探索Linux内核：Kconfig/kbuild的秘密

在下面这个简化的 makefile 代码的帮助下，了解如何在 Linux 内核中实现递归 make：

# In top Makefile
vmlinux: scripts/link-vmlinux.sh $(vmlinux-deps)
                +$(call if_changed,link-vmlinux)
 
# Variable assignments
vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN) $(KBUILD_VMLINUX_LIBS)
 
export KBUILD_VMLINUX_INIT := $(head-y) $(init-y)
export KBUILD_VMLINUX_MAIN := $(core-y) $(libs-y2) $(drivers-y) $(net-y) $(virt-y)
export KBUILD_VMLINUX_LIBS := $(libs-y1)
export KBUILD_LDS          := arch/$(SRCARCH)/kernel/vmlinux.lds
 
init-y          := init/
drivers-y       := drivers/ sound/ firmware/
net-y           := net/
libs-y          := lib/
core-y          := usr/
virt-y          := virt/
 
# Transform to corresponding built-in.a
init-y          := $(patsubst %/, %/built-in.a, $(init-y))
core-y          := $(patsubst %/, %/built-in.a, $(core-y))
drivers-y       := $(patsubst %/, %/built-in.a, $(drivers-y))
net-y           := $(patsubst %/, %/built-in.a, $(net-y))
libs-y1         := $(patsubst %/, %/lib.a, $(libs-y))
libs-y2         := $(patsubst %/, %/built-in.a, $(filter-out %.a, $(libs-y)))
virt-y          := $(patsubst %/, %/built-in.a, $(virt-y))
 
# Setup the dependency. vmlinux-deps are all intermediate objects, vmlinux-dirs
# are phony targets, so every time comes to this rule, the recipe of vmlinux-dirs
# will be executed. Refer "4.6 Phony Targets" of `info make`
$(sort $(vmlinux-deps)): $(vmlinux-dirs) ;
 
# Variable vmlinux-dirs is the directory part of each built-in.a
vmlinux-dirs    := $(patsubst %/,%,$(filter %/, $(init-y) $(init-m) 
                     $(core-y) $(core-m) $(drivers-y) $(drivers-m) 
                     $(net-y) $(net-m) $(libs-y) $(libs-m) $(virt-y)))
 
# The entry of recursive make
$(vmlinux-dirs):
                $(Q)$(MAKE) $(build)=$@ need-builtin=1

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