linux/scripts/Kbuild.include, branch master

kbuild: add $(objtree)/ prefix to some in-kernel build artifacts

2024-11-27T00:38:27Z

$(objtree) refers to the top of the output directory of kernel builds. This commit adds the explicit $(objtree)/ prefix to build artifacts needed for building external modules. This change has no immediate impact, as the top-level Makefile currently defines: objtree := . This commit prepares for supporting the building of external modules in a different directory. Signed-off-by: Masahiro Yamada Reviewed-by: Nicolas Schier

kbuild: raise the minimum GNU Make requirement to 4.0

2024-07-16T07:07:14Z

RHEL/CentOS 7, popular distributions that install GNU Make 3.82, reached EOM/EOL on June 30, 2024. While you may get extended support, it is a good time to raise the minimum GNU Make version. The new requirement, GNU Make 4.0, was released in October, 2013. I did not touch the Makefiles under tools/ because I do not know the requirements for building tools. I do not find any GNU Make version checks under tools/. Signed-off-by: Masahiro Yamada Reviewed-by: Nathan Chancellor

kbuild: use $(src) instead of $(srctree)/$(src) for source directory

2024-05-09T19:34:52Z

Kbuild conventionally uses $(obj)/ for generated files, and $(src)/ for checked-in source files. It is merely a convention without any functional difference. In fact, $(obj) and $(src) are exactly the same, as defined in scripts/Makefile.build: src := $(obj) When the kernel is built in a separate output directory, $(src) does not accurately reflect the source directory location. While Kbuild resolves this discrepancy by specifying VPATH=$(srctree) to search for source files, it does not cover all cases. For example, when adding a header search path for local headers, -I$(srctree)/$(src) is typically passed to the compiler. This introduces inconsistency between upstream and downstream Makefiles because $(src) is used instead of $(srctree)/$(src) for the latter. To address this inconsistency, this commit changes the semantics of $(src) so that it always points to the directory in the source tree. Going forward, the variables used in Makefiles will have the following meanings: $(obj) - directory in the object tree $(src) - directory in the source tree (changed by this commit) $(objtree) - the top of the kernel object tree $(srctree) - the top of the kernel source tree Consequently, $(srctree)/$(src) in upstream Makefiles need to be replaced with $(src). Signed-off-by: Masahiro Yamada Reviewed-by: Nicolas Schier

kbuild: simplify dtbs_install by reading the list of compiled DTBs

2024-02-19T09:20:39Z

Retrieve the list of *.dtb(o) files from arch/*/boot/dts/dtbs-list instead of traversing the directory tree again. Please note that 'make dtbs_install' installs *.dtb(o) files directly added to dtb-y because scripts/Makefile.dtbinst installs $(dtb-y) without expanding the -dtbs suffix. This commit preserves this behavior. Signed-off-by: Masahiro Yamada

kbuild: replace $(dot-target).tmp in filechk with $(tmp-target)

2023-01-22T14:43:33Z

$(tmp-target) is a better fit for local use like this. Signed-off-by: Masahiro Yamada Reviewed-by: Nicolas Schier

kbuild: add more comments for KBUILD_NOCMDDEP=1

2023-01-22T14:43:33Z

The cmd-check for KBUILD_NOCMDDEP=1 may not be clear until you see commit c4d5ee13984f ("kbuild: make KBUILD_NOCMDDEP=1 handle empty built-in.o"). When a phony target (i.e. FORCE) is the only prerequisite, Kbuild uses a tricky way to detect that the target does not exist. Add more comments. Signed-off-by: Masahiro Yamada Reviewed-by: Nicolas Schier

kbuild: rename cmd_$@ to savedcmd_$@ in *.cmd files

2023-01-22T14:43:33Z

The cmd-check macro compares $(cmd_$@) and $(cmd_$1), but a pitfall is that you cannot use cmd_ as the variable name for the command. For example, the following code will not work in the top Makefile or ./Kbuild. quiet_cmd_foo = GEN $@ cmd_foo = touch $@ targets += foo foo: FORCE $(call if_changed,foo) In this case, both $@ and $1 are expanded to 'foo', so $(cmd_check) is always empty. We do not need to use the same prefix for cmd_$@ and cmd_$1. Rename the former to savedcmd_$@. Signed-off-by: Masahiro Yamada Reviewed-by: Nicolas Schier

kbuild: allow to combine multiple V= levels

2023-01-22T14:43:32Z

Commit a6de553da01c ("kbuild: Allow to combine multiple W= levels") supported W=123 to enable all the extra warning groups. I think a similar idea is applicable to the V= option. V=1 echos the whole command V=2 prints the reason for rebuilding These are orthogonal, and can be enabled at the same time. This commit supports V=12 to enable both of them. Signed-off-by: Masahiro Yamada Tested-by: Nicolas Schier Reviewed-by: Nicolas Schier

