How to add a dependency on a generated source file in Kbuild? - linux-kernel

Question

Let generate_testapi.py be a script in my Linux kernel module's source tree, that ingests mymod_test.h and generates a interface source file toward userland (ioctl, debugfs, you name it), and lets name this $(obj)/mymod_test_interfaces.gen.c.
In the Kbuild makefile let mymod-y be the variable containing the list of object files that form the module, i.e.
How does a Kbuild-ish rule look like, that adds the object file compiles from the generates source as a dependency of mymod and describes the generation process.
The following, my first naive attempt on such a rule set does not work.
obj-m := mymod.o
mymod-y := \
mymod_kmod.o \
$(obj)/mymod_test_interfaces.gen.o
$(obj)/mymod_test_interfaces.gen.o: $(src)/mymod_test.h $(src)/generate_testapi.py
$(src)/generate_testapi.py < $(src)/mymod_test.h > $<
Trying to make with that, the resulting error is, that there is no rule to make mymod_test_interfaces.gen.o.
Update (due to comment by Alexandre Belloni)
Yes, I also tried a generator rule of the form
$(obj)/mymod_test_interfaces.gen.c: $(src)/mymod_test.h $(src)/generate_testapi.py
$(src)/generate_testapi.py < $(src)/mymod_test.h > $<
with the same result, which is, that it does not work.

Answer 1

There is another issue: you can't have a C file that has the same name as the module when trying to build a multiple file module. Rename mymod.c to mymod-core.c
Then, the following should work:
obj-m := mymod.o
mymod-objs := mymod-core.o mymod_test_interfaces.gen.o
$(obj)/mymod_test_interfaces.gen.c: $(src)/mymod_test.h $(src)/generate_testapi.py
$(src)/generate_testapi.py < $(src)/mymod_test.h > $#
I've actually tested the following:
diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
index 4387ccb79e64..47fdc3a73c75 100644
--- a/drivers/misc/Makefile
+++ b/drivers/misc/Makefile
## -71,3 +71,8 ## OBJCOPYFLAGS_lkdtm_rodata_objcopy.o := \
--rename-section .text=.rodata
$(obj)/lkdtm_rodata_objcopy.o: $(obj)/lkdtm_rodata.o
$(call if_changed,objcopy)
+
+obj-m := mymod.o
+mymod-objs := mymod-core.o mymod_test_interfaces.gen.o
+$(obj)/mymod_test_interfaces.gen.c:
+ echo "int variable;" > $#
diff --git a/drivers/misc/mymod-core.c b/drivers/misc/mymod-core.c
new file mode 100644
index 000000000000..b39aa4b9ae8d
--- /dev/null
+++ b/drivers/misc/mymod-core.c
## -0,0 +1,19 ##
+#include <linux/module.h>
+#include <linux/platform_device.h>
+
+extern int mymod_variable;
+
+static int mymod_probe(struct platform_device *pdev)
+{
+ mymod_variable = 2;
+
+ return 0;
+}
+
+static struct platform_driver mymod_driver = {
+ .driver = {
+ .name = "mymod",
+ },
+ .probe = mymod_probe,
+};
+module_platform_driver(mymod_driver);
It properly results in:
$ make
CHK include/config/kernel.release
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
CHK include/generated/bounds.h
CHK include/generated/timeconst.h
CHK include/generated/asm-offsets.h
CALL scripts/checksyscalls.sh
CHK include/generated/compile.h
CC [M] drivers/misc/mymod-core.o
echo "int variable;" > drivers/misc/mymod_test_interfaces.gen.c
CC [M] drivers/misc/mymod_test_interfaces.gen.o
LD [M] drivers/misc/mymod.o
LD arch/x86/boot/compressed/vmlinux
ZOFFSET arch/x86/boot/zoffset.h
AS arch/x86/boot/header.o
LD arch/x86/boot/setup.elf
OBJCOPY arch/x86/boot/setup.bin
OBJCOPY arch/x86/boot/vmlinux.bin
BUILD arch/x86/boot/bzImage