Fix opcode for the "fence" instruction

OpenOCD currently uses improper "fence" instruction:
"FENCE" opcode with empty predecessor and successor sets.

Such instruction has no effect and is reserved for future use
as a HINT instruction (RISC-V Unprivileged ISA spec V20191213,
section 2.9).

This patch fixes it by using the proper "fence rw,rw"
instruction.

Change-Id: Ia2a66059009153efef27279410850ddfd73dae38
Signed-off-by: Jan Matyas <jan.matyas@codasip.com>

src/target/riscv/opcodes.h

@@ -294,10 +294,11 @@ static uint32_t srli(unsigned int dest, unsigned int src, uint8_t shamt)
 	return inst_rs2(shamt) | inst_rs1(src) | inst_rd(dest) | MATCH_SRLI;
 }
 
-static uint32_t fence(void) __attribute__((unused));
-static uint32_t fence(void)
+static uint32_t fence_rw_rw(void) __attribute__((unused));
+static uint32_t fence_rw_rw(void)
 {
-	return MATCH_FENCE;
+	/* fence rw,rw */
+	return MATCH_FENCE | 0x3300000;
 }
 
 static uint32_t auipc(unsigned int dest) __attribute__((unused));

src/target/riscv/program.c

@@ -151,9 +151,9 @@ int riscv_program_fence_i(struct riscv_program *p)
 	return riscv_program_insert(p, fence_i());
 }
 
-int riscv_program_fence(struct riscv_program *p)
+int riscv_program_fence_rw_rw(struct riscv_program *p)
 {
-	return riscv_program_insert(p, fence());
+	return riscv_program_insert(p, fence_rw_rw());
 }
 
 int riscv_program_ebreak(struct riscv_program *p)

src/target/riscv/program.h

@@ -63,7 +63,7 @@ int riscv_program_csrr(struct riscv_program *p, enum gdb_regno d, enum gdb_regno
 int riscv_program_csrw(struct riscv_program *p, enum gdb_regno s, enum gdb_regno csr);
 
 int riscv_program_fence_i(struct riscv_program *p);
-int riscv_program_fence(struct riscv_program *p);
+int riscv_program_fence_rw_rw(struct riscv_program *p);
 int riscv_program_ebreak(struct riscv_program *p);
 
 int riscv_program_addi(struct riscv_program *p, enum gdb_regno d, enum gdb_regno s, int16_t i);

src/target/riscv/riscv-013.c

@@ -2532,7 +2532,7 @@ static int execute_fence(struct target *target)
 	struct riscv_program program;
 	riscv_program_init(&program, target);
 	riscv_program_fence_i(&program);
-	riscv_program_fence(&program);
+	riscv_program_fence_rw_rw(&program);
 	int result = riscv_program_exec(&program, target);
 	if (result != ERROR_OK)
 		LOG_TARGET_DEBUG(target, "Unable to execute pre-fence");