✨ Add SiFive debug rom

LICENSE.SiFive

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src/debug/debug_rom/Makefile

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+# See LICENSE.SiFive for license details
+# Recursive make is bad, but in this case we're cross compiling which is a
+# pretty unusual use case.
+
+CC = $(RISCV)/bin/riscv64-unknown-elf-gcc
+OBJCOPY = $(RISCV)/bin/riscv64-unknown-elf-objcopy
+
+COMPILE = $(CC) -nostdlib -nostartfiles -I$(RISCV)/include/ -Tlink.ld
+
+ELFS = debug_rom
+DEPS = debug_rom.S link.ld
+
+all: $(patsubst %,%.h,$(ELFS))
+
+publish: debug_rom.scala
+	mv $< ../../src/main/scala/uncore/devices/debug/DebugRomContents.scala
+
+%.scala: %.raw
+	xxd -i $^ | sed -e "s/^unsigned char debug_rom_raw\[\] = {/\/\/ This file was auto-generated by 'make publish' in debug\/ directory.\n\npackage uncore.devices\n\nobject DebugRomContents {\n\n  def apply() : Array[Byte] = { Array (/" \
+	-e  "s/};/  ).map(_.toByte) }\n\n}/" \
+	-e  "s/^unsigned int debug_rom_raw_len.*//"  > $@
+
+
+%.raw:	%
+	$(OBJCOPY) -O binary --only-section .text $^ $@
+
+debug_rom:	$(DEPS)
+	$(COMPILE) -o $@ $^
+
+clean:
+	rm -f $(ELFS) debug_rom*.raw debug_rom*.h

src/debug/debug_rom/debug_rom.S

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+// See LICENSE.SiFive for license details.
+
+#include "spike/encoding.h"
+
+// These are implementation-specific addresses in the Debug Module
+#define HALTED    0x100
+#define GOING     0x104
+#define RESUMING  0x108
+#define EXCEPTION 0x10C
+
+// Region of memory where each hart has 1
+// byte to read.
+#define FLAGS 0x400
+#define FLAG_GO     0
+#define FLAG_RESUME 1
+
+        .option norvc
+        .global entry
+        .global exception
+
+        // Entry location on ebreak, Halt, or Breakpoint
+        // It is the same for all harts. They branch when 
+        // their GO or RESUME bit is set.
+
+entry:
+       jal zero, _entry
+resume:
+       jal zero, _resume
+exception:
+       jal zero, _exception
+
+_entry:
+        // This fence is required because the execution may have written something
+        // into the Abstract Data or Program Buffer registers.
+        fence
+        csrw CSR_DSCRATCH, s0  // Save s0 to allow signaling MHARTID
+
+        // We continue to let the hart know that we are halted in order that
+        // a DM which was reset is still made aware that a hart is halted.
+        // We keep checking both whether there is something the debugger wants
+        // us to do, or whether we should resume.
+entry_loop:
+        csrr s0, CSR_MHARTID
+        sw   s0, HALTED(zero)
+        lbu  s0, FLAGS(s0) // 1 byte flag per hart. Only one hart advances here.
+        andi s0, s0, (1 << FLAG_GO)
+        bnez s0, going
+        csrr s0, CSR_MHARTID
+        lbu  s0, FLAGS(s0) // multiple harts can resume  here
+        andi s0, s0, (1 << FLAG_RESUME)
+        bnez s0, resume
+        jal  zero, entry_loop
+
+_exception:
+        sw      zero, EXCEPTION(zero) // Let debug module know you got an exception.
+        ebreak
+
+going:
+        csrr s0, CSR_DSCRATCH          // Restore s0 here
+        sw zero, GOING(zero)           // When debug module sees this write, the GO flag is reset.
+        jalr zero, zero, %lo(whereto)          // Rocket-Chip has a specific hack which is that jalr in
+                                       // Debug Mode will flush the I-Cache. We need that so that the
+                                       // remainder of the variable instructions will be what Debug Module
+                                       // intends.
+_resume:
+        csrr s0, CSR_MHARTID
+        sw   s0, RESUMING(zero) // When Debug Module sees this write, the RESUME flag is reset.
+        csrr s0, CSR_DSCRATCH   // Restore s0
+        dret
+
+        // END OF ACTUAL "ROM" CONTENTS. BELOW IS JUST FOR LINKER SCRIPT.
+
+.section .whereto
+whereto:
+        nop
+        // Variable "ROM" This is : jal x0 abstract, jal x0 program_buffer,
+        //                or jal x0 resume, as desired.
+        //                Debug Module state machine tracks what is 'desired'.
+        //                We don't need/want to use jalr here because all of the
+        //                Variable ROM contents are set by
+        //                Debug Module before setting the OK_GO byte.

src/debug/debug_rom/link.ld

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+/* See LICENSE.SiFive for license details. */
+OUTPUT_ARCH( "riscv" )
+ENTRY( entry )
+SECTIONS
+{
+    .whereto 0x300 :
+    {
+        *(.whereto)
+    }   
+    . = 0x800;
+    .text :
+    {
+        *(.text)
+    }
+    _end = .;
+}