This functionally reverts commit bfd77145 ("Makefile: Convert
-Wimplicit-fallthrough=3 to just -Wimplicit-fallthrough for clang").

clang enabled support for -Wimplicit-fallthrough in C in r369414 [1],
which causes a lot of warnings when building the kernel for two reasons:

1. Clang does not support the /* fall through */ comments. There seems
   to be a general consensus in the LLVM community that this is not
   something they want to support. Joe Perches wrote a script to convert
   all of the comments to a "fallthrough" keyword that will be added to
   compiler_attributes.h [2] [3], which catches the vast majority of the
   comments. There doesn't appear to be any consensus in the kernel
   community when to do this conversion.

2. Clang and GCC disagree about falling through to final case statements
   with no content or cases that simply break:

   https://godbolt.org/z/c8csDu

   This difference contributes at least 50 warnings in an allyesconfig
   build for x86, not considering other architectures. This difference
   will need to be discussed to see which compiler is right [4] [5].

[1]: https://github.com/llvm/llvm-project/commit/1e0affb6e564b7361b0aadb38805f26deff4ecfc
[2]: https://lore.kernel.org/lkml/61ddbb86d5e68a15e24ccb06d9b399bbf5ce2da7.camel@perches.com/
[3]: https://lore.kernel.org/lkml/1d2830aadbe9d8151728a7df5b88528fc72a0095.1564549413.git.joe@perches.com/
[4]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91432
[5]: https://github.com/ClangBuiltLinux/linux/issues/636

Given these two problems need discussion and coordination, do not enable
-Wimplicit-fallthrough with clang right now. Add a comment to explain
what is going on as well. This commit should be reverted once these two
issues are fully flushed out and resolved.
Suggested-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NNathan Chancellor <natechancellor@gmail.com>
Acked-by: NMiguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Acked-by: NNick Desaulniers <ndesaulniers@google.com>
Acked-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Move the outputmakefile target to the leftmost in the prerequisite list
so that this is checked first. GNU Make processes the prerequisites
left to right.

GNU Make will keep to stick to this behavior, and it seems even
POSIX standard, according to this:
https://lists.gnu.org/archive/html/bug-make/2019-08/msg00030.html

The POSIX standard of make is available here:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/make.html

Of course, when the parallel option -j given, other targets will be run
simultaneously but it is nice to show the error as early as possible.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Now prepare3 does nothing but depends on include/config/kernel.release
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

With this commit, the error report is shown earlier, even before
running kconfig.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

If you try out-of-tree build with an unclean source tree, Kbuild
suggests to run make mrproper. The path to the source tree may be
shown with a relative path, for example, "make O=foo" emits the
following:

  .. is not clean, please run 'make mrproper'
  in the '..' directory.

This is somewhat confusing if you ran "make O=foo" in the source tree.
Using the absolute path will be clearer.

This commit changes the error message like follows:

***
*** The source tree is not clean, please run 'make mrproper'
*** in /absolute/path/to/linux
***
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Since commit 3a475b21 ("kbuild: Inform user to pass ARCH= for make
mrproper"), if you try out-of-tree build with an unclean source tree,
it suggests to run 'make ARCH=<ARCH> mrproper'.

This looks odd when you are not cross-compiling the kernel. Show the
'ARCH=<ARCH>' part only when ARCH= was given from the command line.
If ARCH is the default (native build) or came from the environment,
it should simply suggest 'make mrproper' as before.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

You already know the location of the source tree without this message.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Commit 415008af ("docs-rst: convert lsm from DocBook to ReST")
stopped using if_changed_rule. There is no more users of if_changed*
for the doc targets. Hence, fixdep is unneeded. Remove the dependency
on scripts_basic.

All the doc targets are phony. The dependency on FORCE is not needed
either.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

This line contains $(MAKE), so Make knows that it will invoke sub-make
without help of the '+' marker.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

scripts/package/Makefile does not use $(obj) or $(src) at all.
It actually generates files and directories in the top of $(objtree).
I do not see much sense in descending into scripts/package/.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

I am not a big fan of the $(objtree)/ hack for clean-files/clean-dirs.

These are created in the top of $(objtree), so let's clean them up
from the top Makefile.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

I think splitting the modpost and linking modules into separate
Makefiles will be useful especially when more complex build steps
come in. The main motivation of this commit is to integrate the
proposed klp-convert feature cleanly.

I moved the logging 'Building modules, stage 2.' to Makefile.modpost
to avoid the code duplication although I do not know whether or not
this message is needed in the first place.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Currently, the timestamp of module linker scripts are not checked.
Add them to the dependency of modules so they are correctly rebuilt.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Currently, the single target build directly descends into the directory
of the target. For example,

  $ make foo/bar/baz.o

... directly descends into foo/bar/.

