openwrt Makefile subdir.mk 详解

2021-01-13Embedded

之前讲述了 openwrt Makefile 的整体框架，主要包括主 Makefile 的描述，，最后简单描述了 subdir.mk , 本文就来详细剖析这个文件。

但是在解析 subdir.mk 之前，先来看两个 Makefile,

debug.mk: 这也是主 Makefile 引入的第一个 .mk 文件，这里定义的调试函数在 subdir.mk 中被调用
target/Makefile: 这是调用到 subdir.mk 的首个 Makefile, 本文将以此为例进行说明

debug.mk

debug.mk 定义了几个调试函数。

debug
warn
debug_eval
warn_eval

define debug
$$(findstring $(2),$$(if $$(DEBUG_SCOPE_DIR),$$(if $$(filter $$(DEBUG_SCOPE_DIR)%,$(1)),$(build_debug)),$(build_debug)))
endef

define warn
$$(if $(call debug,$(1),$(2)),$$(warning $(3)))
endef

define debug_eval
$$(if $(call debug,$(1),$(2)),$(3))
endef

define warn_eval
$(call warn,$(1),$(2),$(3)        $(4))
$(4)
endef

其中：

DEBUG_SCOPE_DIR: 默认没有定义, 可以在执行 make 时指定
build_debug: 对应的是 $(DEBUG) 的值，是 "dltvr" 字符串的子集

要使这几个函数生效，需要定义变量 DEBUG, 可以在执行 make 时定义.

make DEBUG=d V=s
# debug flags:
#
# d: show subdirectory tree
# t: show added targets
# l: show legacy targets
# r: show autorebuild messages
# v: verbose (no .SILENCE for common targets)

默认情况下，DEBUG 是没有定义的，通常不需要管，之所以在这拎出来，是因为 subdir.mk 有调用以上提及的调试函数。

target/Makefile

target/Makefile 与 tools, toolchain, package 目录的 Makefile 使用的是相同的结构，通过调用 subdir.mk 内定义的两个函数 stampfile, subdir 动态生成编译目标规则。

curdir:=target

$(curdir)/subtargets:=install
$(curdir)/builddirs:=linux sdk imagebuilder toolchain
$(curdir)/builddirs-default:=linux
$(curdir)/builddirs-install:=linux $(if $(CONFIG_SDK),sdk) $(if $(CONFIG_IB),imagebuilder) $(if $(CONFIG_MAKE_TOOLCHAIN),toolchain)

$(curdir)/sdk/install:=$(curdir)/linux/install
$(curdir)/imagebuilder/install:=$(curdir)/linux/install

$(eval $(call stampfile,$(curdir),target,prereq,.config))
$(eval $(call stampfile,$(curdir),target,compile,$(TMP_DIR)/.build))
$(eval $(call stampfile,$(curdir),target,install,$(TMP_DIR)/.build))

$($(curdir)/stamp-install): $($(curdir)/stamp-compile) 

$(eval $(call subdir,$(curdir)))

其中 curdir 代表当前目录. 该 Makefile 前半部分定义了一些变量，这些变量在后续的 stampfile, subdir 函数中被调用。target, package 等目录的 Makefile 都是先调用 stampfile 生成特定目标的依赖和指令，然后再调用 subdir 生成各个子目录下目标的编译规则。至于具体怎么生成，继续往后看。

subdir.mk

接下来看本文的主角 —— subdir.mk, 这是编译项目过程中 非常重要 的文件，它可以动态定义和生成编译子目录相关目标的规则，包括 package, target, tools, toolchain 等子目录相关目标的规则。 subdir.mk 也是编译生成 world 目标必不可少的文件，主 Makefile 在 include subdir.mk 之后会 include 以下 Makefile:

target/Makefile
package/Makefile
tools/Makefile
toolchain/Makefile

这些子目录的 Makefile 会调用 subdir.mk 定义的以下两个函数：

subdir
stampfile

这两个函数是配合起来一起用的，先通过 stampfile 生成 stamp-$(target) 相关规则，然后通过 subddir 生成各个子目录相关目标的规则（如 target/linux/clean）。

stampfile

先来看 stampfile.

