ARMV7M: handle bkpt instruction on resume/step

Skip over a bkpt instruction if found on resume/step.
Only software breakpoints known to OpenOCD are currently handled.

So this handles the special case of either a user added bkpt
or library added, eg. semi-hosting support.

Signed-off-by: Spencer Oliver <ntfreak@users.sourceforge.net>

src/target/armv7m.c

@@ -694,6 +694,44 @@ int armv7m_blank_check_memory(struct target *target,
 	return ERROR_OK;
 }
 
+int armv7m_maybe_skip_bkpt_inst(struct target *target, bool *inst_found)
+{
+	struct armv7m_common *armv7m = target_to_armv7m(target);
+	struct reg *r = armv7m->core_cache->reg_list + 15;
+	bool result = false;
+
+
+	/* if we halted last time due to a bkpt instruction
+	 * then we have to manually step over it, otherwise
+	 * the core will break again */
+
+	if (target->debug_reason == DBG_REASON_BREAKPOINT)
+	{
+		uint16_t op;
+		uint32_t pc = buf_get_u32(r->value, 0, 32);
+
+		pc &= ~1;
+		if (target_read_u16(target, pc, &op) == ERROR_OK)
+		{
+			if ((op & 0xFF00) == 0xBE00)
+			{
+				pc = buf_get_u32(r->value, 0, 32) + 2;
+				buf_set_u32(r->value, 0, 32, pc);
+				r->dirty = true;
+				r->valid = true;
+				result = true;
+				LOG_DEBUG("Skipping over BKPT instruction");
+			}
+		}
+	}
+
+	if (inst_found) {
+		*inst_found = result;
+	}
+
+	return ERROR_OK;
+}
+
 /*--------------------------------------------------------------------------*/
 
 /*

src/target/armv7m.h

@@ -171,6 +171,8 @@ int armv7m_checksum_memory(struct target *target,
 int armv7m_blank_check_memory(struct target *target,
 		uint32_t address, uint32_t count, uint32_t* blank);
 
+int armv7m_maybe_skip_bkpt_inst(struct target *target, bool *inst_found);
+
 extern const struct command_registration armv7m_command_handlers[];
 
 #endif /* ARMV7M_H */

src/target/cortex_m3.c

@@ -638,6 +638,16 @@ static int cortex_m3_resume(struct target *target, int current,
 		r->valid = true;
 	}
 
+	/* if we halted last time due to a bkpt instruction
+	 * then we have to manually step over it, otherwise
+	 * the core will break again */
+
+	if (!breakpoint_find(target, buf_get_u32(r->value, 0, 32))
+			&& !debug_execution)
+	{
+		armv7m_maybe_skip_bkpt_inst(target, NULL);
+	}
+
 	resume_pc = buf_get_u32(r->value, 0, 32);
 
 	armv7m_restore_context(target);
@@ -690,6 +700,7 @@ static int cortex_m3_step(struct target *target, int current,
 	struct swjdp_common *swjdp = &armv7m->swjdp_info;
 	struct breakpoint *breakpoint = NULL;
 	struct reg *pc = armv7m->core_cache->reg_list + 15;
+	bool bkpt_inst_found = false;
 
 	if (target->state != TARGET_HALTED)
 	{
@@ -709,14 +720,23 @@ static int cortex_m3_step(struct target *target, int current,
 			cortex_m3_unset_breakpoint(target, breakpoint);
 	}
 
+	armv7m_maybe_skip_bkpt_inst(target, &bkpt_inst_found);
+
 	target->debug_reason = DBG_REASON_SINGLESTEP;
 
 	armv7m_restore_context(target);
 
 	target_call_event_callbacks(target, TARGET_EVENT_RESUMED);
 
-	/* set step and clear halt */
-	cortex_m3_write_debug_halt_mask(target, C_STEP, C_HALT);
+	/* if no bkpt instruction is found at pc then we can perform
+	 * a normal step, otherwise we have to manually step over the bkpt
+	 * instruction - as such simulate a step */
+	if (bkpt_inst_found == false)
+	{
+		/* set step and clear halt */
+		cortex_m3_write_debug_halt_mask(target, C_STEP, C_HALT);
+	}
+
 	mem_ap_read_atomic_u32(swjdp, DCB_DHCSR, &cortex_m3->dcb_dhcsr);
 
 	/* registers are now invalid */
@@ -735,6 +755,7 @@ static int cortex_m3_step(struct target *target, int current,
 	LOG_DEBUG("target stepped dcb_dhcsr = 0x%" PRIx32
 			" nvic_icsr = 0x%" PRIx32,
 			cortex_m3->dcb_dhcsr, cortex_m3->nvic_icsr);
+
 	return ERROR_OK;
 }