_acquire() function added & no duplication in format/freq calls

1. Private _acquire() function is added to avoid multiple locking/unlocking
2. format and frequency functions updated to use appropriate function calls
instead of a aquire()

drivers/SPI.cpp

@@ -36,23 +36,23 @@ SPI::SPI(PinName mosi, PinName miso, PinName sclk, PinName ssel) :
     // No lock needed in the constructor
 
     spi_init(&_spi, mosi, miso, sclk, ssel);
-    aquire();
+    _acquire();
 }
 
 void SPI::format(int bits, int mode) {
     lock();
     _bits = bits;
     _mode = mode;
-    SPI::_owner = NULL; // Not that elegant, but works. rmeyer
-    aquire();
+    spi_format(&_spi, _bits, _mode, 0);
+    _owner = this;
     unlock();
 }
 
 void SPI::frequency(int hz) {
     lock();
     _hz = hz;
-    SPI::_owner = NULL; // Not that elegant, but works. rmeyer
-    aquire();
+    spi_frequency(&_spi, _hz);
+    _owner = this;
     unlock();
 }
 
@@ -70,9 +70,18 @@ void SPI::aquire() {
     unlock();
 }
 
+// Note: Private function with no locking
+void SPI::_acquire() {
+     if (_owner != this) {
+        spi_format(&_spi, _bits, _mode, 0);
+        spi_frequency(&_spi, _hz);
+        _owner = this;
+    }
+}
+
 int SPI::write(int value) {
     lock();
-    aquire();
+    _acquire();
     int ret = spi_master_write(&_spi, value);
     unlock();
     return ret;
@@ -80,7 +89,7 @@ int SPI::write(int value) {
 
 int SPI::write(const char *tx_buffer, int tx_length, char *rx_buffer, int rx_length) {
     lock();
-    aquire();
+    _acquire();
     int ret = spi_master_block_write(&_spi, tx_buffer, tx_length, rx_buffer, rx_length);
     unlock();
     return ret;
@@ -167,7 +176,7 @@ int SPI::queue_transfer(const void *tx_buffer, int tx_length, void *rx_buffer, i
 
 void SPI::start_transfer(const void *tx_buffer, int tx_length, void *rx_buffer, int rx_length, unsigned char bit_width, const event_callback_t& callback, int event)
 {
-    aquire();
+    _acquire();
     _callback = callback;
     _irq.callback(&SPI::irq_handler_asynch);
     spi_master_transfer(&_spi, tx_buffer, tx_length, rx_buffer, rx_length, bit_width, _irq.entry(), event , _usage);

drivers/SPI.h

@@ -270,6 +270,12 @@ protected:
     int _bits;
     int _mode;
     int _hz;
+
+private:
+    /* Private acquire fucntion without locking/unlocking
+     * Implemented in order to avoid duplicate locking and boost performance
+     */
+    void _acquire(void);
 };
 
 } // namespace mbed