# Parameters: <subdir> <name> <target> <depends> <config options> <stampfile location>
define stampfile
  $(1)/stamp-$(3):=$(if $(6),$(6),$(STAGING_DIR))/stamp/.$(2)_$(3)$(5)
  $$($(1)/stamp-$(3)): $(TMP_DIR)/.build $(4)
        @+$(SCRIPT_DIR)/timestamp.pl -n $$($(1)/stamp-$(3)) $(1) $(4) || \
                $(MAKE) $(if $(QUIET),--no-print-directory) $$($(1)/flags-$(3)) $(1)/$(3)
        @mkdir -p $$$$(dirname $$($(1)/stamp-$(3)))
        @touch $$($(1)/stamp-$(3))

  $$(if $(call debug,$(1),v),,.SILENT: $$($(1)/stamp-$(3)))

  .PRECIOUS: $$($(1)/stamp-$(3)) # work around a make bug

  $(1)//clean:=$(1)/stamp-$(3)/clean
  $(1)/stamp-$(3)/clean: FORCE
        @rm -f $$($(1)/stamp-$(3))

endef

首行注释了 stampfile 的参数列表，以 target/Makefile 首个 stampfile 调用为例。

# Parameters: <subdir> <name> <target> <depends> <config options> <stampfile location>
$(eval $(call stampfile,$(curdir),target,prereq,.config))
# subdir: $(curdir) - "target"
# name: "target"
# target: "prereq"
# depends: ".config"
# config options: ""
# stampfile location: ""

为了方便调试，可以在该行前添加一行, 将 eval 改成 warning，这样就可以在编译 log 中看到解析后的编译规则啦。

# add below function call to get the details of stampfile
$(warning $(call stampfile,$(curdir),target,prereq,.config))
$(eval $(call stampfile,$(curdir),target,prereq,.config))

ok, 接下来清除所有编译结果，然后重新编译 defconfig, 编译过程就会调用 target/Makefile 并执行以上语句。

make distclean
make -d DEBUG=vltdr defconfig > log 2>&1

截取 log 中打印以上 warning 信息的部分如下：

target/stamp-prereq:=/home/litreily/openwrt/staging_dir/target-_/stamp/.target_prereq
$(target/stamp-prereq): /home/litreily/openwrt/tmp/.build .config
        @+/home/litreily/openwrt/scripts/timestamp.pl -n $(target/stamp-prereq) target .config || make  $(target/flags-prereq) target/prereq
        @mkdir -p $$(dirname $(target/stamp-prereq))
        @touch $(target/stamp-prereq)

$(if $(findstring v,$(if $(DEBUG_SCOPE_DIR),$(if $(filter $(DEBUG_SCOPE_DIR)%,target),vltdr),vltdr)),,.SILENT: $(target/stamp-prereq))

.PRECIOUS: $(target/stamp-prereq) # work around a make bug

target//clean:=target/stamp-prereq/clean
target/stamp-prereq/clean: FORCE
        @rm -f $(target/stamp-prereq)

由于默认没有 .config 文件，所以解析出来的 STAGING_DIR 为 target-_, 如果是 .config 已存在的情况下，得到的结果更完整，当然这不是我们研究的重点。

从以上解析结果可以看出，stampfile 会生成 4 个目标规则：

$(target/stamp-prereq) : 依赖 tmp/.build, .config
.SILENT: 是在没有定义 DEBUG 参数时才会生成的目标，其依赖就是刚定义的 $(target/stamp-prereq), 可以忽略
.PRECIOUS: 根据注释像是解决某个 bug 的 workaround, 可以忽略
target/stamp-prereq/clean: 用来删除目标 $(target/stamp-prereq) 的伪目标

