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>

8 files changed, 91 insertions, 54 deletions

diff --git a/src/arch/x86/lib/romcc_console.c b/src/arch/x86/lib/romcc_console.c
index db84f9e642..c5e688258b 100644
--- a/src/arch/x86/lib/romcc_console.c
+++ b/src/arch/x86/lib/romcc_console.c
@@ -32,6 +32,7 @@
 #endif
 
 #if CONFIG_CONSOLE_SERIAL8250
+#include "drivers/uart/util.c"
 #include "lib/uart8250.c"
 #endif
 #if CONFIG_CONSOLE_NE2K
diff --git a/src/drivers/oxford/oxpcie/oxpcie_early.c b/src/drivers/oxford/oxpcie/oxpcie_early.c
index d04e9d4df0..4f6f9649f5 100644
--- a/src/drivers/oxford/oxpcie/oxpcie_early.c
+++ b/src/drivers/oxford/oxpcie/oxpcie_early.c
@@ -21,6 +21,7 @@
 #include <arch/io.h>
 #include <arch/early_variables.h>
 #include <delay.h>
+#include <uart.h>
 #include <uart8250.h>
 #include <device/pci_def.h>
 
@@ -116,7 +117,14 @@ void oxford_init(void)
 	/* Now the UART initialization */
 	u32 uart0_base = CONFIG_OXFORD_OXPCIE_BASE_ADDRESS + 0x1000;
 
-	uart8250_mem_init(uart0_base, (4000000 / CONFIG_TTYS0_BAUD));
+	unsigned int div = uart_baudrate_divisor(default_baudrate(),
+		uart_platform_refclk(), 16);
+	uart8250_mem_init(uart0_base, div);
 }
 
 #endif
+
+unsigned int uart_platform_refclk(void)
+{
+	return 62500000;
+}
diff --git a/src/drivers/uart/Makefile.inc b/src/drivers/uart/Makefile.inc
new file mode 100644
index 0000000000..c797d4b44d
--- /dev/null
+++ b/src/drivers/uart/Makefile.inc
@@ -0,0 +1,6 @@
+ifeq ($(CONFIG_CONSOLE_SERIAL),y)
+romstage-y += util.c
+ramstage-y += util.c
+bootblock-y += util.c
+smm-y += util.c
+endif
diff --git a/src/drivers/uart/util.c b/src/drivers/uart/util.c
new file mode 100644
index 0000000000..c577048fb5
--- /dev/null
+++ b/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;
+}
diff --git a/src/include/uart.h b/src/include/uart.h
index b520e09bdb..997fe185ce 100644
--- a/src/include/uart.h
+++ b/src/include/uart.h
@@ -20,6 +20,23 @@
 #ifndef UART_H
 #define UART_H
 
+/* Return the clock frequency UART uses as reference clock for
+ * baudrate generator. */
+unsigned int uart_platform_refclk(void);
+
+/* Return the baudrate determined from option_table, or when that is
+ * not used, CONFIG_TTYS0_BAUD.
+ */
+unsigned int default_baudrate(void);
+
+/* Returns the divisor value for a given baudrate.
+ * The formula to satisfy is:
+ *    refclk / divisor = baudrate * oversample
+ */
+unsigned int uart_baudrate_divisor(unsigned int baudrate,
+	unsigned int refclk, unsigned int oversample);
+
+
 unsigned char uart_rx_byte(void);
 void uart_tx_byte(unsigned char data);
 void uart_tx_flush(void);
diff --git a/src/include/uart8250.h b/src/include/uart8250.h
index a48c948138..112cd44945 100644
--- a/src/include/uart8250.h
+++ b/src/include/uart8250.h
@@ -105,10 +105,6 @@
 #define UART_SCR 0x07
 #define UART_SPR 0x07
 
-#if ((115200 % CONFIG_TTYS0_BAUD) != 0)
-#error Bad ttyS0 baud rate
-#endif
-
 #if CONFIG_CONSOLE_SERIAL8250
 unsigned char uart8250_rx_byte(unsigned base_port);
 int uart8250_can_rx_byte(unsigned base_port);
diff --git a/src/lib/uart8250.c b/src/lib/uart8250.c
index c9075a227a..d136eb8867 100644
--- a/src/lib/uart8250.c
+++ b/src/lib/uart8250.c
@@ -21,15 +21,16 @@
 #include <arch/io.h>
 #include <uart.h>
 #include <uart8250.h>
-#include <pc80/mc146818rtc.h>
 #include <trace.h>
 
