rp2/machine_uart: Fix handling of serial break condition.

The FIFO reports not only the bytes read, but also 4 error bits. These were
not checked, leading to NUL value read in case of break and possible
garbage bytes being written on parity/framing error.

This patch addresses the issue that NUL bytes are incorrectly read on
break, and at least provides the boilerplate code and comments for error
handling, that may be implemented in the future.
Signed-off-by: Maarten van der Schrieck <maarten@thingsconnected.nl>

ports/rp2/machine_uart.c

@@ -140,8 +140,27 @@ static inline void read_mutex_unlock(machine_uart_obj_t *u) {
 STATIC void uart_drain_rx_fifo(machine_uart_obj_t *self) {
     if (read_mutex_try_lock(self)) {
         while (uart_is_readable(self->uart) && ringbuf_free(&self->read_buffer) > 0) {
-            // get a byte from uart and put into the buffer
-            ringbuf_put(&(self->read_buffer), uart_get_hw(self->uart)->dr);
+            // Get a byte from uart and put into the buffer. Every entry from
+            // the FIFO is accompanied by 4 error bits, that may be used for
+            // error handling.
+            uint16_t c = uart_get_hw(self->uart)->dr;
+            if (c & UART_UARTDR_OE_BITS) {
+                // Overrun Error: We missed at least one byte. Not much we can do here.
+            }
+            if (c & UART_UARTDR_BE_BITS) {
+                // Break Error: RX was held low for longer than one character
+                // (11 bits). We did *not* read the zero byte that we seemed to
+                // read from dr.
+                continue;
+            }
+            if (c & UART_UARTDR_PE_BITS) {
+                // Parity Error: The byte we read is invalid.
+            }
+            if (c & UART_UARTDR_FE_BITS) {
+                // Framing Error: We did not receive a valid stop bit.
+            }
+
+            ringbuf_put(&(self->read_buffer), c);
         }
         read_mutex_unlock(self);
     }