根据分析，以上结果可以进一步简化。

target/stamp-prereq:=/home/litreily/openwrt/staging_dir/target-_/stamp/.target_prereq
$(target/stamp-prereq): /home/litreily/openwrt/tmp/.build .config
        @+/home/litreily/openwrt/scripts/timestamp.pl -n $(target/stamp-prereq) target .config || make  $(target/flags-prereq) target/prereq
        @mkdir -p $$(dirname $(target/stamp-prereq))
        @touch $(target/stamp-prereq)

target//clean:=target/stamp-prereq/clean
target/stamp-prereq/clean: FORCE
        @rm -f $(target/stamp-prereq)

同理也可以分析 target/Makefile 中剩余两个 stampfile 调用。

$(warning $(call stampfile,$(curdir),target,compile,$(TMP_DIR)/.build))
$(eval $(call stampfile,$(curdir),target,compile,$(TMP_DIR)/.build))
$(warning $(call stampfile,$(curdir),target,install,$(TMP_DIR)/.build))
$(eval $(call stampfile,$(curdir),target,install,$(TMP_DIR)/.build))

同样进行简化，得到编译目标及其规则：

# $(eval $(call stampfile,$(curdir),target,compile,$(TMP_DIR)/.build))
target/stamp-compile:=/home/litreily/openwrt/staging_dir/target-mips_24kc_musl/stamp/.target_compile
$(target/stamp-compile): /home/litreily/openwrt/tmp/.build /home/litreily/openwrt/tmp/.build
        @+/home/litreily/openwrt/scripts/timestamp.pl -n $(target/stamp-compile) target /home/litreily/openwrt/tmp/.build || make  $(target/flags-compile) target/compile
        @mkdir -p $$(dirname $(target/stamp-compile))
        @touch $(target/stamp-compile)

target//clean:=target/stamp-compile/clean
target/stamp-compile/clean: FORCE
        @rm -f $(target/stamp-compile)

# $(eval $(call stampfile,$(curdir),target,install,$(TMP_DIR)/.build))
target/stamp-install:=/home/litreily/openwrt/staging_dir/target-mips_24kc_musl/stamp/.target_install
$(target/stamp-install): /home/litreily/openwrt/tmp/.build /home/litreily/openwrt/tmp/.build
        @+/home/litreily/openwrt/scripts/timestamp.pl -n $(target/stamp-install) target /home/litreily/openwrt/tmp/.build || make  $(target/flags-install) target/install
        @mkdir -p $$(dirname $(target/stamp-install))
        @touch $(target/stamp-install)

target//clean:=target/stamp-install/clean
target/stamp-install/clean: FORCE

注意： $(target/stamp-compile) 和 $(target/stamp-install) 的依赖包含两个重复的 tmp/.build, 看起来像是冗余的，或许可以将 target/Makefile 优化下。

分析到这一步，stampfile 对 target 目录生成的目标已经都解析出来了，就是下面这些.

$(target/stamp-prereq):
target/stamp-prereq/clean:
$(target/stamp-compile):
target/stamp-compile/clean:
$(target/stamp-install):
target/stamp-install/clean:

