Speed up SPI by not reinitting if the paramters are the same as before (#394)

Leave the SPI port initted after a transaction, and when a new one comes in
check if it is the same settings we're already running.  If so, do nothing.
If not, deinit and reinit.  In general the settings will be identical, so
this will speed things up massively.

Fixes #392

Author: earlephilhower

libraries/SPI/SPI.cpp

@@ -179,23 +179,22 @@ void SPIClassRP2040::transfer(void *txbuf, void *rxbuf, size_t count) {
 
 void SPIClassRP2040::beginTransaction(SPISettings settings) {
     DEBUGSPI("SPI::beginTransaction(clk=%d, bo=%s\n", _spis.getClockFreq(), (_spis.getBitOrder() == MSBFIRST) ? "MSB" : "LSB");
-    _spis = settings;
-    if (_initted) {
-        DEBUGSPI("SPI: deinitting currently active SPI\n");
-        spi_deinit(_spi);
+    if (_initted && settings == _spis) {
+        DEBUGSPI("SPI: Reusing existing initted SPI\n");
+    } else {
+        _spis = settings;
+        if (_initted) {
+            DEBUGSPI("SPI: deinitting currently active SPI\n");
+            spi_deinit(_spi);
+        }
+        DEBUGSPI("SPI: initting SPI\n");
+        spi_init(_spi, _spis.getClockFreq());
+        _initted = true;
     }
-    DEBUGSPI("SPI: initting SPI\n");
-    spi_init(_spi, _spis.getClockFreq());
-    _initted = true;
 }
 
 void SPIClassRP2040::endTransaction(void) {
     DEBUGSPI("SPI::endTransaction()\n");
-    if (_initted) {
-        DEBUGSPI("SPI: deinitting currently active SPI\n");
-        spi_deinit(_spi);
-    }
-    _initted = false;
 }
 
 bool SPIClassRP2040::setRX(pin_size_t pin) {
@@ -281,6 +280,11 @@ void SPIClassRP2040::begin(bool hwCS) {
 
 void SPIClassRP2040::end() {
     DEBUGSPI("SPI::end()\n");
+    if (_initted) {
+        DEBUGSPI("SPI: deinitting currently active SPI\n");
+        _initted = false;
+        spi_deinit(_spi);
+    }
     gpio_set_function(_RX, GPIO_FUNC_SIO);
     if (_hwCS) {
         gpio_set_function(_CS, GPIO_FUNC_SIO);