Message-Id: <20240911112017.14427-1-3090101217@zju.edu.cn>
Date: Wed, 11 Sep 2024 19:20:17 +0800
From: Jing Leng <3090101217@....edu.cn>
To: masahiroy@...nel.org,
	nathan@...nel.org,
	nicolas@...sle.eu,
	corbet@....net
Cc: linux-kbuild@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jing Leng <3090101217@....edu.cn>
Subject: [PATCH] kbuild: Support separating source and output of the external module

Revert commit "9a0ebe5011f4", originally when compiling external modules,
the compiled output can be separated from the source code with the
following command:
    make -C <kernel_src> O=<kernel_out> src=<module_src> M=<module_out>
It is used to https://github.com/lengjingzju/cbuild-ng (inc.mod.mk).

But "src" is an internal variable, so this patch adds a variable "S"
which is specifically used to change the default value of "src".

Fixes: 9a0ebe5011f4 ("kbuild: use $(obj)/ instead of $(src)/ for common pattern rules")
Signed-off-by: Jing Leng <3090101217@....edu.cn>
---
 Documentation/kbuild/modules.rst | 45 +++++++++++++++++++++++---------
 scripts/Makefile.build           | 28 ++++++++++----------
 scripts/Makefile.clean           |  2 +-
 scripts/Makefile.host            |  6 ++---
 scripts/Makefile.lib             |  2 +-
 scripts/Makefile.modpost         |  2 +-
 6 files changed, 52 insertions(+), 33 deletions(-)

diff --git a/Documentation/kbuild/modules.rst b/Documentation/kbuild/modules.rst
index 131863142cbb..1c4d3810b1dd 100644
--- a/Documentation/kbuild/modules.rst
+++ b/Documentation/kbuild/modules.rst
@@ -72,14 +72,23 @@ executed to make module versioning work.
 2.1 Command Syntax
 ==================
 
-	The command to build an external module is::
+	The full command to build an external module is::
 
-		$ make -C <path_to_kernel_src> M=$PWD
+		$ make -C <kernel_src> O=<kernel_out> S=<module_src> M=<module_out>
 
-	The kbuild system knows that an external module is being built
-	due to the "M=<dir>" option given in the command.
+	If <kernel_out> is the same as <kernel_src>, "O=" can be omitted::
 
-	To build against the running kernel use::
+		$ make -C <kernel_src> S=<module_src> M=<module_out>
+
+	If <module_out> is the same as <module_src>, "S=" can be omitted::
+
+		$ make -C <kernel_src> O=<kernel_out> M=<module_src>
+
+	If both of the above are satisfied, the command is::
+
+		$ make -C <kernel_src> M=<module_src>
+
+	So to build against the running kernel can use::
 
 		$ make -C /lib/modules/`uname -r`/build M=$PWD
 
@@ -91,20 +100,29 @@ executed to make module versioning work.
 2.2 Options
 ===========
 
-	($KDIR refers to the path of the kernel source directory.)
-
-	make -C $KDIR M=$PWD
+	make -C <kernel_src> O=<kernel_out> S=<module_src> M=<module_out>
 
-	-C $KDIR
+	-C <kernel_src>
 		The directory where the kernel source is located.
 		"make" will actually change to the specified directory
 		when executing and will change back when finished.
 
-	M=$PWD
+	O=<kernel_out>
+		The value given to "O" is the absolute path of the
+		directory where the kernel output (.config file) is located.
+		If <kernel_out> is the same as <kernel_src>,
+		"O=<kernel_out>" can be omitted.
+
+	S=<module_src>
+		The value given to "S" is the absolute path of the
+		directory where the external module source (kbuild file)
+		is located. If <module_out> is the same as <module_src>,
+		"S=<module_src>" can be omitted.
+
+	M=<module_out>
 		Informs kbuild that an external module is being built.
 		The value given to "M" is the absolute path of the
-		directory where the external module (kbuild file) is
-		located.
+		directory where the external module output is located.
 
 2.3 Targets
 ===========
@@ -112,7 +130,7 @@ executed to make module versioning work.
 	When building an external module, only a subset of the "make"
 	targets are available.
 
-	make -C $KDIR M=$PWD [target]
+	make -C <kernel_src> O=<kernel_out> S=<module_src> M=<module_out> [target]
 
 	The default will build the module(s) located in the current
 	directory, so a target does not need to be specified. All
@@ -151,6 +169,7 @@ executed to make module versioning work.
 		make -C $KDIR M=$PWD foo.ko
 		make -C $KDIR M=$PWD ./
 
+	($KDIR refers to the path of the kernel source directory.)
 
