uart8250: Fix and unify baudrate divisor calculation

Divisor is a function of requested baudrate, platform-specific
reference clock and amount of oversampling done on the UART reference.
Calculate this parameter with divisor rounded to nearest integer.

When building without option_table or when there is no entry for
baud_rate, CONFIG_TTYS0_BAUD is used for default baudrate.

For OxPCIe use of 4 MHz for reference was arbitrary giving correct
divisor for 115200 but somewhat inaccurate for lower baudrates.
Actual hardware is 62500000 with 16 times oversampling.

FIXME: Field for baudrate in lb_tables is still incorrect.

Change-Id: I68539738469af780fadd3392263dd9b3d5964d2d
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5229
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/drivers/uart/util.c

@@ -0,0 +1,48 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <console/console.h>
+#include <uart.h>
+#if CONFIG_USE_OPTION_TABLE
+#include <pc80/mc146818rtc.h>
+#include "option_table.h"
+#endif
+
+unsigned int default_baudrate(void)
+{
+#if !defined(__SMM__) && CONFIG_USE_OPTION_TABLE
+	static const unsigned baud[8] =
+		{ 115200, 57600, 38400, 19200, 9600, 4800, 2400, 1200 };
+	unsigned b_index = 0;
+#if defined(__PRE_RAM__)
+	b_index = read_option(baud_rate, 0xff);
+#else
+	if (get_option(&b_index, "baud_rate") != CB_SUCCESS)
+		b_index = 0xff;
+#endif
+	if (b_index < 8)
+		return baud[b_index];
+#endif
+	return CONFIG_TTYS0_BAUD;
+}
+
+/* Calculate divisor. Do not floor but round to nearest integer. */
+unsigned int uart_baudrate_divisor(unsigned int baudrate,
+	unsigned int refclk, unsigned int oversample)
+{
+	return (1 + (2 * refclk) / (baudrate * oversample)) / 2;
+}