tty: make tty_buffer_space_avail return uint

tty_buffer_space_avail returns values >= 0, so make it clear by the
return type.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20210505091928.22010-25-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_aspeed_vuart.c

@@ -329,7 +329,7 @@ static int aspeed_vuart_handle_irq(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 	unsigned int iir, lsr;
-	int space, count;
+	unsigned int space, count;
 
 	iir = serial_port_in(port, UART_IIR);
 
@@ -353,7 +353,7 @@ static int aspeed_vuart_handle_irq(struct uart_port *port)
 					  jiffies + unthrottle_timeout);
 
 		} else {
-			count = min(space, 256);
+			count = min(space, 256U);
 
 			do {
 				serial8250_read_char(up, lsr);

drivers/tty/tty_buffer.c

@@ -88,7 +88,7 @@ EXPORT_SYMBOL_GPL(tty_buffer_unlock_exclusive);
  *	pre-allocate if memory guarantee is required).
  */
 
-int tty_buffer_space_avail(struct tty_port *port)
+unsigned int tty_buffer_space_avail(struct tty_port *port)
 {
 	int space = port->buf.mem_limit - atomic_read(&port->buf.mem_used);
 	return max(space, 0);

include/linux/tty_flip.h

@@ -3,7 +3,7 @@
 #define _LINUX_TTY_FLIP_H
 
 extern int tty_buffer_set_limit(struct tty_port *port, int limit);
-extern int tty_buffer_space_avail(struct tty_port *port);
+extern unsigned int tty_buffer_space_avail(struct tty_port *port);
 extern int tty_buffer_request_room(struct tty_port *port, size_t size);
 extern int tty_insert_flip_string_flags(struct tty_port *port,
 		const unsigned char *chars, const char *flags, size_t size);