i2c: Prevent priority inversion on top of bus lock

Low priority thread holding the i2c bus mutex could block higher
priority threads to access the bus resulting in unacceptable
latencies. Change the mutex type to rt_mutex preventing priority
inversion.

Tested-by: Peter Ujfalusi <[e-mail]>
Signed-off-by: Mika Kuoppala <[e-mail]>
Signed-off-by: Jean Delvare <[e-mail]>

drivers/i2c/Kconfig

···
 menuconfig I2C
 	tristate "I2C support"
 	depends on HAS_IOMEM
+	select RT_MUTEXES
 	---help---
 	  I2C (pronounce: I-square-C) is a slow serial bus protocol used in
 	  many micro controller applications and developed by Philips.  SMBus,

drivers/i2c/i2c-core.c

···
 		goto out_list;
 	}
 
-	mutex_init(&adap->bus_lock);
+	rt_mutex_init(&adap->bus_lock);
 
 	/* Set default timeout to 1 second if not already set */
 	if (adap->timeout == 0)
···
 #endif
 
 		if (in_atomic() || irqs_disabled()) {
-			ret = mutex_trylock(&adap->bus_lock);
+			ret = rt_mutex_trylock(&adap->bus_lock);
 			if (!ret)
 				/* I2C activity is ongoing. */
 				return -EAGAIN;
 		} else {
-			mutex_lock_nested(&adap->bus_lock, adap->level);
+			rt_mutex_lock(&adap->bus_lock);
 		}
 
 		/* Retry automatically on arbitration loss */
···
 			if (time_after(jiffies, orig_jiffies + adap->timeout))
 				break;
 		}
-		mutex_unlock(&adap->bus_lock);
+		rt_mutex_unlock(&adap->bus_lock);
 
 		return ret;
 	} else {
···
 	flags &= I2C_M_TEN | I2C_CLIENT_PEC;
 
 	if (adapter->algo->smbus_xfer) {
-		mutex_lock(&adapter->bus_lock);
+		rt_mutex_lock(&adapter->bus_lock);
 
 		/* Retry automatically on arbitration loss */
 		orig_jiffies = jiffies;
···
 				       orig_jiffies + adapter->timeout))
 				break;
 		}
-		mutex_unlock(&adapter->bus_lock);
+		rt_mutex_unlock(&adapter->bus_lock);
 	} else
 		res = i2c_smbus_xfer_emulated(adapter,addr,flags,read_write,
 					      command, protocol, data);

include/linux/i2c.h

···
 	void *algo_data;
 
 	/* data fields that are valid for all devices	*/
-	u8 level; 			/* nesting level for lockdep */
-	struct mutex bus_lock;
+	struct rt_mutex bus_lock;
 
 	int timeout;			/* in jiffies */
 	int retries;
···
  */
 static inline void i2c_lock_adapter(struct i2c_adapter *adapter)
 {
-	mutex_lock(&adapter->bus_lock);
+	rt_mutex_lock(&adapter->bus_lock);
 }
 
 /**
···
  */
 static inline void i2c_unlock_adapter(struct i2c_adapter *adapter)
 {
-	mutex_unlock(&adapter->bus_lock);
+	rt_mutex_unlock(&adapter->bus_lock);
 }
 
 /*flags for the client struct: */