On the other hand, the normal build usually descends one directory at
a time, i.e. descends into foo/, and then foo/bar/.

This difference causes some problems.

[1] miss subdir-asflags-y, subdir-ccflags-y in upper Makefiles

    The options in subdir-{as,cc}flags-y take effect in the current
    and its sub-directories. In other words, they are inherited
    downward. In the example above, the single target will miss
    subdir-{as,cc}flags-y if they are defined in foo/Makefile.

[2] could be built in a different directory

    As Documentation/kbuild/modules.rst section 4.3 says, Kbuild can
    handle files that are spread over several sub-directories.

    The build rule of foo/bar/baz.o may not necessarily be specified in
    foo/bar/Makefile. It might be specifies in foo/Makefile as follows:

    [foo/Makefile]
    obj-y := bar/baz.o

    This often happens when a module is so big that its source files
    are divided into sub-directories.

    In this case, there is no Makefile in the foo/bar/ directory, yet
    the single target descends into foo/bar/, then fails due to the
    missing Makefile. You can still do 'make foo/bar/' for partial
    building, but cannot do 'make foo/bar/baz.s'. I believe the single
    target '%.s' is a useful feature for inspecting the compiler output.

    Some modules work around this issue by putting an empty Makefile
    in every sub-directory.

This commit fixes those problems by making the single target build
descend in the same way as the normal build does.

Another change is the single target build will observe the CONFIG
options. Previously, it allowed users to build the foo.o even when
the corresponding CONFIG_FOO is disabled:

   obj-$(CONFIG_FOO) += foo.o

In the new behavior, the single target build will just fail and show
"No rule to make target ..." (or "Nothing to be done for ..." if the
stale object already exists, but cannot be updated).

The disadvantage of this commit is the build speed. Now that the
single target build visits every directory and parses lots of
Makefiles, it is slower than before. (But, I hope it will not be
too slow.)
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Factor out the duplicated code for in-kernel and external module
cleaning.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

The in-kernel build and external module build have similar code
for descending into sub-directories.

Factor out the code into the common place.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

There is no need to set 0 to variables such as config-targets,
mixed-targets, etc.

Unset instead of setting 0 in order to use 'ifdef' to test them.

I also renamed:

  config-targets  ->  config-build
  mixed-targets   ->  mixed-build
  dot-config      ->  need-config

to clarify what we are doing.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

'make clean' descends into ./Kbuild, but does not clean anything
since everything is added to no-clean-files.

There is no need to descend to ./Kbuild in the first place.
We can drop the no-clean-files assignment.

With this, there is no more user of no-clean-files. I will keep it
for a while to see whether a new user will appear.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

'make /' is just an alias for 'make ./'; this builds all objects of an
external module, but skips the modpost stage.

I am not a big fan of 'make /' since it looks as if it were touching
the root directory of the system. I like 'make ./' better.

I do not know how many people are using it, but let's show a hint if
it is used. Also, move it close to the external module rules since
this only makes sense for external modules.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

$(objtree)/Module.symvers is not required for descending into
sub-directories. It is needed for the modpost stage.

Move the Module.symvers check to the right place.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

A compilation -Wimplicit-fallthrough warning was enabled by commit
a035d552 ("Makefile: Globally enable fall-through warning")

Even though clang 10.0.0 does not currently support this warning without
a patch, clang currently does not support a value for this option.

  Link: https://bugs.llvm.org/show_bug.cgi?id=39382

The gcc default for this warning is 3 so removing the =3 has no effect
for gcc and enables the warning for patched versions of clang.

Also remove the =3 from an existing use in a parisc Makefile:
arch/parisc/math-emu/Makefile
Signed-off-by: NJoe Perches <joe@perches.com>
Reviewed-and-tested-by: NNathan Chancellor <natechancellor@gmail.com>
Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Since commit ff9b45c5 ("kbuild: modpost: read modules.order instead
of $(MODVERDIR)/*.mod"), a module is no longer built in the following
pattern:

  [Makefile]
  subdir-y := some-module

  [some-module/Makefile]
  obj-m := some-module.o

You cannot write Makefile this way in upstream because modules.order is
not correctly generated. subdir-y is used to descend to a sub-directory
that builds tools, device trees, etc.

For external modules, the modules order does not matter. So, the
Makefile above was known to work.

I believe the Makefile should be re-written as follows:

  [Makefile]
  obj-m := some-module/

  [some-module/Makefile]
  obj-m := some-module.o

However, people will have no idea if their Makefile suddenly stops
working. In fact, I received questions from multiple people.

