serial: cpm_uart: return immediately from console poll

BugLink: https://bugs.launchpad.net/bugs/1798770

commit be28c1e3 upstream.

kgdb expects poll function to return immediately and
returning NO_POLL_CHAR when no character is available.

Fixes: f5316b4a ("kgdb,8250,pl011: Return immediately from console poll")
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/tty/serial/cpm_uart/cpm_uart_core.c

@@ -1068,8 +1068,8 @@ static int poll_wait_key(char *obuf, struct uart_cpm_port *pinfo)
 	/* Get the address of the host memory buffer.
 	 */
 	bdp = pinfo->rx_cur;
-	while (bdp->cbd_sc & BD_SC_EMPTY)
-		;
+	if (bdp->cbd_sc & BD_SC_EMPTY)
+		return NO_POLL_CHAR;
 
 	/* If the buffer address is in the CPM DPRAM, don't
 	 * convert it.
@@ -1104,7 +1104,11 @@ static int cpm_get_poll_char(struct uart_port *port)
 		poll_chars = 0;
 	}
 	if (poll_chars <= 0) {
-		poll_chars = poll_wait_key(poll_buf, pinfo);
+		int ret = poll_wait_key(poll_buf, pinfo);
+
+		if (ret == NO_POLL_CHAR)
+			return ret;
+		poll_chars = ret;
 		pollp = poll_buf;
 	}
 	poll_chars--;