GNU Make automatically deletes intermediate files that are updated
in a chain of pattern rules.

Example 1) %.dtb.o <- %.dtb.S <- %.dtb <- %.dts
Example 2) %.o <- %.c <- %.c_shipped

A couple of makefiles mark such targets as .PRECIOUS to prevent Make
from deleting them, but the correct way is to use .SECONDARY.

  .SECONDARY
    Prerequisites of this special target are treated as intermediate
    files but are never automatically deleted.

  .PRECIOUS
    When make is interrupted during execution, it may delete the target
    file it is updating if the file was modified since make started.
    If you mark the file as precious, make will never delete the file
    if interrupted.

Both can avoid deletion of intermediate files, but the difference is
the behavior when Make is interrupted; .SECONDARY deletes the target,
but .PRECIOUS does not.

The use of .PRECIOUS is relatively rare since we do not want to keep
partially constructed (possibly corrupted) targets.

Another difference is that .PRECIOUS works with pattern rules whereas
.SECONDARY does not.

  .PRECIOUS: $(obj)/%.lex.c

works, but

  .SECONDARY: $(obj)/%.lex.c

has no effect.  However, for the reason above, I do not want to use
.PRECIOUS which could cause obscure build breakage.

The targets specified as .SECONDARY must be explicit.  $(targets)
contains all targets that need to include .*.cmd files.  So, the
intermediates you want to keep are mostly in there.  Therefore, mark
$(targets) as .SECONDARY.  It means primary targets are also marked
as .SECONDARY, but I do not see any drawback for this.

I replaced some .SECONDARY / .PRECIOUS markers with 'targets'.  This
will make Kbuild search for non-existing .*.cmd files, but this is
not a noticeable performance issue.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Frank Rowand <frowand.list@gmail.com>
Acked-by: Ingo Molnar <mingo@kernel.org>

Our convention is to distinguish file types by suffixes with a period
as a separator.

*-asn1.[ch] is a different pattern from other generated sources such
as *.lex.c, *.tab.[ch], *.dtb.S, etc.  More confusing, files with
'-asn1.[ch]' are generated files, but '_asn1.[ch]' are checked-in
files:
  net/netfilter/nf_conntrack_h323_asn1.c
  include/linux/netfilter/nf_conntrack_h323_asn1.h
  include/linux/sunrpc/gss_asn1.h

Rename generated files to *.asn1.[ch] for consistency.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Another common pattern that consists of chained commands is to compile
a DTB as binary data into the kernel image or a module.  It is used in
several places in the source tree.  Support it in the core Makefile.

$(call if_changed,dt_S_dtb) is more suitable than $(call cmd,dt_S_dtb)
in case cmd_dt_S_dtb is changed in the future.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Frank Rowand <frowand.list@gmail.com>

Files generated by if_changed* must be added to 'targets' to include
*.cmd files.  Otherwise, they would be regenerated every time.

The build system automatically adds objects to 'targets' where
appropriate, such as obj-y, extra-y, etc. but does nothing for
intermediate files.  So, each Makefile needs to add them by itself.

There are some common cases where objects are generated by chained
rules.  Lexers and parsers are compiled like follows:

   %.lex.o <- %.lex.c <- %.l
   %.tab.o <- %.tab.c <- %.y

They are common patterns, so it is reasonable to take care of them
in the core Makefile instead of requiring each Makefile to do so.

At this moment, you cannot delete 'target += zconf.lex.c' in the
Kconfig Makefile because zconf.lex.c is included from zconf.tab.c
instead of being compiled separately.  It should be deleted after
Kconfig is more refactored.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Frank Rowand <frowand.list@gmail.com>

cmd_link_multi-link is used only for cmd_link_multi-m.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

With the incremental linking entirely dropped, we can simplify
the Makefile.

While I am here, I renamed cmd_link_o_target to cmd_ar_builtin.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

When built-in.o was incrementally linked with 'ld -r', the section
mismatch analysis for the individual built-in.o was possible when
CONFIG_DEBUG_SECTION_MISMATCH was enabled.

With the migration to the thin archive, built-in.a (former, built-in.o)
is no longer an ELF file.  So, the modpost does nothing useful.
scripts/mod/modpost.c just checks the header to bail out, as follows:

        /* Is this a valid ELF file? */
        if ((hdr->e_ident[EI_MAG0] != ELFMAG0) ||
            (hdr->e_ident[EI_MAG1] != ELFMAG1) ||
            (hdr->e_ident[EI_MAG2] != ELFMAG2) ||
            (hdr->e_ident[EI_MAG3] != ELFMAG3)) {
                /* Not an ELF file - silently ignore it */
                return 0;
        }

We have the full analysis in the final link stage anyway, so we would
not miss the section mismatching.

I do not see a good reason to require extra linking only for the
purpose of the per-directory analysis.  Just get rid of this part.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

In Kbuild, Makefiles can add the same object to obj-y multiple
times.  So,

   obj-y += foo.o
   obj-y += foo.o

is fine.

However, this is not true when the same object is added multiple
times via composite objects.  For example,

   obj-y    += foo.o bar.o
   foo-objs := foo-bar-common.o foo-only.o
   bar-objs := foo-bar-common.o bar-only.o

causes build error because two instances of foo-bar-common.o are
linked into the vmlinux.

Makefiles tend to invent ugly work-around, for example
  - lib/zstd/Makefile
  - drivers/net/ethernet/cavium/liquidio/Makefile

The technique used in Kbuild to avoid the multiple definition error
is to use $(filter $(obj-y), $^).  Here, $^ lists the names of all
the prerequisites with duplicated names removed.

By replacing it with $(filter $(real-obj-y), $^) we can do likewise
for composite objects.  For built-in objects, we do not need to keep
the composite object structure.  We can simply expand them, and link
$(real-obj-y) to built-in.a.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

When I was refactoring Makefiles, I stupidly mistook 'real-obj-y' for
'real-objs-y' over and over again.  Finally, I decide to rename it to
'real-obj-y'.  This is consistent with 'obj-y', 'subdir-obj-y'.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Just a cosmetic change to put related code close together.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Cao jin <caoj.fnst@cn.fujitsu.com>