serial: 8250_early: Support kgdboc_earlycon

Implement the read() function in the early console driver.  With
recent kgdb patches this allows you to use kgdb to debug fairly early
into the system boot.

We only bother implementing this if polling is enabled since kgdb
can't be enabled without that.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200507130644.v4.11.I8f668556c244776523320a95b09373a86eda11b7@changeid
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

diff --git a/drivers/tty/serial/8250/8250_early.c b/drivers/tty/serial/8250/8250_early.c
index 5cd8c36..70d7826 100644 (file)
--- a/drivers/tty/serial/8250/8250_early.c
+++ b/drivers/tty/serial/8250/8250_early.c
@@ -109,6 +109,28 @@ static void early_serial8250_write(struct console *console,
        uart_console_write(port, s, count, serial_putc);
 }
 
+#ifdef CONFIG_CONSOLE_POLL
+static int early_serial8250_read(struct console *console,
+                                char *s, unsigned int count)
+{
+       struct earlycon_device *device = console->data;
+       struct uart_port *port = &device->port;
+       unsigned int status;
+       int num_read = 0;
+
+       while (num_read < count) {
+               status = serial8250_early_in(port, UART_LSR);
+               if (!(status & UART_LSR_DR))
+                       break;
+               s[num_read++] = serial8250_early_in(port, UART_RX);
+       }
+
+       return num_read;
+}
+#else
+#define early_serial8250_read NULL
+#endif
+
 static void __init init_port(struct earlycon_device *device)
 {
        struct uart_port *port = &device->port;
@@ -149,6 +171,7 @@ int __init early_serial8250_setup(struct earlycon_device *device,
                init_port(device);
 
        device->con->write = early_serial8250_write;
+       device->con->read = early_serial8250_read;
        return 0;
 }
 EARLYCON_DECLARE(uart8250, early_serial8250_setup);