 3. Creating a Kbuild File for an External Module
 ================================================
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index a5ac8ed1936f..44b38cdb76df 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -3,7 +3,7 @@
 # Building
 # ==========================================================================
 
-src := $(if $(VPATH),$(VPATH)/)$(obj)
+src := $(if $(S),$(S),$(if $(VPATH),$(VPATH)/)$(obj))
 
 PHONY := $(obj)/
 $(obj)/:
@@ -113,13 +113,13 @@ endif
 quiet_cmd_cc_s_c = CC $(quiet_modtag)  $@
       cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS) $(CC_FLAGS_LTO), $(c_flags)) -fverbose-asm -S -o $@ $<
 
-$(obj)/%.s: $(obj)/%.c FORCE
+$(obj)/%.s: $(src)/%.c FORCE
 	$(call if_changed_dep,cc_s_c)
 
 quiet_cmd_cpp_i_c = CPP $(quiet_modtag) $@
 cmd_cpp_i_c       = $(CPP) $(c_flags) -o $@ $<
 
-$(obj)/%.i: $(obj)/%.c FORCE
+$(obj)/%.i: $(src)/%.c FORCE
 	$(call if_changed_dep,cpp_i_c)
 
 genksyms = scripts/genksyms/genksyms		\
@@ -133,7 +133,7 @@ cmd_gensymtypes_c = $(CPP) -D__GENKSYMS__ $(c_flags) $< | $(genksyms)
 quiet_cmd_cc_symtypes_c = SYM $(quiet_modtag) $@
       cmd_cc_symtypes_c = $(call cmd_gensymtypes_c,true,$@) >/dev/null
 
-$(obj)/%.symtypes : $(obj)/%.c FORCE
+$(obj)/%.symtypes : $(src)/%.c FORCE
 	$(call cmd,cc_symtypes_c)
 
 # LLVM assembly
@@ -141,7 +141,7 @@ $(obj)/%.symtypes : $(obj)/%.c FORCE
 quiet_cmd_cc_ll_c = CC $(quiet_modtag)  $@
       cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -fno-discard-value-names -o $@ $<
 
-$(obj)/%.ll: $(obj)/%.c FORCE
+$(obj)/%.ll: $(src)/%.c FORCE
 	$(call if_changed_dep,cc_ll_c)
 
 # C (.c) files
@@ -240,7 +240,7 @@ define rule_as_o_S
 endef
 
 # Built-in and composite module parts
-$(obj)/%.o: $(obj)/%.c $(recordmcount_source) FORCE
+$(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
 	$(call if_changed_rule,cc_o_c)
 	$(call cmd,force_checksrc)
 
@@ -257,7 +257,7 @@ quiet_cmd_cc_lst_c = MKLST   $@
 		     $(CONFIG_SHELL) $(srctree)/scripts/makelst $*.o \
 				     System.map $(OBJDUMP) > $@
 
-$(obj)/%.lst: $(obj)/%.c FORCE
+$(obj)/%.lst: $(src)/%.c FORCE
 	$(call if_changed_dep,cc_lst_c)
 
 # Compile Rust sources (.rs)
@@ -290,7 +290,7 @@ rust_common_cmd = \
 quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
       cmd_rustc_o_rs = $(rust_common_cmd) --emit=obj=$@ $<
 
-$(obj)/%.o: $(obj)/%.rs FORCE
+$(obj)/%.o: $(src)/%.rs FORCE
 	+$(call if_changed_dep,rustc_o_rs)
 
 quiet_cmd_rustc_rsi_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
@@ -298,19 +298,19 @@ quiet_cmd_rustc_rsi_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
 	$(rust_common_cmd) -Zunpretty=expanded $< >$@; \
 	command -v $(RUSTFMT) >/dev/null && $(RUSTFMT) $@
 
-$(obj)/%.rsi: $(obj)/%.rs FORCE
+$(obj)/%.rsi: $(src)/%.rs FORCE
 	+$(call if_changed_dep,rustc_rsi_rs)
 
 quiet_cmd_rustc_s_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
       cmd_rustc_s_rs = $(rust_common_cmd) --emit=asm=$@ $<
 
-$(obj)/%.s: $(obj)/%.rs FORCE
+$(obj)/%.s: $(src)/%.rs FORCE
 	+$(call if_changed_dep,rustc_s_rs)
 
 quiet_cmd_rustc_ll_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
       cmd_rustc_ll_rs = $(rust_common_cmd) --emit=llvm-ir=$@ $<
 
-$(obj)/%.ll: $(obj)/%.rs FORCE
+$(obj)/%.ll: $(src)/%.rs FORCE
 	+$(call if_changed_dep,rustc_ll_rs)
 
 # Compile assembler sources (.S)
@@ -336,14 +336,14 @@ cmd_gensymtypes_S =                                                         \
 quiet_cmd_cc_symtypes_S = SYM $(quiet_modtag) $@
       cmd_cc_symtypes_S = $(call cmd_gensymtypes_S,true,$@) >/dev/null
 