这些目标在主 Makefile 是作为其它目标的依赖，也是目标 world 编译过程中的中间依赖目标。

注意： 每个 stampfile 除了生成目标规则都，都定义了一个 target//clean 变量，这个在 subdir 中会调用到。

ok, stampfile 分析完了，接下来看下 subdir 要完成哪些工作。

subdir

# Parameters: <subdir>
define subdir
  $(call warn,$(1),d,D $(1))
  $(foreach bd,$($(1)/builddirs),
    $(call warn,$(1),d,BD $(1)/$(bd))
    $(foreach target,$(SUBTARGETS) $($(1)/subtargets),
      $(foreach btype,$(buildtypes-$(bd)),
        $(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(btype)/$(target): $(if $(NO_DEPS)$(QUILT),,$($(1)/$(bd)/$(btype)/$(target)) $(call $(1)//$(btype)/$(target),$(1)/$(bd)/$(btype))))
                $(call log_make,$(1)/$(bd),$(target),$(btype),$(filter-out __default,$(variant))) \
                        || $(call ERROR,$(2),   ERROR: $(1)/$(bd) [$(btype)] failed to build.,$(findstring $(bd),$($(1)/builddirs-ignore-$(btype)-$(target))))
        $(if $(call diralias,$(bd)),$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(call diralias,$(bd))/$(btype)/$(target): $(1)/$(bd)/$(btype)/$(target)))
      )
      $(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(target): $(if $(NO_DEPS)$(QUILT),,$($(1)/$(bd)/$(target)) $(call $(1)//$(target),$(1)/$(bd))))
        $(foreach variant,$(if $(BUILD_VARIANT),$(BUILD_VARIANT),$(if $(strip $($(1)/$(bd)/variants)),$($(1)/$(bd)/variants),$(if $($(1)/$(bd)/default-variant),$($(1)/$(bd)/default-variant),__default))),
                $(if $(BUILD_LOG),@mkdir -p $(BUILD_LOG_DIR)/$(1)/$(bd)/$(filter-out __default,$(variant)))
                $(if $($(1)/autoremove),$(call rebuild_check,$(1)/$(bd),$(target),,$(filter-out __default,$(variant))))
                $(call log_make,$(1)/$(bd),$(target),,$(filter-out __default,$(variant))) \
                        || $(call ERROR,$(1),   ERROR: $(1)/$(bd) failed to build$(if $(filter-out __default,$(variant)), (build variant: $(variant))).,$(findstring $(bd),$($(1)/builddirs-ignore-$(target)))) 
        )
      $(if $(PREREQ_ONLY)$(DUMP_TARGET_DB),,
        # aliases
        $(if $(call diralias,$(bd)),$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(call diralias,$(bd))/$(target): $(1)/$(bd)/$(target)))
          )
        )
  )
  $(foreach target,$(SUBTARGETS) $($(1)/subtargets),$(call subtarget,$(1),$(target)))
endef

别看这么复杂，其中很大一部分是 debug 信息，只有在 DEBUG 值不为空的情况下才会打印，否则就是空值。在编译的时候加上 DEBUG=vltdr, 可以看到以下详细的调试信息，也能方便理解。

