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Diffstat (limited to 'Documentation/kbuild/makefiles.rst')
| -rw-r--r-- | Documentation/kbuild/makefiles.rst | 110 |
1 files changed, 71 insertions, 39 deletions
diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst index a573ee998cf5..3f1697ce1cd4 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-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 --------------------------------------------- |