staging: pi433: cleanup tx_fifo locking

pi433_write requires locking due to multiple writers.  After acquiring
the lock check if enough free space is available in the kfifo to write
the whole message. This check should prevent partial writes to tx_fifo
so kfifo_reset is not needed anymore.

pi433_tx_thread is the only reader so it does not require locking
after kfifo_reset is removed.
Signed-off-by: Valentin Vidic <Valentin.Vidic@CARNet.hr>
Reviewed-by: Marcus Wolf <marcus.wolf@wolf-entwicklungen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/pi433_if.c

@@ -87,7 +87,7 @@ struct pi433_device {
 
 	/* tx related values */
 	STRUCT_KFIFO_REC_1(MSG_FIFO_SIZE) tx_fifo;
-	struct mutex		tx_fifo_lock; // TODO: check, whether necessary or obsolete
+	struct mutex		tx_fifo_lock; /* serialize userspace writers */
 	struct task_struct	*tx_task_struct;
 	wait_queue_head_t	tx_wait_queue;
 	u8			free_in_fifo;
@@ -589,19 +589,15 @@ pi433_tx_thread(void *data)
 		 * - size of message
 		 * - message
 		 */
-		mutex_lock(&device->tx_fifo_lock);
-
 		retval = kfifo_out(&device->tx_fifo, &tx_cfg, sizeof(tx_cfg));
 		if (retval != sizeof(tx_cfg)) {
 			dev_dbg(device->dev, "reading tx_cfg from fifo failed: got %d byte(s), expected %d", retval, (unsigned int)sizeof(tx_cfg));
-			mutex_unlock(&device->tx_fifo_lock);
 			continue;
 		}
 
 		retval = kfifo_out(&device->tx_fifo, &size, sizeof(size_t));
 		if (retval != sizeof(size_t)) {
 			dev_dbg(device->dev, "reading msg size from fifo failed: got %d, expected %d", retval, (unsigned int)sizeof(size_t));
-			mutex_unlock(&device->tx_fifo_lock);
 			continue;
 		}
 
@@ -634,7 +630,6 @@ pi433_tx_thread(void *data)
 				   sizeof(device->buffer) - position);
 		dev_dbg(device->dev,
 			"read %d message byte(s) from fifo queue.", retval);
-		mutex_unlock(&device->tx_fifo_lock);
 
 		/* if rx is active, we need to interrupt the waiting for
 		 * incoming telegrams, to be able to send something.
@@ -818,7 +813,7 @@ pi433_write(struct file *filp, const char __user *buf,
 	struct pi433_instance	*instance;
 	struct pi433_device	*device;
 	int                     retval;
-	unsigned int		copied;
+	unsigned int		required, available, copied;
 
 	instance = filp->private_data;
 	device = instance->device;
@@ -833,6 +828,16 @@ pi433_write(struct file *filp, const char __user *buf,
 	 * - message
 	 */
 	mutex_lock(&device->tx_fifo_lock);
+
+	required = sizeof(instance->tx_cfg) + sizeof(size_t) + count;
+	available = kfifo_avail(&device->tx_fifo);
+	if (required > available) {
+		dev_dbg(device->dev, "write to fifo failed: %d bytes required but %d available",
+			required, available);
+		mutex_unlock(&device->tx_fifo_lock);
+		return -EAGAIN;
+	}
+
 	retval = kfifo_in(&device->tx_fifo, &instance->tx_cfg,
 			  sizeof(instance->tx_cfg));
 	if (retval != sizeof(instance->tx_cfg))
@@ -855,8 +860,8 @@ pi433_write(struct file *filp, const char __user *buf,
 	return copied;
 
 abort:
-	dev_dbg(device->dev, "write to fifo failed: 0x%x", retval);
-	kfifo_reset(&device->tx_fifo); // TODO: maybe find a solution, not to discard already stored, valid entries
+	dev_warn(device->dev,
+		 "write to fifo failed, non recoverable: 0x%x", retval);
 	mutex_unlock(&device->tx_fifo_lock);
 	return -EAGAIN;
 }