kbuild: allow to start building external modules in any directory

Unless an explicit O= option is provided, external module builds must
start from the kernel directory.

This can be achieved by using the -C option:

  $ make -C /path/to/kernel M=/path/to/external/module

This commit allows starting external module builds from any directory,
so you can also do the following:

  $ make -f /path/to/kernel/Makefile M=/path/to/external/module

The key difference is that the -C option changes the working directory
and parses the Makefile located there, while the -f option only
specifies the Makefile to use.

As shown in the examples in Documentation/kbuild/modules.rst, external
modules usually have a wrapper Makefile that allows you to build them
without specifying any make arguments. The Makefile typically contains
a rule as follows:

    KDIR ?= /path/to/kernel
    default:
            $(MAKE) -C $(KDIR) M=$(CURDIR) $(MAKECMDGOALS)

The log will appear as follows:

    $ make
    make -C /path/to/kernel M=/path/to/external/module
    make[1]: Entering directory '/path/to/kernel'
    make[2]: Entering directory '/path/to/external/module'
      CC [M]  helloworld.o
      MODPOST Module.symvers
      CC [M]  helloworld.mod.o
      CC [M]  .module-common.o
      LD [M]  helloworld.ko
    make[2]: Leaving directory '/path/to/external/module'
    make[1]: Leaving directory '/path/to/kernel'

This changes the working directory twice because the -C option first
switches to the kernel directory, and then Kbuild internally recurses
back to the external module directory.

With this commit, the wrapper Makefile can directly include the kernel
Makefile:

    KDIR ?= /path/to/kernel
    export KBUILD_EXTMOD := $(realpath $(dir $(lastword $(MAKEFILE_LIST))))
    include $(KDIR)/Makefile

This avoids unnecessary sub-make invocations:

    $ make
      CC [M]  helloworld.o
      MODPOST Module.symvers
      CC [M]  helloworld.mod.o
      CC [M]  .module-common.o
      LD [M]  helloworld.ko

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

Documentation/kbuild/modules.rst

@@ -59,6 +59,12 @@ Command Syntax
 
 		$ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
 
+	Starting from Linux 6.13, you can use the -f option instead of -C. This
+	will avoid unnecessary change of the working directory. The external
+	module will be output to the directory where you invoke make.
+
+		$ make -f /lib/modules/`uname -r`/build/Makefile M=$PWD
+
 Options
 -------
 
@@ -221,6 +227,21 @@ Separate Kbuild File and Makefile
 	consisting of several hundred lines, and here it really pays
 	off to separate the kbuild part from the rest.
 
+	Linux 6.13 and later support another way. The external module Makefile
+	can include the kernel Makefile directly, rather than invoking sub Make.
+
+	Example 3::
+
+		--> filename: Kbuild
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o
+
+		--> filename: Makefile
+		KDIR ?= /lib/modules/$(shell uname -r)/build
+		export KBUILD_EXTMOD := $(realpath $(dir $(lastword $(MAKEFILE_LIST))))
+		include $(KDIR)/Makefile
+
+
 Building Multiple Modules
 -------------------------
 

Makefile

@@ -189,9 +189,13 @@ ifdef KBUILD_EXTMOD
         objtree := $(realpath $(KBUILD_OUTPUT))
         $(if $(objtree),,$(error specified kernel directory "$(KBUILD_OUTPUT)" does not exist))
     else
-        objtree := $(CURDIR)
+        objtree := $(abs_srctree)
     endif
-    output := $(or $(KBUILD_EXTMOD_OUTPUT),$(KBUILD_EXTMOD))
+    # If Make is invoked from the kernel directory (either kernel
+    # source directory or kernel build directory), external modules
+    # are built in $(KBUILD_EXTMOD) for backward compatibility,
+    # otherwise, built in the current directory.
+    output := $(or $(KBUILD_EXTMOD_OUTPUT),$(if $(filter $(CURDIR),$(objtree) $(abs_srctree)),$(KBUILD_EXTMOD)))
     # KBUILD_EXTMOD might be a relative path. Remember its absolute path before
     # Make changes the working directory.
     srcroot := $(realpath $(KBUILD_EXTMOD))