kbuild: use obj-y instead extra-y for objects placed at the head

The objects placed at the head of vmlinux need special treatments:

 - arch/$(SRCARCH)/Makefile adds them to head-y in order to place
   them before other archives in the linker command line.

 - arch/$(SRCARCH)/kernel/Makefile adds them to extra-y instead of
   obj-y to avoid them going into built-in.a.

This commit gets rid of the latter.

Create vmlinux.a to collect all the objects that are unconditionally
linked to vmlinux. The objects listed in head-y are moved to the head
of vmlinux.a by using 'ar m'.

With this, arch/$(SRCARCH)/kernel/Makefile can consistently use obj-y
for builtin objects.

There is no *.o that is directly linked to vmlinux. Drop unneeded code
in scripts/clang-tools/gen_compile_commands.py.

$(AR) mPi needs 'T' to workaround the llvm-ar bug. The fix was suggested
by Nathan Chancellor [1].

[1]: https://lore.kernel.org/llvm/YyjjT5gQ2hGMH0ni@dev-arch.thelio-3990X/

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

1 files changed, 1 insertions, 17 deletions

diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index ee7e3ea1fbe1..5a6a8426cc97 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -340,19 +340,7 @@ more details, with real examples.
 
 	Examples are:
 
-	1) head objects
-
-	    Some objects must be placed at the head of vmlinux. They are
-	    directly linked to vmlinux without going through built-in.a
-	    A typical use-case is an object that contains the entry point.
-
-	    arch/$(SRCARCH)/Makefile should specify such objects as head-y.
-
-	    Discussion:
-	      Given that we can control the section order in the linker script,
-	      why do we need head-y?
-
-	2) vmlinux linker script
+	1) vmlinux linker script
 
 	    The linker script for vmlinux is located at
 	    arch/$(SRCARCH)/kernel/vmlinux.lds
@@ -360,10 +348,6 @@ more details, with real examples.
 	Example::
 
 		# arch/x86/kernel/Makefile
-		extra-y	:= head_$(BITS).o
-		extra-y	+= head$(BITS).o
-		extra-y	+= ebda.o
-		extra-y	+= platform-quirks.o
 		extra-y	+= vmlinux.lds
 
 	$(extra-y) should only contain targets needed for vmlinux.