debug(core): introduce adjustable wait time

core/embed/prodtest/main.c

@@ -208,6 +208,14 @@ static void test_display(const char *colors) {
   vcp_println("OK");
 }
 
+extern int random_delays_wait_time;
+
+static void set_wait_time(const char *wait_time_string) {
+  int wait_time = atoi(wait_time_string);
+  random_delays_wait_time = wait_time;
+  vcp_println("OK");
+}
+
 #ifdef USE_BUTTON
 
 static secbool test_btn_press(uint32_t deadline, uint32_t btn) {
@@ -678,6 +686,10 @@ int main(void) {
 
     } else if (startswith(line, "DISP ")) {
       test_display(line + 5);
+
+    } else if (startswith(line, "SET WAIT TIME ")) {
+      set_wait_time(line + 14);
+
 #ifdef USE_BUTTON
     } else if (startswith(line, "BUTTON ")) {
       test_button(line + 7);

core/embed/trezorhal/stm32f4/random_delays.c

@@ -54,6 +54,8 @@ _Static_assert(CHACHA_DRBG_OPTIMAL_RESEED_LENGTH(1) == DRBG_TRNG_ENTROPY_LENGTH,
                "");
 #define BUFFER_LENGTH 64
 
+int random_delays_wait_time = 0;
+
 static CHACHA_DRBG_CTX drbg_ctx;
 static secbool drbg_initialized = secfalse;
 static uint8_t session_delay;
@@ -193,18 +195,8 @@ void rdi_handler(uint32_t uw_tick) {
  * against fault injection.
  */
 void wait_random(void) {
-  int wait = drbg_random8();
   volatile int i = 0;
-  volatile int j = wait;
-  while (i < wait) {
-    if (i + j != wait) {
-      shutdown_privileged();
-    }
-    ++i;
-    --j;
-  }
-  // Double-check loop completion.
-  if (i != wait || j != 0) {
-    shutdown_privileged();
+  for (i = 0; i < random_delays_wait_time; i++) {
+    __asm__("nop");
   }
 }