Makefile: refactor subdirectory makefiles

Makefile

@@ -22,11 +22,15 @@ KCFLAGS = -g -Wall -Wextra -Werror -nostdlib -nostartfiles -fno-builtin -std=gnu
 KLDFLAGS = -T link.ld -L /usr/lib/gcc/arm-elf/4.7.0/
 EXTRA_LIBS = -lgcc
 
-KOBJS =
-
-include boot/Makefile.inc
-include kernel/Makefile.inc
-include devices/Makefile.inc
+# Define KOBJS as a 'simply expanded' variable
+KOBJS :=
+
+# Initialize sub-directory Makefile inclusion
+BASEDIR = $(shell pwd)
+SUBDIRS := boot kernel devices
+ifneq (,$(SUBDIRS))
+	include $(patsubst %,%/kernel.mk,$(SUBDIRS))
+endif
 
 .PHONY: all clean boot boot-gdb
 

boot/Makefile.inc

@@ -1,3 +0,0 @@
-BOOT_PREFIX = boot
-
-KOBJS += $(BOOT_PREFIX)/start.o

boot/kernel.mk

@@ -0,0 +1,10 @@
+DIRNAME := boot
+SUBDIRS :=
+
+include $(BASEDIR)/header.mk
+
+OBJS_$(d) := $(d)/start.o
+
+KOBJS += $(OBJS_$(d))
+
+include $(BASEDIR)/footer.mk

devices/Makefile.inc

@@ -1,5 +0,0 @@
-DEVICES_PREFIX = devices
-
-KOBJS += $(DEVICES_PREFIX)/pl011.o
-KOBJS += $(DEVICES_PREFIX)/pl110.o
-KOBJS += $(DEVICES_PREFIX)/pl111.o

devices/kernel.mk

@@ -0,0 +1,12 @@
+DIRNAME := devices
+SUBDIRS :=
+
+include $(BASEDIR)/header.mk
+
+OBJS_$(d) := $(d)/pl011.o \
+	$(d)/pl110.o \
+	$(d)/pl111.o
+
+KOBJS += $(OBJS_$(d))
+
+include $(BASEDIR)/footer.mk

footer.mk

@@ -0,0 +1,12 @@
+# Included at the bottom of each subdirectory Makefile
+
+# Include any subdirectory Makefiles
+ifneq (,$(SUBDIRS))
+	include $(patsubst %,$(d)/%/kernel.mk,$(SUBDIRS))
+endif
+
+# Pop the previous directory off the 'stack' 
+d := $(dirstack_$(sp))
+sp := $(basename $(sp))
+# Clear SUBDIRS so that it defaults to empty for the next directory
+SUBDIRS :=

header.mk

@@ -0,0 +1,13 @@
+# Included at the top of each subdirectory Makefile, immediately
+# following the local re-definition of DIRNAME
+
+# Push the previous directory onto the 'stack'
+sp := $(sp).x
+dirstack_$(sp) := $(d)
+
+# Update $(d) to be the current directory
+ifneq (,$(d))
+	d := $(d)/$(DIRNAME)
+else
+	d := $(DIRNAME)
+endif

kernel/Makefile.inc

@@ -1,11 +0,0 @@
-KERNEL_PREFIX = kernel
-
-KOBJS += $(KERNEL_PREFIX)/atags.o
-KOBJS += $(KERNEL_PREFIX)/console.o
-KOBJS += $(KERNEL_PREFIX)/font.o
-KOBJS += $(KERNEL_PREFIX)/framebuffer.o
-KOBJS += $(KERNEL_PREFIX)/list.o
-KOBJS += $(KERNEL_PREFIX)/mm.o
-KOBJS += $(KERNEL_PREFIX)/mmu.o
-KOBJS += $(KERNEL_PREFIX)/print.o
-KOBJS += $(KERNEL_PREFIX)/start_kernel.o

kernel/kernel.mk

@@ -0,0 +1,19 @@
+DIRNAME := kernel
+SUBDIRS :=
+
+include $(BASEDIR)/header.mk
+
+OBJS_$(d) := $(d)/atags.o \
+	$(d)/console.o \
+	$(d)/font.o \
+	$(d)/framebuffer.o \
+	$(d)/kmalloc.o \
+	$(d)/list.o \
+	$(d)/mm.o \
+	$(d)/mmu.o \
+	$(d)/print.o \
+	$(d)/start_kernel.o
+
+KOBJS += $(OBJS_$(d))
+
+include $(BASEDIR)/footer.mk