openpilot/panda/board/early_init.h

// Early bringup
#define ENTER_BOOTLOADER_MAGIC 0xdeadbeefU
#define ENTER_SOFTLOADER_MAGIC 0xdeadc0deU
#define BOOT_NORMAL 0xdeadb111U

extern void *g_pfnVectors;
extern uint32_t enter_bootloader_mode;

typedef void (*bootloader_fcn)(void);
typedef bootloader_fcn *bootloader_fcn_ptr;

static void jump_to_bootloader(void) {
  // do enter bootloader
  enter_bootloader_mode = 0;

  bootloader_fcn_ptr bootloader_ptr = (bootloader_fcn_ptr)BOOTLOADER_ADDRESS;
  bootloader_fcn bootloader = *bootloader_ptr;

  // jump to bootloader
  enable_interrupts();
  bootloader();

  // reset on exit
  enter_bootloader_mode = BOOT_NORMAL;
  NVIC_SystemReset();
}

void early_initialization(void) {
  // Reset global critical depth
  disable_interrupts();
  global_critical_depth = 0;

  // Init register and interrupt tables
  init_registers();

  // Always clear any bootloader magic left in RAM. A stale value from a
  // previous bootloader/DFU session would otherwise send us back into the ROM
  // bootloader on a plain hardware reset (which then does not enumerate USB).
  enter_bootloader_mode = BOOT_NORMAL;

  // if wrong chip, reboot
  volatile unsigned int id = DBGMCU->IDCODE;
  if ((id & 0xFFFU) != MCU_IDCODE) {
    enter_bootloader_mode = ENTER_BOOTLOADER_MAGIC;
  }

  // setup interrupt table
  SCB->VTOR = (uint32_t)&g_pfnVectors;

  // early GPIOs float everything
  early_gpio_float();

  detect_board_type();

  // Bootloader entry is done directly by the 0xd1 comms handler via
  // jump_to_bootloader(), not through a magic RAM value (which can be stale).
}