SOURCE =		$$($(2)_STAMP_SRCDIR)/.stamp_downloaded
+$(2)_TARGET_ACTUAL_SOURCE =	$$($(2)_STAMP_SRCDIR)/.stamp_actual_downloaded
 $(2)_TARGET_DIRCLEAN =		$$($(2)_BUILDDIR)/.stamp_dircleaned
 
 # default extract command
diff --git a/package/pkg-kconfig.mk b/package/pkg-kconfig.mk
index 8a63d55be033..5e46c62c58e6 100644
--- a/package/pkg-kconfig.mk
+++ b/package/pkg-kconfig.mk
@@ -147,8 +147,12 @@ $$($(2)_KCONFIG_FILE) $$($(2)_KCONFIG_FRAGMENT_FILES): | $(1)-patch
 		fi; \
 	done
 
+# For an out-of-tree build, the first kconfig run is what creates the
+# wrapper Makefile in the build directory, so it must be started from
+# the source directory.
 $(2)_KCONFIG_MAKE = \
-	$$($(2)_MAKE_ENV) $$($(2)_MAKE) -C $$($(2)_BUILDDIR) \
+	$$($(2)_MAKE_ENV) $$($(2)_MAKE) \
+	$$(if $$($(2)_OUT_OF_TREE),-C $$($(2)_SRCDIR) O=$$($(2)_BUILDDIR),-C $$($(2)_BUILDDIR)) \
 		$$(PKG_KCONFIG_COMMON_OPTS) $$($(2)_KCONFIG_OPTS)
 
 # The specified source configuration file and any additional configuration file
-- 
2.34.1

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