tty/serial: atmel: Prevent a warning on suspend

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 <[e-mail]>
Tested-by: Nicolas Ferre <[e-mail]>
Acked-by: Nicolas Ferre <[e-mail]>
Acked-by: Richard Genoud <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/atmel_serial.c

···
 	bool			has_hw_timer;
 	struct timer_list	uart_timer;
 
+	bool			tx_stopped;
 	bool			suspended;
 	unsigned int		pending;
 	unsigned int		pending_status;
···
  */
 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;
···
 	 * 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);
···
 
 	/* re-enable the transmitter */
 	atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN);
+	atmel_port->tx_stopped = false;
 }
 
 /*
···
 	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,
···
 
 	/* 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) {
···
 	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);
···
 
 	/* 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);
 
···
 {
 	int ret;
 	struct uart_port *port = &atmel_ports[co->index].uart;
+	struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
 	int baud = 115200;
 	int bits = 8;
 	int parity = 'n';
···
 	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);