Add custom bootrom

bootrom/.gitignore

@@ -0,0 +1 @@
+build/

bootrom/Makefile

@@ -0,0 +1,23 @@
+CC ?= riscv64-unknown-elf-gcc
+OBJCOPY ?= riscv64-unknown-elf-objcopy
+ROM_BASE ?= 0x00100
+
+LDFLAGS=-Tlinker.ld -nostdlib -nostartfiles -nodefaultlibs -static -Wl,--no-gc-sections -Wl,--defsym=ROM_BASE=$(ROM_BASE)
+
+b=rbootrom
+
+all: $b.elf $b.bin
+
+$b.o: $b.S
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+$b.elf: $b.o linker.ld
+	$(CC) $(LDFLAGS) -o $@ $<
+
+$b.bin: $b.elf
+	$(OBJCOPY) -O binary $b.elf $b.bin
+
+clean:
+	rm -f $b.elf $b.bin $b.o
+
+.PHONY: clean all

bootrom/linker.ld

@@ -0,0 +1,19 @@
+OUTPUT_ARCH( "riscv" )
+ENTRY(_start)
+
+SECTIONS
+{
+    ROM_BASE = DEFINED(ROM_BASE) ? ROM_BASE : 0x00100; /* Default to 0x00100 if ROM_BASE is not defined */
+
+    . = ROM_BASE;
+    .text.start : { *(.text.start) }
+    . = ROM_BASE + 0x80;
+    .text.hang : { *(.text.hang) }
+    /*
+    . = ROM_BASE + 0xC0;
+    .rodata.dtb : { *(.rodata.dtb) }
+    */
+
+    . = ALIGN(0x100);
+    .text : { *(.text) }
+}

bootrom/rbootrom.S

@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2024, Barcelona Supercomputing Center (BSC)
+ * SPDX-License-Identifier: MIT
+ *
+ * RBOOTROM v1.0
+ * Modified by Rodrigo Arias Mallo <rodrigo.arias@bsc.es>
+ *
+ * This is a custom bootrom that prints some information to the UART when
+ * starting, as well as when it hangs. It assumes the UART is at UART_BASE and
+ * it will jump to DRAM_BASE to continue the boot.
+ */
+
+#define DRAM_BASE 0x80000000
+#define UART_BASE 0x40001000
+
+.macro PUTC, ch
+	li a0, \ch
+	jal putchar
+.endm
+
+.section .text.start, "ax", @progbits
+.globl _start
+_start:
+	jal print_hello		// Print initial message on HART 0 only
+	csrr a0, mhartid	// Load HART ID into a0
+	li s0, DRAM_BASE	// Load next address into s0
+	jr s0			// Jump to s0
+
+.section .text.hang, "ax", @progbits
+.globl _hang
+_hang:
+
+	PUTC '\n'
+	PUTC '\r'
+	PUTC 'R'
+	PUTC 'B'
+	PUTC 'O'
+	PUTC 'O'
+	PUTC 'T'
+	PUTC 'R'
+	PUTC 'O'
+	PUTC 'M'
+	PUTC ' '
+	PUTC 'H'
+	PUTC 'A'
+	PUTC 'N'
+	PUTC 'G'
+	PUTC '\n'
+	PUTC '\r'
+
+	/* Hang */
+	csrr a0, mhartid
+1:
+	wfi
+	j 1b
+
+.section .text, "ax", @progbits
+
+putchar:
+	// Wait a bit before writing to UART
+	li t0, 100000		// Delay a bit
+1:
+	addi t0, t0, -1		// Decrement
+	bne  t0, zero, 1b	// Repeat if not zero
+
+	// Write the character now
+	la t0, UART_BASE	// Transmitter Holding Buffer
+	sb a0, 0(t0)		// Write a0 byte
+	ret
+
+print_hello:
+	csrr t0, mhartid	// Load HART ID into a0
+	beq  t0, zero, 1f	// Print message on HART 0 only
+	ret
+1:
+	// Identify bootroom
+	PUTC '\n'
+	PUTC '\r'
+	PUTC 'R'
+	PUTC 'B'
+	PUTC 'O'
+	PUTC 'O'
+	PUTC 'T'
+	PUTC 'R'
+	PUTC 'O'
+	PUTC 'M'
+	PUTC ' '
+	PUTC 'v'
+	PUTC '1'
+	PUTC '.'
+	PUTC '0'
+	PUTC '\n'
+	PUTC '\r'
+	ret

overlay.nix

@@ -107,4 +107,18 @@ final: prev:
     hardeningDisable = [ "all" ];
   };
 
+  rbootrom = prev.pkgsStatic.stdenv.mkDerivation {
+    name = "sa-fpga-tests";
+    src = ./bootrom;
+    dontConfigure = true;
+    buildPhase = ''
+      make
+    '';
+    installPhase = ''
+      mkdir $out
+      cp rbootrom.bin rbootrom.elf $out/
+    '';
+    dontFixup = true;
+    hardeningDisable = [ "all" ];
+  };
 }