target/Makefile:23: D target
target/Makefile:23: BD target/linux
target/Makefile:23: T    target/linux/clean:  target/stamp-install/clean
target/Makefile:23: T    target/linux/download:  
target/Makefile:23: T    target/linux/prepare:  
target/Makefile:23: T    target/linux/compile:  
target/Makefile:23: T    target/linux/update:  
target/Makefile:23: T    target/linux/refresh:  
target/Makefile:23: T    target/linux/prereq:  
target/Makefile:23: T    target/linux/dist:  
target/Makefile:23: T    target/linux/distcheck:  
target/Makefile:23: T    target/linux/configure:  
target/Makefile:23: T    target/linux/check:  
target/Makefile:23: T    target/linux/check-depends:  
target/Makefile:23: T    target/linux/install:  
target/Makefile:23: BD target/sdk
target/Makefile:23: T    target/sdk/clean:  target/stamp-install/clean
target/Makefile:23: T    target/sdk/download:  
target/Makefile:23: T    target/sdk/prepare:  
target/Makefile:23: T    target/sdk/compile:  
target/Makefile:23: T    target/sdk/update:  
target/Makefile:23: T    target/sdk/refresh:  
target/Makefile:23: T    target/sdk/prereq:  
target/Makefile:23: T    target/sdk/dist:  
target/Makefile:23: T    target/sdk/distcheck:  
target/Makefile:23: T    target/sdk/configure:  
target/Makefile:23: T    target/sdk/check:  
target/Makefile:23: T    target/sdk/check-depends:  
target/Makefile:23: T    target/sdk/install: target/linux/install 
target/Makefile:23: BD target/imagebuilder
target/Makefile:23: T    target/imagebuilder/clean:  target/stamp-install/clean
target/Makefile:23: T    target/imagebuilder/download:  
target/Makefile:23: T    target/imagebuilder/prepare:  
target/Makefile:23: T    target/imagebuilder/compile:  
target/Makefile:23: T    target/imagebuilder/update:  
target/Makefile:23: T    target/imagebuilder/refresh:  
target/Makefile:23: T    target/imagebuilder/prereq:  
target/Makefile:23: T    target/imagebuilder/dist:  
target/Makefile:23: T    target/imagebuilder/distcheck:  
target/Makefile:23: T    target/imagebuilder/configure:  
target/Makefile:23: T    target/imagebuilder/check:  
target/Makefile:23: T    target/imagebuilder/check-depends:  
target/Makefile:23: T    target/imagebuilder/install: target/linux/install 
target/Makefile:23: BD target/toolchain
target/Makefile:23: T    target/toolchain/clean:  target/stamp-install/clean
target/Makefile:23: T    target/toolchain/download:  
target/Makefile:23: T    target/toolchain/prepare:  
target/Makefile:23: T    target/toolchain/compile:  
target/Makefile:23: T    target/toolchain/update:  
target/Makefile:23: T    target/toolchain/refresh:  
target/Makefile:23: T    target/toolchain/prereq:  
target/Makefile:23: T    target/toolchain/dist:  
target/Makefile:23: T    target/toolchain/distcheck:  
target/Makefile:23: T    target/toolchain/configure:  
target/Makefile:23: T    target/toolchain/check:  
target/Makefile:23: T    target/toolchain/check-depends:  
target/Makefile:23: T    target/toolchain/install:  
target/Makefile:23: T    target/clean:  target/linux/clean
target/Makefile:23: T    target/download:  target/linux/download
target/Makefile:23: T    target/prepare:  target/linux/prepare
target/Makefile:23: T    target/compile:  target/linux/compile
target/Makefile:23: T    target/update:  target/linux/update
target/Makefile:23: T    target/refresh:  target/linux/refresh
target/Makefile:23: T    target/prereq:  target/linux/prereq
target/Makefile:23: T    target/dist:  target/linux/dist
target/Makefile:23: T    target/distcheck:  target/linux/distcheck
target/Makefile:23: T    target/configure:  target/linux/configure
target/Makefile:23: T    target/check:  target/linux/check
target/Makefile:23: T    target/check-depends:  target/linux/check-depends
target/Makefile:23: T    target/install:  target/linux/install

其中：

D : Directory, 当前目录
BD : builddirs, 在 target/Makefile 中定义的 builddirs 变量
T : subtargets, 具体的目标

结合以上调试信息可以更容易分析 subdir 的执行流程，函数内部主要通过 3 层 foreach 循环逐层遍历以下信息：

$(call warn,$(1),d,D $(1))
# 1. builddirs: linux sdk imagebuilder toolchain`
$(foreach bd,$($(1)/builddirs),
  $(call warn,$(1),d,BD $(1)/$(bd))
  # 2. subtargets: clean download prepare compile update refresh prereq dist distcheck configure check check-depends install
  $(foreach target,$(SUBTARGETS) $($(1)/subtargets),
     # 3. buildtypes: ""
     $(foreach btype,$(buildtypes-$(bd)),

  $(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(target): $(if $(NO_DEPS)$(QUILT),,$($(1)/$(bd)/$(target)) $(call $(1)//$(target),$(1)/$(bd))))
        # 3. variant: __default
        $(foreach variant, ...
#...
$(foreach target,$(SUBTARGETS) $($(1)/subtargets),$(call subtarget,$(1),$(target)))

对其中的个别变量进行说明，$(bd) 对应 target 子目录 linux, sdk 等；$(SUBTARGETS) 是在 rules.mk 中定义的。

# include/subdir.mk
SUBTARGETS:=$(DEFAULT_SUBDIR_TARGETS)

# rules.mk
DEFAULT_SUBDIR_TARGETS:=clean download prepare compile update refresh prereq dist distcheck configure check check-depends

