serial: sprd: Add polling IO support

In order to access the UART without the interrupts, the kernel uses
the basic polling methods for IO with the device. With these methods
implemented, it is now possible to enable kgdb during early boot over serial.
Signed-off-by: Lanqing Liu <liuhhome@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Tested-by: Baolin Wang <baolin.wang@linaro.org>
Link: https://lore.kernel.org/r/f112a741c053ac5fb0637e2f058be81e17f78ccc.1568862391.git.liuhhome@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sprd_serial.c

@@ -919,6 +919,34 @@ static void sprd_pm(struct uart_port *port, unsigned int state,
 	}
 }
 
+#ifdef CONFIG_CONSOLE_POLL
+static int sprd_poll_init(struct uart_port *port)
+{
+	if (port->state->pm_state != UART_PM_STATE_ON) {
+		sprd_pm(port, UART_PM_STATE_ON, 0);
+		port->state->pm_state = UART_PM_STATE_ON;
+	}
+
+	return 0;
+}
+
+static int sprd_poll_get_char(struct uart_port *port)
+{
+	while (!(serial_in(port, SPRD_STS1) & SPRD_RX_FIFO_CNT_MASK))
+		cpu_relax();
+
+	return serial_in(port, SPRD_RXD);
+}
+
+static void sprd_poll_put_char(struct uart_port *port, unsigned char ch)
+{
+	while (serial_in(port, SPRD_STS1) & SPRD_TX_FIFO_CNT_MASK)
+		cpu_relax();
+
+	serial_out(port, SPRD_TXD, ch);
+}
+#endif
+
 static const struct uart_ops serial_sprd_ops = {
 	.tx_empty = sprd_tx_empty,
 	.get_mctrl = sprd_get_mctrl,
@@ -936,6 +964,11 @@ static const struct uart_ops serial_sprd_ops = {
 	.config_port = sprd_config_port,
 	.verify_port = sprd_verify_port,
 	.pm = sprd_pm,
+#ifdef CONFIG_CONSOLE_POLL
+	.poll_init	= sprd_poll_init,
+	.poll_get_char	= sprd_poll_get_char,
+	.poll_put_char	= sprd_poll_put_char,
+#endif
 };
 
 #ifdef CONFIG_SERIAL_SPRD_CONSOLE