kbuild: print short log in addition to the whole command with V=1

2023-01-22T14:43:32Z

"make V=1" prints the whole command instead of the short log, but I think it is nicer to print both so that you can easily spot the build rule of your interest. This commit changes V=1 to print the short log (the line starts with '#'), followed by the full log. In parallel builds, the short/full logs from the same build rule may be interspersed. If you want to avoid it, please add -Otarget option. Kbuild will never set it by default because Make would buffer the logs and lose the escape sequences. (Modern compilers print warnings and errors in color, but only when they write to a terminal.) This is also a preparation for supporting V=12 because V=2 appends the reason for rebuilding to the short log. Signed-off-by: Masahiro Yamada Tested-by: Nicolas Schier Reviewed-by: Nicolas Schier

kbuild: use .NOTINTERMEDIATE for future GNU Make versions

2022-12-13T13:29:10Z

In Kbuild, some files are generated by chains of pattern/implicit rules. For example, *.dtb.o files in drivers/of/unittest-data/Makefile are generated by the chain of 3 pattern rules, like this: %.dts -> %.dtb -> %.dtb.S -> %.dtb.o Here, %.dts is the real source, %.dtb.o is the final target. %.dtb and %.dtb.S are called "intermediate files". As GNU Make manual [1] says, intermediate files are treated differently in two ways: (a) The first difference is what happens if the intermediate file does not exist. If an ordinary file 'b' does not exist, and make considers a target that depends on 'b', it invariably creates 'b' and then updates the target from 'b'. But if 'b' is an intermediate file, then make can leave well enough alone: it won't create 'b' unless one of its prerequisites is out of date. This means the target depending on 'b' won't be rebuilt either, unless there is some other reason to update that target: for example the target doesn't exist or a different prerequisite is newer than the target. (b) The second difference is that if make does create 'b' in order to update something else, it deletes 'b' later on after it is no longer needed. Therefore, an intermediate file which did not exist before make also does not exist after make. make reports the deletion to you by printing a 'rm' command showing which file it is deleting. The combination of these is problematic for Kbuild because most of the build rules depend on FORCE and the if_changed* macros really determine if the target should be updated. So, all missing files, whether they are intermediate or not, are always rebuilt. To see the problem, delete ".SECONDARY:" from scripts/Kbuild.include, and repeat this command: $ make allmodconfig drivers/of/unittest-data/ The intermediate files will be deleted, which results in rebuilding intermediate and final objects in the next run of make. In the old days, people suppressed (b) in inconsistent ways. As commit 54a702f70589 ("kbuild: mark $(targets) as .SECONDARY and remove .PRECIOUS markers") noted, you should not use .PRECIOUS because .PRECIOUS has the following behavior (c), which is not likely what you want. (c) If make is killed or interrupted during the execution of their recipes, the target is not deleted. Also, the target is not deleted on error even if .DELETE_ON_ERROR is specified. .SECONDARY is a much better way to disable (b), but a small problem is that .SECONDARY enables (a), which gives a side-effect to $?; prerequisites marked as .SECONDARY do not appear in $?. This is a drawback for Kbuild. I thought it was a bug and opened a bug report. As Paul, the GNU Make maintainer, concluded in [2], this is not a bug. A good news is that, GNU Make 4.4 added the perfect solution, .NOTINTERMEDIATE, which cancels both (a) and (b). For clarificaton, my understanding of .INTERMEDIATE, .SECONDARY, .PRECIOUS and .NOTINTERMEDIATE are as follows: (a) (b) (c) .INTERMEDIATE enable enable disable .SECONDARY enable disable disable .PRECIOUS disable disable enable .NOTINTERMEDIATE disable disable disable However, GNU Make 4.4 has a bug for the global .NOTINTERMEDIATE. [3] It was fixed by commit 6164608900ad ("[SV 63417] Ensure global .NOTINTERMEDIATE disables all intermediates"), and will be available in the next release of GNU Make. The following is the gain for .NOTINTERMEDIATE: [Current Make] $ make allnoconfig vmlinux [ full build ] $ rm include/linux/device.h $ make vmlinux CALL scripts/checksyscalls.sh Make does not notice the removal of . [Future Make] $ make-latest allnoconfig vmlinux [ full build ] $ rm include/linux/device.h $ make-latest vmlinux CC arch/x86/kernel/asm-offsets.s In file included from ./include/linux/writeback.h:13, from ./include/linux/memcontrol.h:22, from ./include/linux/swap.h:9, from ./include/linux/suspend.h:5, from arch/x86/kernel/asm-offsets.c:13: ./include/linux/blk_types.h:11:10: fatal error: linux/device.h: No such file or directory 11 | #include | ^~~~~~~~~~~~~~~~ compilation terminated. make-latest[1]: *** [scripts/Makefile.build:114: arch/x86/kernel/asm-offsets.s] Error 1 make-latest: *** [Makefile:1282: prepare0] Error 2 Make notices the removal of , and rebuilds objects that depended on . There exists a source file that includes , and it raises an error. To see detailed background information, refer to commit 2d3b1b8f0da7 ("kbuild: drop $(wildcard $^) check in if_changed* for faster rebuild"). [1]: https://www.gnu.org/software/make/manual/make.html#Chained-Rules [2]: https://savannah.gnu.org/bugs/?55532 [3]: https://savannah.gnu.org/bugs/?63417 Signed-off-by: Masahiro Yamada