On Tue, Sep 17, 2024 at 11:16:51PM +0900, Masahiro Yamada wrote: > 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/exernal/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/exernal/module' s/exernal/external/, also above. > 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 = $(CURDIR) > 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@xxxxxxxxxx> > --- > > Documentation/kbuild/modules.rst | 21 +++++++++++++++++++++ > Makefile | 8 ++++++-- > 2 files changed, 27 insertions(+), 2 deletions(-) > > diff --git a/Documentation/kbuild/modules.rst b/Documentation/kbuild/modules.rst > index 03347e13eeb5..5610cfa426bb 100644 > --- a/Documentation/kbuild/modules.rst > +++ b/Documentation/kbuild/modules.rst > @@ -88,6 +88,12 @@ executed to make module versioning work. > > $ 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=$PWD > + > 2.2 Options > =========== > > @@ -246,6 +252,21 @@ module 8123.ko, which is built from the following files:: > consisting of several hundred lines, and here it really pays > off to separate the kbuild part from the rest. > > + Linux 6.13 and later support another way. The external module Makefile > + can include the kernel Makefile directly, rather than invoking sub Make. > + > + Example 3:: > + > + --> filename: Kbuild > + obj-m := 8123.o > + 8123-y := 8123_if.o 8123_pci.o > + > + --> filename: Makefile > + KDIR ?= /lib/modules/`uname -r`/build KDIR ?= /lib/modules/$(shell uname -r)/build This patch is another very good idea, thanks! Reviewed-by: Nicolas Schier <nicolas@xxxxxxxxx> > + export KBUILD_EXTMOD = $(CURDIR) > + include $(KDIR)/Makefile > + > + > 3.4 Building Multiple Modules > ============================= > > diff --git a/Makefile b/Makefile > index 81603a50c757..88c160ac7b3c 100644 > --- a/Makefile > +++ b/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. > export abs_extmodtree := $(realpath $(KBUILD_EXTMOD)) > -- > 2.43.0 > -- Nicolas
