serial: 8250: Use canary to restart console after suspend

When using no_console_suspend, the serial console may be powered off
anyway during system sleep. Upon resume, the port may be in its default
power-on state, but is expected to continue console i/o before the device
has received its pm callback. The resultant garbage i/o can cause all
kinds of havoc on the remote end.

Use the scratch register as a canary to discover if the console
has been powered-off. Write a non-zero value to the scratch register
at port suspend and reprogram the port before any console i/o if the
scratch register != canary before port resume.

This workaround is disabled for omap_8250 (which uses different divisor
programming).

Credit to Doug Anderson <dianders@chromium.org> for the idea of using
the scratch register canary to discover port power-down.

Cc: Doug Anderson <dianders@chromium.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 37 insertions, 1 deletions

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index c8ecfaf49eb4..1449c56506b7 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -3267,6 +3267,27 @@ serial8250_console_write(struct console *co, const char *s, unsigned int count)
         else
                 serial_port_out(port, UART_IER, 0);
 
+        /* check scratch reg to see if port powered off during system sleep */
+        if (up->canary && (up->canary != serial_port_in(port, UART_SCR))) {
+                struct ktermios termios;
+                unsigned int baud, quot, frac = 0;
+
+                termios.c_cflag = port->cons->cflag;
+                if (port->state->port.tty && termios.c_cflag == 0)
+                        termios.c_cflag = port->state->port.tty->termios.c_cflag;
+
+                baud = uart_get_baud_rate(port, &termios, NULL,
+                                          port->uartclk / 16 / 0xffff,
+                                          port->uartclk / 16);
+                quot = serial8250_get_divisor(up, baud, &frac);
+
+                serial8250_set_divisor(port, baud, quot, frac);
+                serial_port_out(port, UART_LCR, up->lcr);
+                serial_port_out(port, UART_MCR, UART_MCR_DTR | UART_MCR_RTS);
+
+                up->canary = 0;
+        }
+
         uart_console_write(port, s, count, serial8250_console_putchar);
 
         /*
@@ -3417,7 +3438,17 @@ int __init early_serial_setup(struct uart_port *port)
  */
 void serial8250_suspend_port(int line)
 {
-        uart_suspend_port(&serial8250_reg, &serial8250_ports[line].port);
+        struct uart_8250_port *up = &serial8250_ports[line];
+        struct uart_port *port = &up->port;
+
+        if (!console_suspend_enabled && uart_console(port) &&
+            port->type != PORT_8250) {
+                unsigned char canary = 0xa5;
+                serial_out(up, UART_SCR, canary);
+                up->canary = canary;
+        }
+
+        uart_suspend_port(&serial8250_reg, port);
 }
 
 /**
@@ -3431,6 +3462,8 @@ void serial8250_resume_port(int line)
         struct uart_8250_port *up = &serial8250_ports[line];
         struct uart_port *port = &up->port;
 
+        up->canary = 0;
+
         if (up->capabilities & UART_NATSEMI) {
                 /* Ensure it's still in high speed mode */
                 serial_port_out(port, UART_LCR, 0xE0);
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index 245b959f1ff6..a8efa235b7c1 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -85,6 +85,9 @@ struct uart_8250_port {
         unsigned char           mcr_force;      /* mask of forced bits */
         unsigned char           cur_iotype;     /* Running I/O type */
         unsigned int            rpm_tx_active;
+        unsigned char           canary;         /* non-zero during system sleep
+                                                 *   if no_console_suspend
+                                                 */
 
         /*
          * Some bits in registers are cleared on a read, so they must