GNU Make correct behaviour when including other makefiles - makefile

Question

I have the following makefile in the root of the project:
Makefile
# Board version
# Available: 3
PI ?= 3
# Kernel binaries
ifeq ($(PI), 3)
KERNEL_IMG := kernel8.img
else ifeq ($(PI), 2)
KERNEL_IMG := kernel7.img
else ifeq ($(PI), 1)
KERNEL_IMG := kernel.img
else
$(error Unsupported Raspberry Pi version)
endif
KERNEL_ELF := $(patsubst %.img,%.elf,$(KERNEL_IMG))
# Directories/paths
BUILD_DIR := build
# Toolchain
TOOLCHAIN ?= aarch64-elf
OBJCOPY := $(TOOLCHAIN)-objcopy
LD := $(TOOLCHAIN)-ld
CC := $(TOOLCHAIN)-gcc
# Misc
LINKER_SCRIPT := linker.ld
# Flags
LDFLAGS := -T $(LINKER_SCRIPT)
ASFLAGS :=
CFLAGS :=
# Source files
C_SRC := $(wildcard *.c)
ASM_SRC := $(wildcard *.S)
# Include
include pi/$(PI)/mod.mk
# Object files
OBJECTS := $(patsubst %,$(BUILD_DIR)/%.o,$(C_SRC))
OBJECTS += $(patsubst %,$(BUILD_DIR)/%.o,$(ASM_SRC))
# Targets
.PHONY: all builddirs clean
all: $(BUILD_DIR)/$(KERNEL_IMG)
$(BUILD_DIR)/$(KERNEL_IMG): $(BUILD_DIR)/$(KERNEL_ELF)
$(OBJCOPY) $< -O binary $#
$(BUILD_DIR)/$(KERNEL_ELF): $(LINKER_SCRIPT) $(OBJECTS)
$(LD) $(OBJECTS) $(LDFLAGS) -o $#
$(OBJECTS): | builddirs
builddirs: $(BUILD_DIR)/pi/$(PI)
$(BUILD_DIR)/pi/$(PI):
mkdir -p $#
$(BUILD_DIR)/%.S.o: %.S
$(CC) -c $< $(ASFLAGS) -o $#
$(BUILD_DIR)/%.c.o: %.c
$(CC) -c $< $(CFLAGS) -o $#
clean:
$(RM) -r $(BUILD_DIR)
It includes pi/3/mod.mk
C_SRC +=
ASM_SRC += pi/3/start.S
$(BUILD_DIR)/pi/3/start.S.o: pi/3/start.S pi/3/include/cpu/sysregs.h
$(CC) -c $< $(ASFLAGS) -o $#
Now here's the problem: whenever I run 'make' in the root of a project, '$(BUILD_DIR)/pi/3/start.S.o' rule invokes, instead of 'all'. If I move 'include pi/$(PI)/mod.mk' to the very bottom of the root makefile, and replace 'C_SRC' and 'ASM_SRC' variables in 'pi/3/mod.mk' with 'OBJECTS += $(BUILD_DIR)/pi/3/start.S.o' and invoke 'make', this rule isn't even invoked, so I get an error that make doesn't know how to build start.S.o.
What am I doing wrong and what is the best way to handle this?

Answer 1

Make's default goal is the first target in your Makefile. In your case the first target is the one defined in the included Makefile: $(BUILD_DIR)/pi/3/start.S.o. Either invoke make all or move the all rule in your Makefile such that it becomes the first one, or tell make that the default goal is all:
.DEFAULT_GOAL := all
(see GNU make manual).