feat(uart): setBufferSize

It makes sure that setting TX buffer size will match availableForWrite() response. It also sets the buffer to the minimum instead of doing nothing and returning an error.

For RX Buffer, it sets the minimum and also don't return an error.

This makes the APIs better and easy to understand its results.

cores/esp32/HardwareSerial.cpp

@@ -537,35 +537,38 @@ bool HardwareSerial::setMode(SerialMode mode)
     return uartSetMode(_uart, mode);
 }
 
+// minimum total RX Buffer size is the UART FIFO space (128 bytes for most SoC) + 1. IDF imposition.
 size_t HardwareSerial::setRxBufferSize(size_t new_size) {
 
     if (_uart) {
-        log_e("RX Buffer can't be resized when Serial is already running.\n");
+        log_e("RX Buffer can't be resized when Serial is already running. Set it before calling begin().");
         return 0;
     }
 
     if (new_size <= SOC_UART_FIFO_LEN) {
-        log_e("RX Buffer must be higher than %d.\n", SOC_UART_FIFO_LEN);  // ESP32, S2, S3 and C3 means higher than 128
-        return 0;
+        log_w("RX Buffer set to minimum value: %d.", SOC_UART_FIFO_LEN + 1);  // ESP32, S2, S3 and C3 means higher than 128
+        new_size = SOC_UART_FIFO_LEN + 1;
     }
 
     _rxBufferSize = new_size;
     return _rxBufferSize;
 }
 
+// minimum total TX Buffer size is the UART FIFO space (128 bytes for most SoC).
 size_t HardwareSerial::setTxBufferSize(size_t new_size) {
 
     if (_uart) {
-        log_e("TX Buffer can't be resized when Serial is already running.\n");
+        log_e("TX Buffer can't be resized when Serial is already running. Set it before calling begin().");
         return 0;
     }
 
-    if (new_size <= SOC_UART_FIFO_LEN) {
-        log_e("TX Buffer must be higher than %d.\n", SOC_UART_FIFO_LEN);  // ESP32, S2, S3 and C3 means higher than 128
-        return 0;
+    if (new_size < SOC_UART_FIFO_LEN) {
+        log_w("TX Buffer set to minimum value: %d.", SOC_UART_FIFO_LEN);  // ESP32, S2, S3 and C3 means higher than 128
+        _txBufferSize = 0; // it will use just UART FIFO with SOC_UART_FIFO_LEN bytes (128 for most SoC)
+        return SOC_UART_FIFO_LEN;
     }
-
-    _txBufferSize = new_size;
-    return _txBufferSize;
+    // if new_size is SOC_UART_FIFO_LEN, _txBufferSize will be zero - just use the UART FIFO space
+    _txBufferSize = new_size - SOC_UART_FIFO_LEN; // for total correct report from "availableForWrite()"  that matches a call to this function
+    return new_size;
 }