cpu/stm32f4: made SPI driver thread safe

cpu/stm32f4/periph/spi.c

@@ -15,6 +15,7 @@
  *
  * @author      Peter Kietzmann <peter.kietzmann@haw-hamburg.de>
  * @author      Fabian Nack <nack@inf.fu-berlin.de>
+ * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
  *
  * @}
  */
@@ -22,6 +23,7 @@
 
 #include "board.h"
 #include "cpu.h"
+#include "mutex.h"
 #include "periph/spi.h"
 #include "periph_conf.h"
 #include "thread.h"
@@ -34,12 +36,18 @@
 /* guard this file in case no SPI device is defined */
 #if SPI_NUMOF
 
+/**
+ * @brief Data-structure holding the state for a SPI device
+ */
 typedef struct {
     char(*cb)(char data);
 } spi_state_t;
 
 static inline void irq_handler_transfer(SPI_TypeDef *spi, spi_t dev);
 
+/**
+ * @brief Reserve memory for saving the SPI device's state
+ */
 static spi_state_t spi_config[SPI_NUMOF];
 
 /* static bus div mapping */
@@ -55,6 +63,21 @@ static const uint8_t spi_bus_div_map[SPI_NUMOF] = {
 #endif
 };
 
+/**
+ * @brief Array holding one pre-initialized mutex for each SPI device
+ */
+static mutex_t locks[] =  {
+#if SPI_0_EN
+    [SPI_0] = MUTEX_INIT,
+#endif
+#if SPI_1_EN
+    [SPI_1] = MUTEX_INIT,
+#endif
+#if SPI_2_EN
+    [SPI_2] = MUTEX_INIT
+#endif
+};
+
 int spi_init_master(spi_t dev, spi_conf_t conf, spi_speed_t speed)
 {
     uint8_t speed_devider;
@@ -270,6 +293,24 @@ int spi_conf_pins(spi_t dev)
     return 0;
 }
 
+int spi_acquire(spi_t dev)
+{
+    if (dev >= SPI_NUMOF) {
+        return -1;
+    }
+    mutex_lock(&locks[dev]);
+    return 0;
+}
+
+int spi_release(spi_t dev)
+{
+    if (dev >= SPI_NUMOF) {
+        return -1;
+    }
+    mutex_unlock(&locks[dev]);
+    return 0;
+}
+
 int spi_transfer_byte(spi_t dev, char out, char *in)
 {
     SPI_TypeDef *spi_port;