uart: support begin and end methods.

A component may need to reset uart buffer/status by using begin() and
end() methods. This is useful for example when a component needs to be
sure it is not reading garbage from previously received data over uart.
For end() methods with software serial, disabling interrupt is
currently impossible because of a bug in esp8266 Core:
esp8266/Arduino#6049

Signed-off-by: 0hax <0hax@protonmail.com>

esphome/components/uart/uart.cpp

@@ -121,6 +121,8 @@ void UARTComponent::write_str(const char *str) {
   this->hw_serial_->write(str);
   ESP_LOGVV(TAG, "    Wrote \"%s\"", str);
 }
+void UARTComponent::end() { this->hw_serial_->end(); }
+void UARTComponent::begin() { this->hw_serial_->begin(this->baud_rate_, get_config()); }
 bool UARTComponent::read_byte(uint8_t *data) {
   if (!this->check_read_timeout_())
     return false;
@@ -272,6 +274,18 @@ void UARTComponent::write_str(const char *str) {
   }
   ESP_LOGVV(TAG, "    Wrote \"%s\"", str);
 }
+void UARTComponent::end() {
+  if (this->hw_serial_ != nullptr)
+    this->hw_serial_->end();
+  else if (this->sw_serial_ != nullptr)
+    this->sw_serial_->end();
+}
+void UARTComponent::begin() {
+  if (this->hw_serial_ != nullptr)
+    this->hw_serial_->begin(this->baud_rate_, static_cast<SerialConfig>(get_config()));
+  else if (this->sw_serial_ != nullptr)
+    this->sw_serial_->begin();
+}
 bool UARTComponent::read_byte(uint8_t *data) {
   if (!this->check_read_timeout_())
     return false;
@@ -337,7 +351,19 @@ void UARTComponent::flush() {
     this->sw_serial_->flush();
   }
 }
-
+void ESP8266SoftwareSerial::end() {
+  /* Because of this bug: https://github.com/esp8266/Arduino/issues/6049
+   * detach_interrupt can't called.
+   * So simply reset rx_in_pos and rx_out_pos even if it's totally racy with
+   * the interrupt.
+   */
+  // this->gpio_rx_pin_->detach_interrupt();
+  this->rx_in_pos_ = 0;
+  this->rx_out_pos_ = 0;
+}
+void ESP8266SoftwareSerial::begin() {
+  this->gpio_rx_pin_->attach_interrupt(ESP8266SoftwareSerial::gpio_intr, this, FALLING);
+}
 void ESP8266SoftwareSerial::setup(int8_t tx_pin, int8_t rx_pin, uint32_t baud_rate, uint8_t stop_bits, uint32_t nr_bits,
                                   std::string &parity) {
   this->bit_time_ = F_CPU / baud_rate;
@@ -346,13 +372,15 @@ void ESP8266SoftwareSerial::setup(int8_t tx_pin, int8_t rx_pin, uint32_t baud_ra
   this->parity_ = parity;
   if (tx_pin != -1) {
     auto pin = GPIOPin(tx_pin, OUTPUT);
+    this->gpio_tx_pin_ = &pin;
     pin.setup();
     this->tx_pin_ = pin.to_isr();
     this->tx_pin_->digital_write(true);
   }
   if (rx_pin != -1) {
     auto pin = GPIOPin(rx_pin, INPUT);
     pin.setup();
+    this->gpio_rx_pin_ = &pin;
     this->rx_pin_ = pin.to_isr();
     this->rx_buffer_ = new uint8_t[this->rx_buffer_size_];
     pin.attach_interrupt(ESP8266SoftwareSerial::gpio_intr, this, FALLING);

esphome/components/uart/uart.h

@@ -22,6 +22,11 @@ class ESP8266SoftwareSerial {
 
   int available();
 
+  void begin();
+  void end();
+  GPIOPin *gpio_tx_pin_{nullptr};
+  GPIOPin *gpio_rx_pin_{nullptr};
+
  protected:
   static void gpio_intr(ESP8266SoftwareSerial *arg);
 
@@ -58,6 +63,8 @@ class UARTComponent : public Component, public Stream {
   void write_array(const std::vector<uint8_t> &data) { this->write_array(&data[0], data.size()); }
 
   void write_str(const char *str);
+  void end();
+  void begin();
 
   bool peek_byte(uint8_t *data);
 
@@ -139,6 +146,8 @@ class UARTDevice : public Stream {
   size_t write(uint8_t data) override { return this->parent_->write(data); }
   int read() override { return this->parent_->read(); }
   int peek() override { return this->parent_->peek(); }
+  void end() { this->parent_->end(); }
+  void begin() { this->parent_->begin(); }
 
   /// Check that the configuration of the UART bus matches the provided values and otherwise print a warning
   void check_uart_settings(uint32_t baud_rate, uint8_t stop_bits = 1);