tty: serial: cpm_uart: Add udbg support for enabling xmon

In order to use xmon with powerpc 8xx, the serial driver
must provide udbg_putc() and udpb_getc().

Provide them via cpm_put_poll_char() and cpm_get_poll_char().

This requires CONFIG_CONSOLE_POLL.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/e4471bf81089252470efb3eed735d71a5b32adbd.1608716197.git.christophe.leroy@csgroup.euSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/cpm_uart/cpm_uart_core.c

@@ -1107,6 +1107,32 @@ static void cpm_put_poll_char(struct uart_port *port,
 	ch[0] = (char)c;
 	cpm_uart_early_write(pinfo, ch, 1, false);
 }
+
+static struct uart_port *udbg_port;
+
+static void udbg_cpm_putc(char c)
+{
+	if (c == '\n')
+		cpm_put_poll_char(udbg_port, '\r');
+	cpm_put_poll_char(udbg_port, c);
+}
+
+static int udbg_cpm_getc_poll(void)
+{
+	int c = cpm_get_poll_char(udbg_port);
+
+	return c == NO_POLL_CHAR ? -1 : c;
+}
+
+static int udbg_cpm_getc(void)
+{
+	int c;
+
+	while ((c = udbg_cpm_getc_poll()) == -1)
+		cpu_relax();
+	return c;
+}
+
 #endif /* CONFIG_CONSOLE_POLL */
 
 static const struct uart_ops cpm_uart_pops = {
@@ -1237,7 +1263,10 @@ static int cpm_uart_init_port(struct device_node *np,
 	}
 
 #ifdef CONFIG_PPC_EARLY_DEBUG_CPM
-	udbg_putc = NULL;
+#ifdef CONFIG_CONSOLE_POLL
+	if (!udbg_port)
+#endif
+		udbg_putc = NULL;
 #endif
 
 	return cpm_uart_request_port(&pinfo->port);
@@ -1358,6 +1387,15 @@ static int __init cpm_uart_console_setup(struct console *co, char *options)
 	uart_set_options(port, co, baud, parity, bits, flow);
 	cpm_line_cr_cmd(pinfo, CPM_CR_RESTART_TX);
 
+#ifdef CONFIG_CONSOLE_POLL
+	if (!udbg_port) {
+		udbg_port = &pinfo->port;
+		udbg_putc = udbg_cpm_putc;
+		udbg_getc = udbg_cpm_getc;
+		udbg_getc_poll = udbg_cpm_getc_poll;
+	}
+#endif
+
 	return 0;
 }