[PATCH 6/7] kbuild: doc: clarify the difference between extra-y and always-y

Masahiro Yamada
2020-11-23T04:55:41Z

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' o o
'make vmlinux' o o
'make modules' x o

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>
---

Documentation/kbuild/makefiles.rst | 110 +++++++++++++++++++----------
1 file changed, 71 insertions(+), 39 deletions(-)

diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index 49afcb1d3695..159e470f2616 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -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-
+ --- 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/$(SRCARCH)/Makefile should specify such objects as head-y.
+
+ Discussion:
+ Given that we can control the section order in our linker script,
+ why do we need head-y?
+
+ 2) vmlinux linker script
+
+ The linker script for vmlinux is located at
+ arch/$(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 visited 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
---------------------------------------------

--
2.25.1