遍历 target 的语句是 $(foreach target, $(SUBTARGETS) $(1)/subtargets), 就是说，除了上面的默认 targets 之外，还要加上 $(1)/subtargets 的值，对应 target/Makefile 中的值是 install. 这与上面展示的 log 是相对应的。

整体结构清楚了，但是还是不好分析，那么先根据以下原则对 subdir 进行精简：

忽略调试信息 warn
解析变量
解析 if 条件判断，仅留下有效分支

举例说明，

subdir 中的 buildtypes-$(bd) 为空，所以其对应 foreach 可以直接删除;
BUILD_VARIANT 未定义，对应 if 判断结果为 __default;
$(BUILD_LOG) 未定义，对应 if 分支删除。
$($(1)/autoremove) 未定义，对应 if 分支删除。

根据这些规则后简化的 subdir 如下：

# Parameters: <subdir>
define subdir
  $(foreach bd,$($(1)/builddirs),
    $(foreach target,$(SUBTARGETS) $($(1)/subtargets),
      $(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(target): $(call $(1)//$(target),$(1)/$(bd)))
        $(foreach variant,__default,
                $(call log_make,$(1)/$(bd),$(target),,$(filter-out __default,$(variant))) 
        )
        # aliases
        $(if $(call diralias,$(bd)),$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(call diralias,$(bd))/$(target): $(1)/$(bd)/$(target)))
    )
  )
  $(foreach target,$(SUBTARGETS) $($(1)/subtargets),$(call subtarget,$(1),$(target)))
endef

example

以遍历 $(bd) 值为 linux 说明以上精简后的操作。

      $(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(target): $(call $(1)//$(target),$(1)/$(bd)))

在定义了 DEBUG=vltdr 的情况下，以上指令会打印 target 相关信息。

target/Makefile:23: T    target/linux/clean:  target/stamp-install/clean
target/Makefile:23: T    target/linux/download:  
target/Makefile:23: T    target/linux/prepare:  
target/Makefile:23: T    target/linux/compile:  
target/Makefile:23: T    target/linux/update:  
target/Makefile:23: T    target/linux/refresh:  
target/Makefile:23: T    target/linux/prereq:  
target/Makefile:23: T    target/linux/dist:  
target/Makefile:23: T    target/linux/distcheck:  
target/Makefile:23: T    target/linux/configure:  
target/Makefile:23: T    target/linux/check:  
target/Makefile:23: T    target/linux/check-depends:  
target/Makefile:23: T    target/linux/install:

从前面 warn_eval 的定义可知，它除了打印 warning 信息外，还会将最后一个参数单独执行一遍。所以它包含两个功能，warn 和 eval, 且 eval 更重要。它实际上也定义了一组编译目标。也就是 warn_eval 的第4个参数:

$(1)/$(bd)/$(target): $(call $(1)//$(target),$(1)/$(bd))

将变量依次替换得到：

target/linux/clean: $(call target//clean,target/linux)

$(target/linux) : 变量没有定义
$(target//clean) : 前面 stampfile 定义的变量，其值为 target/stamp-install/clean

最终解析出来如下：

target/linux/clean: target/stamp-install/clean

该目标除了依赖之外，还有对应的指令，也就是紧随其后的 foreach 语句。

        $(foreach variant,__default,
                $(call log_make,$(1)/$(bd),$(target),,$(filter-out __default,$(variant))) 
        )

看到这应该明了了，就是这个 log_make 调用，顺藤摸瓜，来看下这个函数的定义。

subdir_make_opts = \
        -r -C $(1) \
                BUILD_SUBDIR="$(1)" \
                BUILD_VARIANT="$(4)"

# 1: subdir
# 2: target
# 3: build type
# 4: build variant
log_make = \
        $(if $(call debug,$(1),v),,@)+ \
        $(if $(BUILD_LOG), \
                set -o pipefail; \
                mkdir -p $(BUILD_LOG_DIR)/$(1)$(if $(4),/$(4));) \
      $(SCRIPT_DIR)/time.pl "time: $(1)$(if $(4),/$(4))/$(if $(3),$(3)-)$(2)" \
      $$(SUBMAKE) $(subdir_make_opts) $(if $(3),$(3)-)$(2) \
                $(if $(BUILD_LOG),SILENT= 2>&1 | tee $(BUILD_LOG_DIR)/$(1)$(if $(4),/$(4))/$(if $(3),$(3)-)$(2).txt)

