The atmel serial port driver reported the following warning on suspend:
atmel_usart f8020000.serial: ttyS1: Unable to drain transmitter

As the ATMEL_US_TXEMPTY status bit in ATMEL_US_CSR is always cleared
when the transmitter is disabled, we need to know the transmitter's
state to return the real fifo state. And as ATMEL_US_CR is write-only,
it is necessary to save the state of the transmitter in a local
variable, and update the variable when TXEN and TXDIS is written in
ATMEL_US_CR.

After those changes, atmel_tx_empty can return "empty" on suspend, the
warning in uart_suspend_port disappears, and suspending is 20ms shorter
for each enabled Atmel serial port.

Signed-off-by: Romain Izard <redacted>
---
 drivers/tty/serial/atmel_serial.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index 7551cab438ff..195c0d1b594e 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c

@@ -171,6 +171,7 @@ struct atmel_uart_port {
        bool                    has_hw_timer;
        struct timer_list       uart_timer;

+       bool                    tx_stopped;
        bool                    suspended;
        unsigned int            pending;
        unsigned int            pending_status;

@@ -380,6 +381,10 @@ static int atmel_config_rs485(struct uart_port *port,
  */
 static u_int atmel_tx_empty(struct uart_port *port)
 {
+       struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
+
+       if (atmel_port->tx_stopped)
+               return TIOCSER_TEMT;
        return (atmel_uart_readl(port, ATMEL_US_CSR) & ATMEL_US_TXEMPTY) ?
                TIOCSER_TEMT :
                0;

@@ -485,6 +490,7 @@ static void atmel_stop_tx(struct uart_port *port)
         * is fully transmitted.
         */
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXDIS);
+       atmel_port->tx_stopped = true;

        /* Disable interrupts */
        atmel_uart_writel(port, ATMEL_US_IDR, atmel_port->tx_done_mask);

@@ -492,6 +498,7 @@ static void atmel_stop_tx(struct uart_port *port)
        if ((port->rs485.flags & SER_RS485_ENABLED) &&
            !(port->rs485.flags & SER_RS485_RX_DURING_TX))
                atmel_start_rx(port);
+
 }

 /*

@@ -521,6 +528,7 @@ static void atmel_start_tx(struct uart_port *port)

        /* re-enable the transmitter */
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN);
+       atmel_port->tx_stopped = false;
 }

 /*

@@ -1866,6 +1874,7 @@ static int atmel_startup(struct uart_port *port)
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_RSTSTA | ATMEL_US_RSTRX);
        /* enable xmit & rcvr */
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN | ATMEL_US_RXEN);
+       atmel_port->tx_stopped = false;

        setup_timer(&atmel_port->uart_timer,
                        atmel_uart_timer_callback,

@@ -2122,6 +2131,7 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,

        /* disable receiver and transmitter */
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXDIS | ATMEL_US_RXDIS);
+       atmel_port->tx_stopped = true;

        /* mode */
        if (port->rs485.flags & SER_RS485_ENABLED) {

@@ -2207,6 +2217,7 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
        atmel_uart_writel(port, ATMEL_US_BRGR, quot);
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_RSTSTA | ATMEL_US_RSTRX);
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN | ATMEL_US_RXEN);
+       atmel_port->tx_stopped = false;

        /* restore interrupts */
        atmel_uart_writel(port, ATMEL_US_IER, imr);

@@ -2450,6 +2461,7 @@ static void atmel_console_write(struct console *co, const char *s, u_int count)

        /* Make sure that tx path is actually able to send characters */
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN);
+       atmel_port->tx_stopped = false;

        uart_console_write(port, s, count, atmel_console_putchar);

@@ -2528,6 +2540,7 @@ static int __init atmel_console_setup(struct console *co, char *options)
        atmel_uart_writel(port, ATMEL_US_IDR, -1);
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_RSTSTA | ATMEL_US_RSTRX);
        atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN | ATMEL_US_RXEN);
+       atmel_port->tx_stopped = false;

        if (options)
                uart_parse_options(options, &baud, &parity, &bits, &flow);
--

2.11.0