Show a warning for a while if obj-m is specified in a Makefile visited
by subdir-y or subdir-m.

I touched the %/ rule to avoid false-positive warnings for the single
target.

Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Tom Stonecypher <thomas.edwardx.stonecypher@intel.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: NJan Kiszka <jan.kiszka@siemens.com>

I removed the single target %.ko in commit ff9b45c5 ("kbuild:
modpost: read modules.order instead of $(MODVERDIR)/*.mod") because
the modpost stage does not work reliably. For instance, the module
dependency, modversion, etc. do not work if we lack symbol information
from the other modules.

Yet, some people still want to build only one module in their interest,
and it may be still useful if it is used within those limitations.

Fixes: ff9b45c5 ("kbuild: modpost: read modules.order instead of $(MODVERDIR)/*.mod")
Reported-by: NDon Brace <don.brace@microsemi.com>
Reported-by: NArend Van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Define and export OBJSIZE variable for "size" tool from binutils to be
used in architecture specific Makefiles (naming the variable just "SIZE"
would be too risky). In particular this tool is useful to perform checks
that early boot code is not using bss section (which might have not been
zeroed yet or intersects with initrd or other files boot loader might
have put right after the linux kernel).

Link: http://lkml.kernel.org/r/patch-1.thread-2257a1.git-188f5a3d81d5.your-ad-here.call-01565088755-ext-5120@work.hoursAcked-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NVasily Gorbik <gor@linux.ibm.com>

CLANG_FLAGS is initialized by the following line:

  CLANG_FLAGS     := --target=$(notdir $(CROSS_COMPILE:%-=%))

..., which is run only when CROSS_COMPILE is set.

Some build targets (bindeb-pkg etc.) recurse to the top Makefile.

When you build the kernel with Clang but without CROSS_COMPILE,
the same compiler flags such as -no-integrated-as are accumulated
into CLANG_FLAGS.

If you run 'make CC=clang' and then 'make CC=clang bindeb-pkg',
Kbuild will recompile everything needlessly due to the build command
change.

Fix this by correctly initializing CLANG_FLAGS.

Fixes: 238bcbc4 ("kbuild: consolidate Clang compiler flags")
Cc: <stable@vger.kernel.org> # v5.0+
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NNathan Chancellor <natechancellor@gmail.com>
Acked-by: NNick Desaulniers <ndesaulniers@google.com>

Now that all the fall-through warnings have been addressed in the
kernel, enable the fall-through warning globally.

Also, update the deprecated.rst file to include implicit fall-through
as 'deprecated' so people can be pointed to a single location for
justification.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>

The gcc -fcf-protection=branch option is not compatible with
-mindirect-branch=thunk-extern. The latter is used when
CONFIG_RETPOLINE is selected, and this will fail to build with
a gcc which has -fcf-protection=branch enabled by default. Adding
-fcf-protection=none when building with retpoline enabled
prevents such build failures.
Signed-off-by: NSeth Forshee <seth.forshee@canonical.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Now that there is no rule for 'prepare1', it can go away.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

While descending directories, Kbuild produces objects for modules,
but do not link final *.ko files; it is done in the modpost.

To keep track of modules, Kbuild creates a *.mod file in $(MODVERDIR)
for every module it is building. Some post-processing steps read the
necessary information from *.mod files. This avoids descending into
directories again. This mechanism was introduced in 2003 or so.

Later, commit 551559e1 ("kbuild: implement modules.order") added
modules.order. So, we can simply read it out to know all the modules
with directory paths. This is easier than parsing the first line of
*.mod files.

$(MODVERDIR) has a flat directory structure, that is, *.mod files
are named only with base names. This is based on the assumption that
the module name is unique across the tree. This assumption is really
fragile.

Stephen Rothwell reported a race condition caused by a module name
conflict:

  https://lkml.org/lkml/2019/5/13/991

In parallel building, two different threads could write to the same
$(MODVERDIR)/*.mod simultaneously.

Non-unique module names are the source of all kind of troubles, hence
commit 3a48a919 ("kbuild: check uniqueness of module names")
introduced a new checker script.

However, it is still fragile in the build system point of view because
this race happens before scripts/modules-check.sh is invoked. If it
happens again, the modpost will emit unclear error messages.

To fix this issue completely, create *.mod with full directory path
so that two threads never attempt to write to the same file.

$(MODVERDIR) is no longer needed.

Since modules with directory paths are listed in modules.order, Kbuild
is still able to find *.mod files without additional descending.

