kbuild: doc: clarify the difference between extra-y and always-y

The difference between extra-y and always-y is obscure.

Basically, Kbuild builds targets listed in extra-y and always-y in
visited Makefiles without relying on any dependency.

The difference is that extra-y is used to list the targets needed for
vmlinux whereas always-y is used to list the targets that must be always
built irrespective of final targets.

Kbuild skips extra-y when it is building only modules (i.e.
'make modules'). This is the long-standing behavior since extra-y was
introduced in 2003, and it is explained in that commit log [1].

For clarification, this is the extra-y vs always-y table:

                  extra-y    always-y
  'make'             y          y
  'make vmlinux'     y          y
  'make modules'     n          y

Kbuild skips extra-y also when building external modules since obviously
it never builds vmlinux.

Unfortunately, extra-y is wrongly used in many places of upstream code,
and even in external modules.

Using extra-y in external module Makefiles is wrong. What you should
use is probably always-y or 'targets'.

The current documentation for extra-y is misleading. I rewrote it, and
moved it to the section 3.7.

always-y is not documented anywhere. I added.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=f94e5fd7e5d09a56a60670a9bb211a791654bba8

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
This commit is contained in:
Masahiro Yamada 2020-11-28 20:51:07 +09:00
parent 39bb232ae6
commit d0e628cd81

View File

@ -15,13 +15,15 @@ This document describes the Linux kernel Makefiles.
--- 3.4 Objects which export symbols
--- 3.5 Library file goals - lib-y
--- 3.6 Descending down in directories
--- 3.7 Compilation flags
--- 3.8 Dependency tracking
--- 3.9 Custom Rules
--- 3.10 Command change detection
--- 3.11 $(CC) support functions
--- 3.12 $(LD) support functions
--- 3.13 Script Invocation
--- 3.7 Non-builtin vmlinux targets - extra-y
--- 3.8 Always built goals - always-y
--- 3.9 Compilation flags
--- 3.10 Dependency tracking
--- 3.11 Custom Rules
--- 3.12 Command change detection
--- 3.13 $(CC) support functions
--- 3.14 $(LD) support functions
--- 3.15 Script Invocation
=== 4 Host Program support
--- 4.1 Simple Host Program
@ -321,7 +323,60 @@ more details, with real examples.
names. This allows kbuild to totally skip the directory if the
corresponding `CONFIG_` option is neither 'y' nor 'm'.
3.7 Compilation flags
3.7 Non-builtin vmlinux targets - extra-y
-----------------------------------------
extra-y specifies targets which are needed for building vmlinux,
but not combined into built-in.a.
Examples are:
1) head objects
Some objects must be placed at the head of vmlinux. They are
directly linked to vmlinux without going through built-in.a
A typical use-case is an object that contains the entry point.
arch/$(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
The linker script for vmlinux is located at
arch/$(SRCARCH)/kernel/vmlinux.lds
Example::
# arch/x86/kernel/Makefile
extra-y := head_$(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.
Kbuild skips extra-y when vmlinux is apparently not a final goal.
(e.g. 'make modules', or building external modules)
If you intend to build targets unconditionally, always-y (explained
in the next section) is the correct syntax to use.
3.8 Always built goals - always-y
---------------------------------
always-y specifies targets which are literally always built when
Kbuild visits the Makefile.
Example::
# ./Kbuild
offsets-file := include/generated/asm-offsets.h
always-y += $(offsets-file)
3.9 Compilation flags
---------------------
ccflags-y, asflags-y and ldflags-y
@ -410,8 +465,8 @@ more details, with real examples.
AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt
3.8 Dependency tracking
-----------------------
3.10 Dependency tracking
------------------------
Kbuild tracks dependencies on the following:
@ -422,8 +477,8 @@ more details, with real examples.
Thus, if you change an option to $(CC) all affected files will
be re-compiled.
3.9 Custom Rules
----------------
3.11 Custom Rules
-----------------
Custom rules are used when the kbuild infrastructure does
not provide the required support. A typical example is
@ -499,7 +554,7 @@ more details, with real examples.
will be displayed with "make KBUILD_VERBOSE=0".
3.10 Command change detection
3.12 Command change detection
-----------------------------
When the rule is evaluated, timestamps are compared between the target
@ -545,7 +600,7 @@ more details, with real examples.
unwanted results when the target is up to date and only the
tests on changed commands trigger execution of commands.
3.11 $(CC) support functions
3.13 $(CC) support functions
----------------------------
The kernel may be built with several different versions of
@ -660,7 +715,7 @@ more details, with real examples.
endif
endif
3.12 $(LD) support functions
3.14 $(LD) support functions
----------------------------
ld-option
@ -674,7 +729,7 @@ more details, with real examples.
#Makefile
LDFLAGS_vmlinux += $(call ld-option, -X)
3.13 Script invocation
3.15 Script invocation
----------------------
Make rules may invoke scripts to build the kernel. The rules shall
@ -1304,29 +1359,6 @@ When kbuild executes, the following steps are followed (roughly):
When "make" is executed without arguments, bzImage will be built.
7.6 Building non-kbuild targets
-------------------------------
extra-y
extra-y specifies additional targets created in the current
directory, in addition to any targets specified by `obj-*`.
Listing all targets in extra-y is required for two purposes:
1) Enable kbuild to check changes in command lines
- When $(call if_changed,xxx) is used
2) kbuild knows what files to delete during "make clean"
Example::
#arch/x86/kernel/Makefile
extra-y := head.o init_task.o
In this example, extra-y is used to list object files that
shall be built, but shall not be linked as part of built-in.a.
7.7 Commands useful for building a boot image
---------------------------------------------