media: cec: avoid decrementing transmit_queue_sz if it is 0

WARN if transmit_queue_sz is 0 but do not decrement it.
The CEC adapter will become unresponsive if it goes below
0 since then it thinks there are 4 billion messages in the
queue.

Obviously this should not happen, but a driver bug could
cause this.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: <stable@vger.kernel.org>      # for v4.12 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/cec/cec-adap.c

@@ -380,6 +380,7 @@ static void cec_data_cancel(struct cec_data *data, u8 tx_status)
 	} else {
 		list_del_init(&data->list);
 		if (!(data->msg.tx_status & CEC_TX_STATUS_OK))
+			if (!WARN_ON(!data->adap->transmit_queue_sz))
 				data->adap->transmit_queue_sz--;
 	}
 
@@ -432,6 +433,14 @@ static void cec_flush(struct cec_adapter *adap)
 		 * need to do anything special in that case.
 		 */
 	}
+	/*
+	 * If something went wrong and this counter isn't what it should
+	 * be, then this will reset it back to 0. Warn if it is not 0,
+	 * since it indicates a bug, either in this framework or in a
+	 * CEC driver.
+	 */
+	if (WARN_ON(adap->transmit_queue_sz))
+		adap->transmit_queue_sz = 0;
 }
 
 /*
@@ -522,6 +531,7 @@ int cec_thread_func(void *_adap)
 		data = list_first_entry(&adap->transmit_queue,
 					struct cec_data, list);
 		list_del_init(&data->list);
+		if (!WARN_ON(!data->adap->transmit_queue_sz))
 			adap->transmit_queue_sz--;
 
 		/* Make this the current transmitting message */