I also killed cmd_secanalysis; scripts/mod/sumversion.c computes MD4 hash
for modules with MODULE_VERSION(). When CONFIG_DEBUG_SECTION_MISMATCH=y,
it occurs not only in the modpost stage, but also during directory
descending, where sumversion.c may parse stale *.mod files. It would emit
'No such file or directory' warning when an object consisting a module is
renamed, or when a single-obj module is turned into a multi-obj module or
vice versa.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: NNicolas Pitre <nico@fluxnic.net>

Towards the goal of removing MODVERDIR, read out modules.order to get
the list of modules to be processed. This is simpler than parsing *.mod
files in $(MODVERDIR).

For external modules, $(KBUILD_EXTMOD)/modules.order should be read.

I removed the single target %.ko from the top Makefile. To make sure
modpost works correctly, vmlinux and the other modules must be built.
You cannot build a particular .ko file alone.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Removing the 'kernel/' prefix will make our life easier because we can
simply do 'cat modules.order' to get all built modules with full paths.

Currently, we parse the first line of '*.mod' files in $(MODVERDIR).
Since we have duplicated functionality here, I plan to remove MODVERDIR
entirely.

In fact, modules.order is generated also for external modules in a
broken format. It adds the 'kernel/' prefix to the absolute path of
the module, like this:

  kernel//path/to/your/external/module/foo.ko

This is fine for now since modules.order is not used for external
modules. However, I want to sanitize the format everywhere towards
the goal of removing MODVERDIR.

We cannot change the format of installed module.{order,builtin}.
So, 'make modules_install' will add the 'kernel/' prefix while copying
them to $(MODLIB)/.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Currently, $(objtree)/modules.order is touched in two places.

In the 'prepare0' rule, scripts/Makefile.build creates an empty
modules.order while processing 'obj=.'

In the 'modules' rule, the top-level Makefile overwrites it with
the correct list of modules.

While this might be a good side-effect that modules.order is made
empty every time (probably this is not intended functionality),
I personally do not like this behavior.

Create modules.order only when it is sensible to do so.

This avoids creating the following pointless files:

  scripts/basic/modules.order
  scripts/dtc/modules.order
  scripts/gcc-plugins/modules.order
  scripts/genksyms/modules.order
  scripts/mod/modules.order
  scripts/modules.order
  scripts/selinux/genheaders/modules.order
  scripts/selinux/mdp/modules.order
  scripts/selinux/modules.order

Going forward, $(objtree)/modules.order lists the modules that
was built in the last successful build.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

It takes somewhat long time to generate these tag files.
Keep such precious files until we run 'make distclean'.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

As commit 1e022137 ("mips: vdso: drop unnecessary cc-ldoption")
explained, these flags are supported by the minimal required version
of binutils. They are supported by ld.lld too.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NNathan Chancellor <natechancellor@gmail.com>
Tested-by: NNathan Chancellor <natechancellor@gmail.com>

When cross-compiling an out-of-tree build with an unclean source tree
directory, the build fails with:

  /path/to/kernel/source/tree is not clean, please run 'make mrproper'
  in the '/path/to/kernel/source/tree' directory.

However, doing so does not fix the problem, as "make mrproper" now
requires passing the target architecture to the make command, else it
won't remove $(srctree)/arch/$(SRCARCH)/include/generated.
"git ls-files -o" doesn't give a clue, as it doesn't list (empty)
directories, only files.

Improve usability by including the ARCH= option in the error output.

Fixes: a788b2ed ("kbuild: check arch/$(SRCARCH)/include/generated before out-of-tree build")
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

In old days, Kbuild always used an absolute path for $(srctree).

Since commit 890676c6 ("kbuild: Use relative path when building in
the source tree"), $(srctree) is '.' when O= was not passed from the
command line.

Yet, using absolute paths is useful in some cases even without O=, for
instance, to create a cscope file with absolute path tags.

'O=.' was known to work as a workaround to force Kbuild to use absolute
paths even when you are building in the source tree.

Since commit 25b146c5 ("kbuild: allow Kbuild to start from any
directory"), Kbuild is too clever to be tricked. Even if you pass 'O=.'
Kbuild notices you are building in the source tree, then use '.' for
$(srctree).

So, 'make O=. cscope' is no help to create absolute path tags.

We cannot force one or the other according to commit e93bc1a0
("Revert "kbuild: specify absolute paths for cscope""). Both of
relative path and absolute path have pros and cons.

This commit adds a new flag KBUILD_ABS_SRCTREE to allow users to
choose the absolute path for $(srctree).

'make KBUILD_ABS_SRCTREE=1 cscope' will work as a replacement of
'make O=. cscope'.
Reported-by: NPawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>