On Tue, Feb 02, 2021 at 04:02:16PM +0900, Masahiro Yamada wrote:

CONSOLE_LOGLEVEL_DEFAULT is nothing more than a shorthand of
CONFIG_CONSOLE_LOGLEVEL_DEFAULT.

When you change CONFIG_CONSOLE_LOGLEVEL_DEFAULT from Kconfig, almost
all objects are rebuilt because CONFIG_CONSOLE_LOGLEVEL_DEFAULT is
used in <linux/printk.h>, which is included from most of source files.

In fact, there are only 4 users of CONSOLE_LOGLEVEL_DEFAULT:

  arch/x86/platform/uv/uv_nmi.c
  drivers/firmware/efi/libstub/efi-stub-helper.c
  drivers/tty/sysrq.c
  kernel/printk/printk.c

So, when you change CONFIG_CONSOLE_LOGLEVEL_DEFAULT and rebuild the
kernel, it is enough to recompile those 4 files.

Remove the CONSOLE_LOGLEVEL_DEFAULT definition from <linux/printk.h>,
and use CONFIG_CONSOLE_LOGLEVEL_DEFAULT directly.

With this, the build system will rebuild the minimal number of objects.

Steps to confirm it:

  [1] Do the full build
  [2] Change CONFIG_CONSOLE_LOGLEVEL_DEFAULT from 'make menuconfig' etc.
  [3] Rebuild

  $ make
    SYNC    include/config/auto.conf
    CALL    scripts/checksyscalls.sh
    CALL    scripts/atomic/check-atomics.sh
    DESCEND  objtool
    CHK     include/generated/compile.h
    CC      kernel/printk/printk.o
    AR      kernel/printk/built-in.a
    AR      kernel/built-in.a
    CC      drivers/tty/sysrq.o
    AR      drivers/tty/built-in.a
    CC      drivers/firmware/efi/libstub/efi-stub-helper.o
    STUBCPY drivers/firmware/efi/libstub/efi-stub-helper.stub.o
    AR      drivers/firmware/efi/libstub/lib.a
    AR      drivers/built-in.a
    GEN     .version
    CHK     include/generated/compile.h
    UPD     include/generated/compile.h
    CC      init/version.o
    AR      init/built-in.a
    LD      vmlinux.o
    ...

For the same reason, do likewise for CONSOLE_LOGLEVEL_QUIET and
MESSAGE_LOGLEVEL_DEFAULT.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 arch/x86/platform/uv/uv_nmi.c                  |  2 +-
 drivers/firmware/efi/libstub/efi-stub-helper.c |  6 +++---
 drivers/tty/sysrq.c                            |  4 ++--
 drivers/video/fbdev/core/fbcon.c               |  2 +-
 drivers/video/fbdev/efifb.c                    |  2 +-
 include/linux/printk.h                         | 10 ----------
 init/main.c                                    |  2 +-
 kernel/printk/printk.c                         |  6 +++---
 8 files changed, 12 insertions(+), 22 deletions(-)

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>