periph/i2c: added means to make I2C thread-safe

As discussed in #2289 this changes provide means to
use the i2c interface safely within multible threads.

cpu/stm32f1/periph/i2c.c

@@ -18,6 +18,7 @@
  * For implementation details please refer to STM application note AN2824.
  *
  * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ * @author      Thomas Eichinger <thomas.eichinger@fu-berlin.de>
  *
  * @}
  */
@@ -26,6 +27,7 @@
 
 #include "cpu.h"
 #include "irq.h"
+#include "mutex.h"
 #include "periph_conf.h"
 #include "periph/i2c.h"
 
@@ -44,6 +46,24 @@ static inline void _clear_addr(I2C_TypeDef *dev);
 static inline void _write(I2C_TypeDef *dev, char *data, int length);
 static inline void _stop(I2C_TypeDef *dev);
 
+/**
+ * @brief Array holding one pre-initialized mutex for each I2C device
+ */
+static mutex_t locks[] =  {
+#if I2C_0_EN
+    [I2C_0] = MUTEX_INIT,
+#endif
+#if I2C_1_EN
+    [I2C_1] = MUTEX_INIT,
+#endif
+#if I2C_2_EN
+    [I2C_2] = MUTEX_INIT
+#endif
+#if I2C_3_EN
+    [I2C_3] = MUTEX_INIT
+#endif
+};
+
 int i2c_init_master(i2c_t dev, i2c_speed_t speed)
 {
     I2C_TypeDef *i2c;
@@ -105,6 +125,7 @@ int i2c_init_master(i2c_t dev, i2c_speed_t speed)
         /* enable device */
         _i2c_init(i2c, ccr);
     }
+
     return 0;
 }
 
@@ -173,6 +194,24 @@ int i2c_init_slave(i2c_t dev, uint8_t address)
     return -1;
 }
 
+int i2c_acquire(i2c_t dev)
+{
+    if (dev >= I2C_NUMOF) {
+        return -1;
+    }
+    mutex_lock(&locks[dev]);
+    return 0;
+}
+
+int i2c_release(i2c_t dev)
+{
+    if (dev >= I2C_NUMOF) {
+        return -1;
+    }
+    mutex_unlock(&locks[dev]);
+    return 0;
+}
+
 int i2c_read_byte(i2c_t dev, uint8_t address, char *data)
 {
     return i2c_read_bytes(dev, address, data, 1);

drivers/include/periph/i2c.h

@@ -46,6 +46,7 @@
                 7-bit addressing mode.
  *
  * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ * @author      Thomas Eichinger <thomas.eichinger@fu-berlin.de>
  */
 
 #ifndef __I2C_H
@@ -119,6 +120,28 @@ int i2c_init_master(i2c_t dev, i2c_speed_t speed);
  */
 int i2c_init_slave(i2c_t dev, uint8_t address);
 
+/**
+ * @brief Get mutually exclusive access to the given I2C bus
+ *
+ * In case the I2C device is busy, this function will block until the bus is free again.
+ *
+ * @param[in] dev       I2C device to access
+ *
+ * @return              0 on success
+ * @return              -1 on error
+ */
+int i2c_acquire(i2c_t dev);
+
+/**
+ * @brief Release the given I2C device to be used by others
+ *
+ * @param[in] dev       I2C device to release
+ *
+ * @return              0 on success
+ * @return              -1 on error
+ */
+int i2c_release(i2c_t dev);
+
 /**
  * @brief Read one byte from an I2C device with the given address
  *