tty: serial: 21285: stop using the unused[] variable from struct uart_port

Much like the samsung_tty driver (now I know where they copied the idea
from), the 21285 uart driver uses 2 bytes from the "unused" array of
struct uart_port to keep tx/rx enabled/disabled state.  Those fields are
going away (they were never really needed in the first place), so fix up
the 21285 driver by another horrible hack.

Instead of creating a whole structure for just 2 bytes, just use two
bits from the private_data pointer instead, as that pointer is never
used.  The two bits reflect if tx/rx is now enabled/disabled.

Astute readers will note that once rx is disabled, nothing ever seems to
turn it back on, making one wonder if anyone has ever done this.
Reported-by: kbuild test robot <lkp@intel.com>
Cc: Dmitry Safonov <dima@arista.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20191219145109.GA1962496@kroah.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/21285.c

@@ -41,8 +41,43 @@
 
 static const char serial21285_name[] = "Footbridge UART";
 
-#define tx_enabled(port)	((port)->unused[0])
-#define rx_enabled(port)	((port)->unused[1])
+/*
+ * We only need 2 bits of data, so instead of creating a whole structure for
+ * this, use bits of the private_data pointer of the uart port structure.
+ */
+#define tx_enabled_bit	0
+#define rx_enabled_bit	1
+
+static bool is_enabled(struct uart_port *port, int bit)
+{
+	unsigned long private_data = (unsigned long)port->private_data;
+
+	if (test_bit(bit, &private_data))
+		return true;
+	return false;
+}
+
+static void enable(struct uart_port *port, int bit)
+{
+	unsigned long private_data = (unsigned long)port->private_data;
+
+	set_bit(bit, &private_data);
+}
+
+static void disable(struct uart_port *port, int bit)
+{
+	unsigned long private_data = (unsigned long)port->private_data;
+
+	clear_bit(bit, &private_data);
+}
+
+#define is_tx_enabled(port)	is_enabled(port, tx_enabled_bit)
+#define tx_enable(port)		enable(port, tx_enabled_bit)
+#define tx_disable(port)	disable(port, tx_enabled_bit)
+
+#define is_rx_enabled(port)	is_enabled(port, rx_enabled_bit)
+#define rx_enable(port)		enable(port, rx_enabled_bit)
+#define rx_disable(port)	disable(port, rx_enabled_bit)
 
 /*
  * The documented expression for selecting the divisor is:
@@ -57,25 +92,25 @@ static const char serial21285_name[] = "Footbridge UART";
 
 static void serial21285_stop_tx(struct uart_port *port)
 {
-	if (tx_enabled(port)) {
+	if (is_tx_enabled(port)) {
 		disable_irq_nosync(IRQ_CONTX);
-		tx_enabled(port) = 0;
+		tx_disable(port);
 	}
 }
 
 static void serial21285_start_tx(struct uart_port *port)
 {
-	if (!tx_enabled(port)) {
+	if (!is_tx_enabled(port)) {
 		enable_irq(IRQ_CONTX);
-		tx_enabled(port) = 1;
+		tx_enable(port);
 	}
 }
 
 static void serial21285_stop_rx(struct uart_port *port)
 {
-	if (rx_enabled(port)) {
+	if (is_rx_enabled(port)) {
 		disable_irq_nosync(IRQ_CONRX);
-		rx_enabled(port) = 0;
+		rx_disable(port);
 	}
 }
 
@@ -185,8 +220,8 @@ static int serial21285_startup(struct uart_port *port)
 {
 	int ret;
 
-	tx_enabled(port) = 1;
-	rx_enabled(port) = 1;
+	tx_enable(port);
+	rx_enable(port);
 
 	ret = request_irq(IRQ_CONRX, serial21285_rx_chars, 0,
 			  serial21285_name, port);