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   its prerequisites is out of date.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 54a702f7 ("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 <linux/device.h>.

  [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 <linux/device.h>
            |          ^~~~~~~~~~~~~~~~
      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 <linux/device.h>, and rebuilds objects
  that depended on <linux/device.h>. There exists a source file that
  includes <linux/device.h>, and it raises an error.

To see detailed background information, refer to commit 2d3b1b8f
("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/?63417Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>