IO: Fix access to free memory

The free memory was accessed from interrupt context where it should not be accessed from. We build the statistic now at a fixed rate while not armed.

src/modules/px4iofirmware/px4io.c

@@ -89,6 +89,7 @@ static char msg[NUM_MSG][CONFIG_USART1_TXBUFSIZE];
 
 static void heartbeat_blink(void);
 static void ring_blink(void);
+static void update_mem_usage(void);
 
 /*
  * add a debug message to be printed on the console
@@ -128,6 +129,27 @@ show_debug_messages(void)
 	}
 }
 
+/*
+ * Get the memory usage at 2 Hz while not armed
+ */
+static void
+update_mem_usage(void)
+{
+	if (/* IO armed */ (r_status_flags & PX4IO_P_STATUS_FLAGS_SAFETY_OFF)
+			   /* and FMU is armed */ && (r_setup_arming & PX4IO_P_SETUP_ARMING_FMU_ARMED)) {
+		return;
+	}
+
+	static uint64_t last_mem_time = 0;
+	uint64_t now = hrt_absolute_time();
+
+	if (now - last_mem_time > (500 * 1000)) {
+		struct mallinfo minfo = mallinfo();
+		r_page_status[PX4IO_P_STATUS_FREEMEM] = minfo.fordblks;
+		last_mem_time = now;
+	}
+}
+
 static void
 heartbeat_blink(void)
 {
@@ -311,6 +333,7 @@ user_start(int argc, char *argv[])
 	perf_counter_t loop_perf = perf_alloc(PC_INTERVAL, "loop");
 
 	struct mallinfo minfo = mallinfo();
+	r_page_status[PX4IO_P_STATUS_FREEMEM] = minfo.mxordblk;
 	syslog(LOG_INFO, "MEM: free %u, largest %u\n", minfo.mxordblk, minfo.fordblks);
 
 	/* initialize PWM limit lib */
@@ -418,6 +441,8 @@ user_start(int argc, char *argv[])
 			LED_BLUE(true);
 		}
 
+		update_mem_usage();
+
 		ring_blink();
 
 		check_reboot();

src/modules/px4iofirmware/registers.c

@@ -880,10 +880,6 @@ registers_get(uint8_t page, uint8_t offset, uint16_t **values, unsigned *num_val
 	 */
 	case PX4IO_PAGE_STATUS:
 		/* PX4IO_P_STATUS_FREEMEM */
-		{
-			struct mallinfo minfo = mallinfo();
-			r_page_status[PX4IO_P_STATUS_FREEMEM] = minfo.fordblks;
-		}
 
 		/* XXX PX4IO_P_STATUS_CPULOAD */