MCUBOOT_SERIAL_WAIT_FOR_DFU

[FaustynaS]

Hi,
I noticed that the MCUBOOT_SERIAL_WAIT_FOR_DFU option might not work as expected. In the boot_serial_read_console function (boot_serial.c), the start time is assigned inside the while loop. If the elapsed time between setting the start time and the check_timeout part of the loop is less than 1 ms, the elapsed time will very often be 0, which can cause the device to get stuck in this loop for a very long time.

 ...
    while (timeout_in_ms > 0 || bs_entry) {
...
#ifdef MCUBOOT_SERIAL_WAIT_FOR_DFU
        uint32_t start = k_uptime_get_32();
#endif
...
check_timeout:
        /* Subtract elapsed time */
#ifdef MCUBOOT_SERIAL_WAIT_FOR_DFU
        elapsed_in_ms = (k_uptime_get_32() - start);
#endif
        timeout_in_ms -= elapsed_in_ms;
    }

My suggested change:

@@ -1514,6 +1521,11 @@ boot_serial_read_console(const struct boot_uart_funcs *f,int timeout_in_ms)
     max_input = sizeof(in_buf);
 
     off = 0;
+
+#ifdef MCUBOOT_SERIAL_WAIT_FOR_DFU
+    uint32_t start = k_uptime_get_32();
+#endif
+
     while (timeout_in_ms > 0 || bs_entry) {
         /*
          * Don't enter CPU idle state here if timeout based serial recovery is
@@ -1527,9 +1539,8 @@ boot_serial_read_console(const struct boot_uart_funcs *f,int timeout_in_ms)
         }
 #endif
         MCUBOOT_WATCHDOG_FEED();
-#ifdef MCUBOOT_SERIAL_WAIT_FOR_DFU
-        uint32_t start = k_uptime_get_32();
-#endif
         rc = f->read(in_buf + off, sizeof(in_buf) - off, &full_line);
         if (rc <= 0 && !full_line) {
 #ifndef MCUBOOT_SERIAL_WAIT_FOR_DFU
@@ -1564,7 +1575,11 @@ boot_serial_read_console(const struct boot_uart_funcs *f,int timeout_in_ms)
 check_timeout:
         /* Subtract elapsed time */
 #ifdef MCUBOOT_SERIAL_WAIT_FOR_DFU
-        elapsed_in_ms = (k_uptime_get_32() - start);
+        uint32_t end_time = k_uptime_get_32();
+        elapsed_in_ms = end_time - start;
+        if (end_time != start) {
+            start = end_time;
+        }
 #endif
         timeout_in_ms -= elapsed_in_ms;
     }