同样，先进行简化，DEBUG, BUILD_LOG 均未定义, 对应分支可删除；已知全局变量可以替换掉。

log_make = \
        @+ \
        scripts/time.pl "time: $(1)$(if $(4),/$(4))/$(if $(3),$(3)-)$(2)" \
        $$(SUBMAKE) -r -C $(1) BUILD_SUBDIR="$(1)" BUILD_VARIANT="$(4)" \
        $(if $(3),$(3)-)$(2)

接着回到 subdir 中的 log_make 的调用处：

$(call log_make,$(1)/$(bd),$(target),,$(filter-out __default,$(variant)))

按照简化后的 log_make 代入参数解析以上指令。

+ scripts/time.pl "time: target/linux/clean" make -w -r -C target/linux BUILD_SUBDIR="target/linux" BUILD_VARIANT="" clean

将目标、依赖及其指令组合起来就是这样的：

target/linux/clean: target/stamp-install/clean
        + scripts/time.pl "time: target/linux/clean" make -w -r -C target/linux BUILD_SUBDIR="target/linux" BUILD_VARIANT="" clean

看看这个目标定义，依赖项 target/stamp-install/clean 的规则在 stampfile 中定义好了；其指令是通过 perl 脚本 time.pl 执行 make 指令并记录时间信息。

到此就完成了对 subdir 遍历 subtargets 之一 target/linux/clean 的解析，其它 target 也是类似的。

# target/linux
target/linux/clean: target/stamp-install/clean
        + scripts/time.pl "time: target/linux/clean" make -w -r -C target/linux BUILD_SUBDIR="target/linux" BUILD_VARIANT="" clean
target/linux/download: 
        + scripts/time.pl "time: target/linux/download" make -w -r -C target/linux BUILD_SUBDIR="target/linux" BUILD_VARIANT="" download
target/linux/prepare: 
        + scripts/time.pl "time: target/linux/download" make -w -r -C target/linux BUILD_SUBDIR="target/linux" BUILD_VARIANT="" prepare
#...
target/linux/install: 
        + scripts/time.pl "time: target/linux/install" make -w -r -C target/linux BUILD_SUBDIR="target/linux" BUILD_VARIANT="" install

# target/sdk
target/sdk/clean: target/stamp-install/clean
        + scripts/time.pl "time: target/sdk/clean" make -w -r -C target/sdk BUILD_SUBDIR="target/sdk" BUILD_VARIANT="" clean
target/sdk/download: 
        + scripts/time.pl "time: target/sdk/download" make -w -r -C target/sdk BUILD_SUBDIR="target/sdk" BUILD_VARIANT="" download
#...
target/sdk/install: 
        + scripts/time.pl "time: target/sdk/install" make -w -r -C target/sdk BUILD_SUBDIR="target/sdk" BUILD_VARIANT="" install

# target/imagebuilder
target/imagebuilder/clean: target/stamp-install/clean
        + scripts/time.pl "time: target/imagebuilder/clean" make -w -r -C target/imagebuilder BUILD_SUBDIR="target/imagebuilder" BUILD_VARIANT="" clean
target/imagebuilder/download: 
        + scripts/time.pl "time: target/imagebuilder/download" make -w -r -C target/imagebuilder BUILD_SUBDIR="target/imagebuilder" BUILD_VARIANT="" download
#...
target/imagebuilder/install: 
        + scripts/time.pl "time: target/imagebuilder/install" make -w -r -C target/imagebuilder BUILD_SUBDIR="target/imagebuilder" BUILD_VARIANT="" install

