tty: serial: owl: Add support for kernel debugger

Implement 'poll_put_char' and 'poll_get_char' callbacks in struct
'owl_uart_ops' that enables OWL UART to be used for kernel debugging
over serial line.
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Link: https://lore.kernel.org/r/026543195b9aeefb339d90abc5660a6ac7463c63.1610484108.git.cristian.ciocaltea@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/owl-uart.c

@@ -12,6 +12,7 @@
 #include <linux/console.h>
 #include <linux/delay.h>
 #include <linux/io.h>
+#include <linux/iopoll.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>
@@ -62,6 +63,9 @@
 #define OWL_UART_STAT_TRFL_MASK		GENMASK(16, 11)
 #define OWL_UART_STAT_UTBB		BIT(17)
 
+#define OWL_UART_POLL_USEC		5
+#define OWL_UART_TIMEOUT_USEC		10000
+
 static struct uart_driver owl_uart_driver;
 
 struct owl_uart_info {
@@ -461,6 +465,36 @@ static void owl_uart_config_port(struct uart_port *port, int flags)
 	}
 }
 
+#ifdef CONFIG_CONSOLE_POLL
+
+static int owl_uart_poll_get_char(struct uart_port *port)
+{
+	if (owl_uart_read(port, OWL_UART_STAT) & OWL_UART_STAT_RFEM)
+		return NO_POLL_CHAR;
+
+	return owl_uart_read(port, OWL_UART_RXDAT);
+}
+
+static void owl_uart_poll_put_char(struct uart_port *port, unsigned char ch)
+{
+	u32 reg;
+	int ret;
+
+	/* Wait while FIFO is full or timeout */
+	ret = readl_poll_timeout_atomic(port->membase + OWL_UART_STAT, reg,
+					!(reg & OWL_UART_STAT_TFFU),
+					OWL_UART_POLL_USEC,
+					OWL_UART_TIMEOUT_USEC);
+	if (ret == -ETIMEDOUT) {
+		dev_err(port->dev, "Timeout waiting while UART TX FULL\n");
+		return;
+	}
+
+	owl_uart_write(port, ch, OWL_UART_TXDAT);
+}
+
+#endif /* CONFIG_CONSOLE_POLL */
+
 static const struct uart_ops owl_uart_ops = {
 	.set_mctrl = owl_uart_set_mctrl,
 	.get_mctrl = owl_uart_get_mctrl,
@@ -476,6 +510,10 @@ static const struct uart_ops owl_uart_ops = {
 	.request_port = owl_uart_request_port,
 	.release_port = owl_uart_release_port,
 	.verify_port = owl_uart_verify_port,
+#ifdef CONFIG_CONSOLE_POLL
+	.poll_get_char = owl_uart_poll_get_char,
+	.poll_put_char = owl_uart_poll_put_char,
+#endif
 };
 
 #ifdef CONFIG_SERIAL_OWL_CONSOLE