target: cortex-m: defer cache identification on Cortex-M7 under reset

On Cortex-M7 only, several registers in System Control Space (SCS)
are not accessible when the CPU is under reset, generating a bus
error.
This cause OpenOCD to fail examining the CPU when the board reset
button is pressed or when the flag 'connect_assert_srst' is used
on 'reset_config' command.

Introduce a deferred identification of the cache and run it during
polling and at target halted (just in case of polling disabled).

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Ahmed HAOUES <ahmed.haoues@st.com>

Author: borneoa

src/target/armv7m_cache.c

@@ -68,7 +68,7 @@ static struct armv7m_cache_size decode_ccsidr(uint32_t ccsidr)
 	return size;
 }
 
-int armv7m_identify_cache(struct target *target)
+static int armv7m_identify_cache_internal(struct target *target)
 {
 	struct armv7m_common *armv7m = target_to_armv7m(target);
 	struct armv7m_cache_common *cache = &armv7m->armv7m_cache;
@@ -189,6 +189,54 @@ int armv7m_identify_cache(struct target *target)
 	return ERROR_OK;
 }
 
+/*
+ * On Cortex-M7 only, when the CPU is kept in reset, several registers of the
+ * System Control Space (SCS) are not accessible and return bus error.
+ * The list of accessible registers is:
+ * - 0xE000ED00
+ * - 0xE000ED30
+ * - 0xE000EDF0 ... 0xE000EEFC
+ * - 0xE000EF40 ... 0xE000EF48
+ * - 0xE000EFD0 ... 0xE000EFFC
+ * This makes impossible detecting the cache during the reset.
+ * Use a deferred mechanism to detect the cache during polling or when the
+ * Cortex-M7 halts.
+ */
+int armv7m_identify_cache(struct target *target)
+{
+	struct cortex_m_common *cortex_m = target_to_cm(target);
+	struct armv7m_common *armv7m = target_to_armv7m(target);
+	struct armv7m_cache_common *cache = &armv7m->armv7m_cache;
+
+	if (cache->info_valid)
+		return ERROR_OK;
+
+	if (cortex_m->core_info->impl_part == CORTEX_M7_PARTNO
+			&& cortex_m->dcb_dhcsr & S_RESET_ST) {
+		cache->defer_identification = true;
+		return ERROR_OK;
+	}
+
+	return armv7m_identify_cache_internal(target);
+}
+
+int armv7m_deferred_identify_cache(struct target *target)
+{
+	struct armv7m_common *armv7m = target_to_armv7m(target);
+	struct armv7m_cache_common *cache = &armv7m->armv7m_cache;
+
+	if (cache->info_valid || !cache->defer_identification)
+		return ERROR_OK;
+
+	int retval = armv7m_identify_cache_internal(target);
+	if (retval != ERROR_OK)
+		return retval;
+
+	cache->defer_identification = false;
+
+	return ERROR_OK;
+}
+
 extern struct command_context *global_cmd_ctx;
 
 static int armv7m_cache_helper(struct target *target, const char *helper,
@@ -334,6 +382,11 @@ int armv7m_handle_cache_info_command(struct command_invocation *cmd,
 		return ERROR_FAIL;
 	}
 
+	if (cache->defer_identification) {
+		command_print(cmd, "Cache not detected yet");
+		return ERROR_OK;
+	}
+
 	if (!cache->info_valid) {
 		command_print(cmd, "No cache detected");
 		return ERROR_OK;

src/target/armv7m_cache.h

@@ -38,6 +38,7 @@ struct armv7m_arch_cache {
 // common cache information
 struct armv7m_cache_common {
 	bool info_valid;
+	bool defer_identification;
 	unsigned int loc;					// level of coherency
 	uint32_t d_min_line_len;			// minimum d-cache line_len
 	uint32_t i_min_line_len;			// minimum i-cache line_len
@@ -47,6 +48,7 @@ struct armv7m_cache_common {
 };
 
 int armv7m_identify_cache(struct target *target);
+int armv7m_deferred_identify_cache(struct target *target);
 int armv7m_d_cache_flush(struct target *target, uint32_t address,
 	unsigned int length);
 int armv7m_i_cache_inval(struct target *target, uint32_t address,

src/target/cortex_m.c

@@ -832,6 +832,9 @@ static int cortex_m_debug_entry(struct target *target)
 	}
 
 	// read caches state
+	retval = armv7m_deferred_identify_cache(target);
+	if (retval != ERROR_OK)
+		return retval;
 	uint32_t ccr = 0;
 	if (armv7m->armv7m_cache.info_valid) {
 		retval = mem_ap_read_u32(armv7m->debug_ap, CCR, &ccr);
@@ -951,6 +954,10 @@ static int cortex_m_poll_one(struct target *target)
 			LOG_TARGET_INFO(target, "external reset detected");
 		}
 		return ERROR_OK;
+	} else {
+		retval = armv7m_deferred_identify_cache(target);
+		if (retval != ERROR_OK)
+			return retval;
 	}
 
 	if (target->state == TARGET_RESET) {
@@ -2698,6 +2705,16 @@ int cortex_m_examine(struct target *target)
 		if (retval != ERROR_OK)
 			return retval;
 
+		/*
+		 * Use a safe value of sticky S_RESET_ST for cache detection, before
+		 * clearing it below.
+		 */
+		retval = armv7m_identify_cache(target);
+		if (retval != ERROR_OK) {
+			LOG_ERROR("Cannot detect cache");
+			return retval;
+		}
+
 		/*  Don't cumulate sticky S_RESET_ST at the very first read of DHCSR
 		 *  as S_RESET_ST may indicate a reset that happened long time ago
 		 *  (most probably the power-on reset before OpenOCD was started).
@@ -2767,12 +2784,6 @@ int cortex_m_examine(struct target *target)
 		LOG_TARGET_INFO(target, "target has %d breakpoints, %d watchpoints",
 			cortex_m->fp_num_code,
 			cortex_m->dwt_num_comp);
-
-		retval = armv7m_identify_cache(target);
-		if (retval != ERROR_OK) {
-			LOG_ERROR("Cannot detect cache");
-			return retval;
-		}
 	}
 
 	return ERROR_OK;