Do full flush on uart8250 only at end of printk.

The previous code does a full flush of the uart after every character.
Unfortunately, this can cause transmission delays on some serial
ports.

This patch changes the code so that it does a flush at the end of
every printk instead of at the end of every character.  This reduces
the time it takes to transmit serial messages (up to 9% on my Asrock
e350m1 board).  It also makes the transmission time more consistent
which is important when performing timing tests via serial
transmissions.

Change-Id: I6b28488b905da68c6d68d7c517cc743cde567d70
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Reviewed-on: http://review.coreboot.org/90
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)
Reviewed-by: Rudolf Marek <r.marek@assembler.cz>
Reviewed-by: Sven Schnelle <svens@stackframe.org>

src/console/uart8250_console.c

@@ -31,6 +31,11 @@ static void ttyS0_tx_byte(unsigned char data)
 	uart8250_tx_byte(CONFIG_TTYS0_BASE, data);
 }
 
+static void ttyS0_tx_flush(void)
+{
+	uart8250_tx_flush(CONFIG_TTYS0_BASE);
+}
+
 static unsigned char ttyS0_rx_byte(void)
 {
 	return uart8250_rx_byte(CONFIG_TTYS0_BASE);
@@ -44,6 +49,7 @@ static int ttyS0_tst_byte(void)
 static const struct console_driver uart8250_console __console = {
 	.init     = ttyS0_init,
 	.tx_byte  = ttyS0_tx_byte,
+	.tx_flush = ttyS0_tx_flush,
 	.rx_byte  = ttyS0_rx_byte,
 	.tst_byte = ttyS0_tst_byte,
 };

src/console/uart8250mem_console.c

@@ -36,6 +36,11 @@ static void uartmem_tx_byte(unsigned char data)
 	uart8250_mem_tx_byte(uart_bar, data);
 }
 
+static void uartmem_tx_flush(void)
+{
+	uart8250_mem_tx_flush(uart_bar);
+}
+
 static unsigned char uartmem_rx_byte(void)
 {
 	if (!uart_bar)
@@ -55,6 +60,7 @@ static int uartmem_tst_byte(void)
 static const struct console_driver uart8250mem_console __console = {
 	.init     = uartmem_init,
 	.tx_byte  = uartmem_tx_byte,
+	.tx_flush = uartmem_tx_flush,
 	.rx_byte  = uartmem_rx_byte,
 	.tst_byte = uartmem_tst_byte,
 };