Provide an improved example on early boot stage

This also implied a renaming arch/riscv/bios -> arch/riscv/machine.

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

1 files changed, 125 insertions, 0 deletions

diff --git a/arch/riscv/machine/machine.S b/arch/riscv/machine/machine.S
new file mode 100644
index 0000000..8a7ce8d
--- /dev/null
+++ b/arch/riscv/machine/machine.S
@@ -0,0 +1,125 @@
+.section .text
+
+# Returns the physical address that was identified to be for the serial port. If
+# any error was found then it just returns 0. This function accepts one argument
+# which is a pointer to the FDT structure as given by the firmware. You can
+# consider this function to have this equivalent C signature:
+#
+#   uint64_t get_serial_address(void *fdt);
+#
+.type get_serial_address, @function
+get_serial_address:
+	# Leave early if the caller passed a NULL pointer.
+	beqz a0, .parse_error
+
+	# First loop. This one simply iterates over the FDT structure in search for
+    # 'serial@', which marks the beginning of the address definition.
+.search_serial_loop_reset:
+	la t1, .serial
+.search_serial_loop:
+	lbu t2, (t1)
+	beqz t2, .parse_hex
+
+	lbu t3, (a0)
+	addi a0, a0, 1
+	bne t2, t3, .search_serial_loop_reset
+	addi t1, t1, 1
+	j .search_serial_loop
+
+	# Second loop. On each digit shift left one nibble and add the character
+    # converted to an integer. If the character cannot be converted, then we go
+    # to `parse_error` which sets the return value to 0.
+.parse_hex:
+	mv t1, zero
+	li t2, 10
+	li t3, 0x30	# Character '0'
+
+.parse_hex_loop:
+	# The FDT specification guarantees that strings are NULL-terminated. Thus,
+    # whenever we find the NULL character, then we are done parsing.
+	lbu t0, (a0)
+	beqz t0, .get_serial_address_end
+
+	# Trying to parse a numeric character. Note that this parser is grossly
+    # uncapable of handling Aa-Ff hexadecimal values, and only knows numeric
+    # digits. This can be improved but I did not have an example to work
+	# on this case.
+	sub t0, t0, t3
+	bltz t0, .parse_error
+	blt t2, t0, .parse_error
+
+	# Shift one nibble on the accumulator and add the computed value to it.
+	sll t1, t1, 4
+	add t1, t1, t0
+
+	# Next loop.
+	addi a0, a0, 1
+	j .parse_hex_loop
+
+.parse_error:
+	mv t1, zero
+.get_serial_address_end:
+	mv a0, t1
+	ret
+
+# Send the `message` to the given `address` so it's printed there. You can
+# consider this function to have this equivalent C signature:
+#
+#   void printm(uint64_t address, char *message);
+#
+.type printm, @function
+printm:
+	mv t1, a1
+
+.printm_loop:
+	lbu t0, (t1)
+	beqz t0, .printm_end
+
+	sb t0, (a0)
+	addi t1, t1, 1
+	j .printm_loop
+
+.printm_end:
+	ret
+
+.section .text.bios
+
+# The entry point: the firmware will blindly jump here. We expect the firmware
+# to pass up two arguments, the first one being the hart ID, and the other being
+# a pointer to the FDT describing this machine. You can consider this function
+# to have this equivalent C signature:
+#
+#   void _start(uint64_t hart_id, void *fdt) __attribute__((noreturn));
+#
+.global _start
+.type _start, @function
+_start:
+	# We expect the configuration to be given as a devicetree blob pointed by
+	# the second argument (a1). Hence, the configuration pointer from RISC-V
+	# should be set to NULL indicating that. If that's not the case, then it's
+	# not a supported scenario and we jump to the infinite loop gracefully.
+	csrr t0, mconfigptr
+	bnez t0, .end
+
+	# We don't care about the hard ID, but we need the FDT pointer as an
+    # argument for the `get_serial_address` function.
+	mv a0, a1
+	call get_serial_address
+
+	# If the return value of `get_serial_address` is a NULL value, then
+    # something went wrong there and we just have to end it here.
+	beqz a0, .end
+
+	# And now we can print our message.
+	la a1, .msg1
+	call printm
+
+	# Infinite loop so not to crash :)
+.end:
+	j .end
+
+.section .rodata
+.msg1:
+	.string "Hello, world!\n"
+.serial:
+	.string "serial@"