# target/toolchain
target/toolchain/clean: target/stamp-install/clean
        + scripts/time.pl "time: target/toolchain/clean" make -w -r -C target/toolchain BUILD_SUBDIR="target/toolchain" BUILD_VARIANT="" clean
target/toolchain/download: 
        + scripts/time.pl "time: target/toolchain/download" make -w -r -C target/toolchain BUILD_SUBDIR="target/toolchain" BUILD_VARIANT="" download
#...
target/toolchain/install: 
        + scripts/time.pl "time: target/toolchain/install" make -w -r -C target/toolchain BUILD_SUBDIR="target/toolchain" BUILD_VARIANT="" install

in the end

最后，还有一个小尾巴没讲到。注意看前面给出 log 的最后一部分。

target/Makefile:23: T    target/clean:  target/linux/clean
target/Makefile:23: T    target/download:  target/linux/download
target/Makefile:23: T    target/prepare:  target/linux/prepare
target/Makefile:23: T    target/compile:  target/linux/compile
target/Makefile:23: T    target/update:  target/linux/update
target/Makefile:23: T    target/refresh:  target/linux/refresh
target/Makefile:23: T    target/prereq:  target/linux/prereq
target/Makefile:23: T    target/dist:  target/linux/dist
target/Makefile:23: T    target/distcheck:  target/linux/distcheck
target/Makefile:23: T    target/configure:  target/linux/configure
target/Makefile:23: T    target/check:  target/linux/check
target/Makefile:23: T    target/check-depends:  target/linux/check-depends
target/Makefile:23: T    target/install:  target/linux/install

这里打印的信息是哪里来的呢？其实也是 subdir 函数，是该函数最后一个 foreach 循环遍历出来的。

  $(foreach target,$(SUBTARGETS) $($(1)/subtargets),$(call subtarget,$(1),$(target)))

经过前面的推导，这部分简直是小 case, 无非是遍历所有 target, 然后使用函数 subtarget 去生成规则。

subtarget-default = $(filter-out ., \
        $(if $($(1)/builddirs-$(2)),$($(1)/builddirs-$(2)), \
        $(if $($(1)/builddirs-default),$($(1)/builddirs-default), \
        $($(1)/builddirs))))

define subtarget
  $(call warn_eval,$(1),t,T,$(1)/$(2): $($(1)/) $(foreach bd,$(call subtarget-default,$(1),$(2)),$(1)/$(bd)/$(2)))
endef

同样对其进行简化。

# $(1): target
# $(2): $(target) -> "clean, download, ..., install"
# $(target/builddirs-clean): ""
# $(target/builddirs-default): linux # defined in target/Makefile
subtarget-default = $(filter-out ., $(if $($(1)/builddirs-$(2)),$($(1)/builddirs-$(2)), linux))

define subtarget
  $(1)/$(2): $($(1)/) $(foreach bd,$(call subtarget-default,$(1),$(2)),$(1)/$(bd)/$(2))
endef

然后代入参数 $(1): target, $(2): clean, 得到：

subtarget-default = linux

define subtarget
  target/clean: target/linux/clean
endef

这与 log 中的一致，其它 target 的规则也是类似，这里最主要的就是一个默认编译规则，相当于在不指定具体 target 哪个子目录时，根据默认值进行编译，target/Makefile 定义的 $(builddirs-default) 就是 linux, 也就是说，执行 make target/clean 与 make target/linux/clean 是一样的。

除 target 目录外，package, tools, toolchain 目录也是类似的。

到这，最后的小尾巴也讲完啦。

Tips

使用 makefile 的 warning 函数打印信息可以快速梳理 Makefile 的执行流程。

$(warning info)

# Example
# target/Makefile
$(warning $(call subdir,$(curdir)))

配合 make -n V=s 能够打印指令信息, 如果加上 DEBUG 可以显示更加详细的调试信息。

make -d V=s DEBUG=dltvr
# -n to print command only
make -d -n V=s DEBUG=dltvr

Reference

OpenWRT编译系统分析之subdir函数的用法

本文作者：litreily
本文链接：https://www.litreily.top/2021/01/13/mk-subdir/
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