usr: Bootstrap the initramfs for the kernel

Introduce a sample `foo` program which already calls the
`write_and_block` system call which is yet to be implemented by this
kernel. Moreover, the same CPIO format is used for the initramfs as with
Linux, so we can pass it with QEMU without worries.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

4 files changed, 57 insertions, 12 deletions

diff --git a/.gitignore b/.gitignore
index f1ccbff..d87610d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,8 @@
 kernel/fbos.ld
 /fbos
 .cache
+usr/bin/*
+usr/initramfs.cpio
 
 # You can generate it with Bear: `$ bear -- make`.
 compile_commands.json
diff --git a/Makefile b/Makefile
index df82342..6799281 100644
--- a/Makefile
+++ b/Makefile
@@ -18,15 +18,19 @@ endif
 # no `-mcpu`, no `-mtune`, no funny business.
 
 CC    = $(CROSS_COMPILE)gcc$(CC_SUFFIX)
+AS    = $(CROSS_COMPILE)as$(CC_SUFFIX)
 LD    = $(CROSS_COMPILE)ld
 QEMU ?= qemu-system-riscv64
 
 ISA     ?= rv64imafdc_zicntr_zicsr_zifencei_zihpm_zca_zcd_zba_zbb
-CCFLAGS  = -march=$(ISA) -Iinclude/ -mabi=lp64d
+ASFLAGS  = -march=$(ISA) -mabi=lp64d
+CCFLAGS  = $(ASFLAGS) -Iinclude/
 CCFLAGS += -Werror -Wpedantic -Wall -Wextra -Wcast-align -Wcast-qual -Winit-self \
            -Wmissing-include-dirs -Wredundant-decls -Wshadow -Wsign-conversion \
-           -Wswitch-default -Wundef -Wunreachable-code -Wmissing-noreturn
-LDFLAGS  = -Iinclude/ -static -nostdlib -melf64lriscv -z noexecstack
+           -Wswitch-default -Wundef -Wunreachable-code -Wmissing-noreturn \
+           -nostdinc -nostdlib -std=gnu17
+LDFLAGS  = -Iinclude/ -static -melf64lriscv -z noexecstack
+USRFLAGS = -static -melf64lriscv
 
 ##
 # Optional parameters for QEMU and gdb.
@@ -42,26 +46,29 @@ GDB_EXTRA_FLAGS ?=
 DEBUG =
 ifeq ($(strip $(DEBUG)),)
 	CCFLAGS += -O3
+	QEMU_FLAGS += -nographic
 else
 	CCFLAGS += -g
 	QEMU_FLAGS += -s -S
 endif
 
 ##
-# Paths.
+# Paths
 
 SRC    = $(wildcard kernel/head.S kernel/*.c)
 OBJ    = $(patsubst %.c,%.o,$(patsubst %.S,%.o,$(SRC)))
 LINKER = kernel/fbos.ld
 KRNL   = fbos
+USR    = usr/bin/foo
+INIT   = usr/initramfs.cpio
 
 LDFLAGS += -T $(LINKER)
 
 ##
-# Targets
+# Kernel
 
 .PHONY: all
-all: clean $(KRNL)
+all: clean $(KRNL) usr
 
 .PHONY: $(KRNL)
 $(KRNL): $(OBJ) $(LINKER).S
@@ -78,12 +85,34 @@ $(KRNL): $(OBJ) $(LINKER).S
 	$(E) "	CC	" $(*F)
 	$(Q) $(CC) $(CCFLAGS) -c $< -o $@
 
+##
+# User space
+
+.PHONY: usr
+usr: $(USR)
+	$(E) "	CPIO	" $(INIT)
+	$(Q) find usr/bin/ -type f -executable | cpio -o --quiet -H newc > $(INIT)
+
+usr/src/%.o: usr/src/%.S
+	$(E) "	AS	" $(basename $@)
+	$(Q) $(AS) $(ASFLAGS) -c $< -o $@
+
+usr/bin/%: usr/src/%.o
+	$(E) "	LD	" $@
+	$(Q) $(LD) $(USRFLAGS) $< -o $@
+
+# HACK: do not remove object files from usr/src/.
+.SECONDARY:
+
+##
+# Hacking
+
 .PHONY: qemu
-qemu: $(KRNL)
+qemu: $(KRNL) usr
 ifeq ($(strip $(QEMU_BIOS)),)
-	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -kernel $(KRNL) -nographic
+	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -kernel $(KRNL) -initrd $(INIT)
 else
-	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -bios $(QEMU_BIOS) -kernel $(KRNL) -nographic
+	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -bios $(QEMU_BIOS) -kernel $(KRNL) -initrd $(INIT)
 endif
 
 .PHONY: gdb
@@ -92,7 +121,7 @@ gdb:
 
 .PHONY: clean
 clean:
-	$(Q) rm -f $(OBJ) $(KRNL) $(LINKER)
+	$(Q) rm -f $(OBJ) $(KRNL) $(LINKER) $(USR) usr/src/*.o $(INIT)
 
 .PHONY: lint
 lint:
diff --git a/README.md b/README.md
index 5833b55..8ce947f 100644
--- a/README.md
+++ b/README.md
@@ -8,8 +8,9 @@ are unaware of each other and it's up to the kernel to schedule them by using
 the timer interrupts as given by openSBI (`fizz` on % 3 seconds, `buzz` on % 5
 seconds, and `fizzbuzz` on % 15 seconds).
 
-This kernel provides just one system call, `write`, which allows any program to
-pass the string to be written into the serial port.
+This kernel provides just one system call, `write_and_block`, which allows any
+program to pass the string to be written into the serial port and wait for the
+kernel to re-schedule it.
 
 ## Build
 
diff --git a/usr/src/foo.S b/usr/src/foo.S
new file mode 100644
index 0000000..57e82a1
--- /dev/null
+++ b/usr/src/foo.S
@@ -0,0 +1,13 @@
+.global _start
+.text
+
+_start:
+	li a0, 1
+	la a1, foo
+	ecall
+.Loop:
+	j .Loop
+
+.section .rodata
+foo:
+	.string "foo\n"