-#if CONFIG_USE_OPTION_TABLE
-#include "option_table.h"
-#endif
-
 /* Should support 8250, 16450, 16550, 16550A type UARTs */
 
+/* Nominal values only, good for the range of choices Kconfig offers for
+ * set of standard baudrates.
+ */
+#define BAUDRATE_REFCLK		(115200)
+#define BAUDRATE_OVERSAMPLE	(1)
+
 /* Expected character delay at 1200bps is 9ms for a working UART
  * and no flow-control. Assume UART as stuck if shift register
  * or FIFO takes more than 50ms per character to appear empty.
@@ -108,26 +109,8 @@ void uart8250_init(unsigned base_port, unsigned divisor)
 
 void uart_init(void)
 {
-	/* TODO the divisor calculation is hard coded to standard UARTs. Some
-	 * UARTs won't work with these values. This should be a property of the
-	 * UART used, worst case a Kconfig variable. For now live with hard
-	 * codes as the only devices that might be different are the iWave
-	 * iRainbowG6 and the OXPCIe952 card (and the latter is memory mapped)
-	 */
-	unsigned int div = (115200 / CONFIG_TTYS0_BAUD);
-
-#if !defined(__SMM__) && CONFIG_USE_OPTION_TABLE
-	static const unsigned char divisor[8] = { 1, 2, 3, 6, 12, 24, 48, 96 };
-	unsigned b_index = 0;
-#if defined(__PRE_RAM__)
-	b_index = read_option(baud_rate, 0);
-	b_index &= 7;
-	div = divisor[b_index];
-#else
-	if (get_option(&b_index, "baud_rate") == CB_SUCCESS)
-		div = divisor[b_index];
-#endif
-#endif
-
+	unsigned int div;
+	div = uart_baudrate_divisor(default_baudrate(), BAUDRATE_REFCLK,
+		BAUDRATE_OVERSAMPLE);
 	uart8250_init(CONFIG_TTYS0_BASE, div);
 }
diff --git a/src/lib/uart8250mem.c b/src/lib/uart8250mem.c
index 040e732df3..5e96825834 100644
--- a/src/lib/uart8250mem.c
+++ b/src/lib/uart8250mem.c
@@ -21,10 +21,6 @@
 #include <arch/io.h>
 #include <uart.h>
 #include <uart8250.h>
-#include <pc80/mc146818rtc.h>
-#if CONFIG_USE_OPTION_TABLE
-#include "option_table.h"
-#endif
 #include <device/device.h>
 #include <delay.h>
 
@@ -96,7 +92,6 @@ void uart8250_mem_init(unsigned base_port, unsigned divisor)
 	/* DLAB on */
 	write8(base_port + UART_LCR, UART_LCR_DLAB | CONFIG_TTYS0_LCS);
 
-	/* Set Baud Rate Divisor. 12 ==> 9600 Baud */
 	write8(base_port + UART_DLL, divisor & 0xFF);
 	write8(base_port + UART_DLM, (divisor >> 8) & 0xFF);
 
@@ -106,27 +101,11 @@ void uart8250_mem_init(unsigned base_port, unsigned divisor)
 
 u32 uart_mem_init(void)
 {
-	unsigned uart_baud = CONFIG_TTYS0_BAUD;
 	u32 uart_bar = 0;
 	unsigned div;
 
-	/* find out the correct baud rate */
-#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, 0);
-	b_index &= 7;
-	uart_baud = baud[b_index];
-#else
-	if (get_option(&b_index, "baud_rate") == CB_SUCCESS)
-		uart_baud = baud[b_index];
-#endif
-#endif
-
 	/* Now find the UART base address and calculate the divisor */
 #if CONFIG_DRIVERS_OXFORD_OXPCIE
-
 #if defined(MORE_TESTING) && !defined(__SIMPLE_DEVICE__)
 	device_t dev = dev_find_device(0x1415, 0xc158, NULL);
 	if (!dev)
@@ -145,10 +124,9 @@ u32 uart_mem_init(void)
 #endif
 	uart_bar = CONFIG_OXFORD_OXPCIE_BASE_ADDRESS + 0x1000; // 1st UART
 	// uart_bar = CONFIG_OXFORD_OXPCIE_BASE_ADDRESS + 0x2000; // 2nd UART
-
-	div = 4000000 / uart_baud;
 #endif
 
+	div = uart_baudrate_divisor(default_baudrate(), uart_platform_refclk(), 16);
 	if (uart_bar)
 		uart8250_mem_init(uart_bar, div);