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>
zero-sugar-mainline-defconfig
Masahiro Yamada
parent
39bb232ae6
commit
d0e628cd81
|
|
@ -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
|
||||
---------------------------------------------
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue