Add support for reconfiguring a Serial port

Example use case is to implement a 1-Wire Bus "Master" [1]. In that
example, the UART is used instead of bit-banging the 1-Wire protocol.
However, not all signals can be generated with a single baud rate. In
particular, the 1-wire reset strobe is much longer than the 1-wire data
strobes, requiring to use a different, slower baud rate to reset
devices.

   [1]: https://www.maximintegrated.com/en/design/technical-documents/tutorials/2/214.html

Signed-off-by: Jonas Schäfer <j.wielicki@sotecware.net>

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Added
+
+- Serial can now be reconfigured, allowing to change e.g. the baud rate after initialisation.
+
 ## [v0.8.0] - 2021-12-29
 
 ### Breaking changes

src/serial.rs

@@ -235,6 +235,19 @@ where
         USART::reset(rcc);
 
         remap();
+        self.apply_config(config, clocks);
+
+        // UE: enable USART
+        // RE: enable receiver
+        // TE: enable transceiver
+        self.usart
+            .cr1
+            .modify(|_r, w| w.ue().set_bit().re().set_bit().te().set_bit());
+
+        self
+    }
+
+	fn apply_config(&self, config: Config, clocks: Clocks) {
         // Configure baud rate
         let brr = USART::get_frequency(&clocks).0 / config.baudrate.0;
         assert!(brr >= 16, "impossible baud rate");
@@ -267,16 +280,24 @@ where
             StopBits::STOP1P5 => 0b11,
         };
         self.usart.cr2.modify(|_r, w| w.stop().bits(stop_bits));
-
-        // UE: enable USART
-        // RE: enable receiver
-        // TE: enable transceiver
-        self.usart
-            .cr1
-            .modify(|_r, w| w.ue().set_bit().re().set_bit().te().set_bit());
-
-        self
-    }
+	}
+
+	/// Reconfigure the USART instance.
+	///
+	/// If a transmission is currently in progress, this returns
+	/// [`nb::Error::WouldBlock`].
+	pub fn reconfigure(&mut self, config: impl Into<Config>, clocks: Clocks) -> nb::Result<(), ()> {
+		let sr = self.usart.sr.read();
+		// if we're currently busy transmitting, we have to wait until that is
+		// over -- regarding reception, we assume that the caller -- with
+		// exclusive access to the Serial instance due to &mut self -- knows
+		// what they're doing.
+		if sr.tc().bit_is_clear() {
+			return nb::Result::Err(nb::Error::WouldBlock)
+		}
+		self.apply_config(config.into(), clocks);
+		nb::Result::Ok(())
+	}
 
     /// Starts listening to the USART by enabling the _Received data
     /// ready to be read (RXNE)_ interrupt and _Transmit data