drivers/uart: Enable override for input clock divider

Allow the platform to override the input clock divider by adding the
uart_input_clock_divider routine.  This routine combines the baud-rate
oversample divider with any other input clock divider.  The default
routine returns 16 which is the standard baud-rate oversampling value.
A platform may override this default "weak" routine by providing a new
routine and selecting UART_OVERRIDE_INPUT_CLOCK_DIVIDER.  This works
around ROMCC not supporting weak routines.

Testing on Galileo:
*  Edit the src/mainboard/intel/galileo/Makefile.inc file:
   *  Add "select ADD_FSP_PDAT_FILE"
   *  Add "select ADD_FSP_RAW_BIN"
   *  Add "select ADD_RMU_FILE"
*  Place the FSP.bin file in the location specified by CONFIG_FSP_FILE
*  Place the pdat.bin files in the location specified by
   CONFIG_FSP_PDAT_FILE
*  Place the rmu.bin file in the location specified by CONFIG_RMU_FILE
*  Build EDK2 CorebootPayloadPkg/CorebootPayloadPkgIa32.dsc to generate
   UEFIPAYLOAD.fd
*  Testing is successful when CorebootPayloadPkg is able to properly
   initialize the serial port without using built-in values.

Change-Id: Ieb6453b045d84702b8f730988d0fed9f253f63e2
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/14611
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/drivers/uart/Kconfig

@@ -13,6 +13,13 @@ config DRIVERS_UART_8250IO
 config NO_UART_ON_SUPERIO
 	def_bool n
 
+config UART_OVERRIDE_INPUT_CLOCK_DIVIDER
+	boolean
+	default n
+	help
+	  Set to "y" when the platform overrides the uart_input_clock_divider
+	  routine.
+
 config DRIVERS_UART_8250MEM
 	bool
 	default n

src/drivers/uart/uart8250io.c

@@ -30,8 +30,12 @@
 /* Nominal values only, good for the range of choices Kconfig offers for
  * set of standard baudrates.
  */
-#define BAUDRATE_REFCLK		(115200)
-#define BAUDRATE_OVERSAMPLE	(1)
+
+/* Multiply the maximim baud-rate by the default oversample rate to compute
+ * the default input clock to the UART.  The uart_baudrate_divisor divides
+ * by the oversample clock to determine the final baud-rate.
+ */
+#define BAUDRATE_REFCLK		(115200 * 16)
 
 /* Expected character delay at 1200bps is 9ms for a working UART
  * and no flow-control. Assume UART as stuck if shift register
@@ -112,7 +116,7 @@ void uart_init(int idx)
 {
 	unsigned int div;
 	div = uart_baudrate_divisor(default_baudrate(), BAUDRATE_REFCLK,
-		BAUDRATE_OVERSAMPLE);
+		uart_input_clock_divider());
 	uart8250_init(uart_platform_base(idx), div);
 }
 

src/drivers/uart/uart8250mem.c

@@ -117,7 +117,8 @@ void uart_init(int idx)
 		return;
 
 	unsigned int div;
-	div = uart_baudrate_divisor(default_baudrate(), uart_platform_refclk(), 16);
+	div = uart_baudrate_divisor(default_baudrate(),
+		uart_platform_refclk(), uart_input_clock_divider());
 	uart8250_mem_init(base, div);
 }
 

src/drivers/uart/util.c

@@ -42,3 +42,20 @@ unsigned int uart_baudrate_divisor(unsigned int baudrate,
 {
 	return (1 + (2 * refclk) / (baudrate * oversample)) / 2;
 }
+
+#if !IS_ENABLED(CONFIG_UART_OVERRIDE_INPUT_CLOCK_DIVIDER)
+unsigned int uart_input_clock_divider(void)
+{
+	/* Specify the default oversample rate for the UART.
+	 *
+	 * UARTs oversample the receive data.  The UART's input clock first
+	 * enters the baud-rate divider to generate the oversample clock.  Then
+	 * the UART typically divides the result by 16.  The asynchronous
+	 * receive data is synchronized with the oversample clock and when a
+	 * start bit is detected the UART delays half a bit time using the
+	 * oversample clock.  Samples are then taken to verify the start bit and
+	 * if present, samples are taken for the rest of the frame.
+	 */
+	return 16;
+}
+#endif

src/include/console/uart.h

@@ -35,6 +35,10 @@ unsigned int default_baudrate(void);
 unsigned int uart_baudrate_divisor(unsigned int baudrate,
 	unsigned int refclk, unsigned int oversample);
 
+/* Returns the oversample divisor multiplied by any other divisors that act
+ * on the input clock
+ */
+unsigned int uart_input_clock_divider(void);
 
 void uart_init(int idx);
 void uart_tx_byte(int idx, unsigned char data);