-$(obj)/%.symtypes : $(obj)/%.S FORCE
+$(obj)/%.symtypes : $(src)/%.S FORCE
 	$(call cmd,cc_symtypes_S)
 
 
 quiet_cmd_cpp_s_S = CPP $(quiet_modtag) $@
 cmd_cpp_s_S       = $(CPP) $(a_flags) -o $@ $<
 
-$(obj)/%.s: $(obj)/%.S FORCE
+$(obj)/%.s: $(src)/%.S FORCE
 	$(call if_changed_dep,cpp_s_S)
 
 quiet_cmd_as_o_S = AS $(quiet_modtag)  $@
@@ -358,7 +358,7 @@ cmd_gen_symversions_S = $(call gen_symversions,S)
 
 endif
 
-$(obj)/%.o: $(obj)/%.S FORCE
+$(obj)/%.o: $(src)/%.S FORCE
 	$(call if_changed_rule,as_o_S)
 
 targets += $(filter-out $(subdir-builtin), $(real-obj-y))
diff --git a/scripts/Makefile.clean b/scripts/Makefile.clean
index 4fcfab40ed61..6ce7514aefc8 100644
--- a/scripts/Makefile.clean
+++ b/scripts/Makefile.clean
@@ -3,7 +3,7 @@
 # Cleaning up
 # ==========================================================================
 
-src := $(if $(VPATH),$(VPATH)/)$(obj)
+src := $(if $(S),$(S),$(if $(VPATH),$(VPATH)/)$(obj))
 
 PHONY := __clean
 __clean:
diff --git a/scripts/Makefile.host b/scripts/Makefile.host
index e85be7721a48..3c17e6ba421c 100644
--- a/scripts/Makefile.host
+++ b/scripts/Makefile.host
@@ -112,7 +112,7 @@ endif
 quiet_cmd_host-csingle 	= HOSTCC  $@
       cmd_host-csingle	= $(HOSTCC) $(hostc_flags) $(KBUILD_HOSTLDFLAGS) -o $@ $< \
 		$(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(target-stem))
-$(host-csingle): $(obj)/%: $(obj)/%.c FORCE
+$(host-csingle): $(obj)/%: $(src)/%.c FORCE
 	$(call if_changed_dep,host-csingle)
 
 # Link an executable based on list of .o files, all plain c
@@ -129,7 +129,7 @@ $(call multi_depend, $(host-cmulti), , -objs)
 # host-cobjs -> .o
 quiet_cmd_host-cobjs	= HOSTCC  $@
       cmd_host-cobjs	= $(HOSTCC) $(hostc_flags) -c -o $@ $<
-$(host-cobjs): $(obj)/%.o: $(obj)/%.c FORCE
+$(host-cobjs): $(obj)/%.o: $(src)/%.c FORCE
 	$(call if_changed_dep,host-cobjs)
 
 # Link an executable based on list of .o files, a mixture of .c and .cc
@@ -146,7 +146,7 @@ $(call multi_depend, $(host-cxxmulti), , -objs -cxxobjs)
 # Create .o file from a single .cc (C++) file
 quiet_cmd_host-cxxobjs	= HOSTCXX $@
       cmd_host-cxxobjs	= $(HOSTCXX) $(hostcxx_flags) -c -o $@ $<
-$(host-cxxobjs): $(obj)/%.o: $(obj)/%.cc FORCE
+$(host-cxxobjs): $(obj)/%.o: $(src)/%.cc FORCE
 	$(call if_changed_dep,host-cxxobjs)
 
 # Create executable from a single Rust crate (which may consist of
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 207325eaf1d1..6f961aa5790d 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -430,7 +430,7 @@ DT_TMP_SCHEMA := $(objtree)/$(DT_BINDING_DIR)/processed-schema.json
 dtb-check-enabled = $(if $(filter %.dtb, $@),y)
 endif
 
-$(obj)/%.dtb: $(obj)/%.dts $(DTC) $(DT_TMP_SCHEMA) FORCE
+$(obj)/%.dtb: $(src)/%.dts $(DTC) $(DT_TMP_SCHEMA) FORCE
 	$(call if_changed_dep,dtc)
 
 $(obj)/%.dtbo: $(src)/%.dtso $(DTC) FORCE
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 44936ebad161..12afe2acbe05 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -112,7 +112,7 @@ else
 
 # set src + obj - they may be used in the modules's Makefile
 obj := $(KBUILD_EXTMOD)
-src := $(if $(VPATH),$(VPATH)/)$(obj)
+src := $(if $(S),$(S),$(if $(VPATH),$(VPATH)/)$(obj))
 
 # Include the module's Makefile to find KBUILD_EXTRA_SYMBOLS
 include $(kbuild-file)
-- 
2.25.1

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