Interface for programming Option Bytes on stm32g family.

This should probably work for f family too. Untested.

embassy-stm32/src/flash/g.rs

@@ -30,6 +30,51 @@ pub(crate) unsafe fn unlock() {
     }
 }
 
+/// This locks the option bytes, but doesn't "enable" or flash them.
+pub(crate) unsafe fn opt_lock() {
+    pac::FLASH.cr().modify(|w| w.set_optlock(true));
+}
+
+/// Unlock option bytes registers according to RM0440 page 206.
+/// Flash needs to be unlocked first to use this.
+pub(crate) unsafe fn opt_unlock() {
+    // Unlock option bytes
+    if pac::FLASH.cr().read().optlock() {
+        pac::FLASH.optkeyr().write_value(0x0819_2A3B);
+        pac::FLASH.optkeyr().write_value(0x4C5D_6E7F);
+    }
+}
+
+/// This should flash the option bytes and restart the device.
+/// If it returns - something went wrong (eg. option bytes are locked).
+pub(crate) unsafe fn opt_reload() {
+    pac::FLASH.cr().modify(|w| w.set_optstrt(true));
+    while pac::FLASH.sr().read().bsy() {}
+}
+
+/// Program the option bytes according to procedure from RM0440. Pass a function
+/// that changes required bits within the option bytes. On success, this
+/// function doesn't return - it resets the device. Before calling it, check if
+/// the option bytes were already programmed.
+pub unsafe fn program_option_bytes(setter: impl FnOnce() -> ()) {
+    // Unlocking flash also waits for all flash operations to cease.
+    unlock();
+    opt_unlock();
+    assert_eq!(pac::FLASH.cr().read().optlock(), false);
+
+    // Call user setter and modify the bits.
+    setter();
+
+    // This should reset and configured the bits and not return.
+    opt_reload();
+
+    // But if we failed: cleanup.
+    pac::FLASH.cr().modify(|w| w.set_obl_launch(true));
+    opt_lock();
+    lock();
+}
+
+
 pub(crate) unsafe fn enable_blocking_write() {
     assert_eq!(0, WRITE_SIZE % 4);
     pac::FLASH.cr().write(|w| w.set_pg(true));