bootrom: follow SBI (a0=hartid, a1=dtb)

bootrom/Makefile

@@ -5,8 +5,11 @@ OBJCOPY=riscv64-unknown-elf-objcopy
 
 all: $(bootrom_img)
 
-%.img: %.elf
-	$(OBJCOPY) -O binary --change-addresses=-0x1000 --only-section .text $< $@
+%.img: %.bin
+	dd if=$< of=$@ bs=128 count=1
+
+%.bin: %.elf
+	$(OBJCOPY) -O binary $< $@
 
 %.elf: %.S linker.ld
-	$(GCC) -Tlinker.ld $< -nostdlib -static -o $@
+	$(GCC) -Tlinker.ld $< -nostdlib -static -Wl,--no-gc-sections -o $@

bootrom/bootrom.S

@@ -1,13 +1,19 @@
-.text
-.global _start
+.section .text.start, "ax", @progbits
+.globl _start
 _start:
-  // This boot ROM doesn't know about any boot devices, so it just spins,
-  // waiting for the debugger to load a program and change the PC.
-  j _start // reset vector
-  .word 0 // reserved
-  .word 0 // reserved
-  .word 0 // pointer to config string
-  .word 0 // default trap vector
-  .word 0
-  .word 0
-  .word 0
+  la s0, DRAM_BASE
+  csrr a0, mhartid
+  la a1, _dtb
+  jr s0
+
+.section .text.hang, "ax", @progbits
+.globl _hang
+_hang:
+  wfi
+  j _hang
+
+.section .rodata.dtb, "a", @progbits
+.globl _dtb
+.align 5, 0
+_dtb:
+.ascii "DTB goes here"

bootrom/linker.ld

@@ -1,5 +1,12 @@
 SECTIONS
 {
-    . = 0x1000;
-    .text : { *(.text) }
+    DRAM_BASE = 0x80000000;
+    ROM_BASE = 0x1000;
+
+    . = ROM_BASE;
+    .text.start : { *(.text.start) }
+    . = ROM_BASE + 0x40;
+    .text.hang : { *(.text.hang) }
+    . = ROM_BASE + 0x80;
+    .rodata.dtb : { *(.rodata.dtb) }
 }