uart/sifive: make divisor configurable

The SiFive UART on the HiFive Unleashed uses the tlclk as input clock
which runs at coreclk / 2.

The input frequency is configured in the board code depending on the
current stage. (bootblock + romstage run at 33.33Mhz, ramstage at 1Ghz)

Change-Id: Iaf66723dba3d308f809fde5b05dfc3e43f43bd42
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/27440
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

src/drivers/uart/sifive.c

@@ -46,11 +46,10 @@ struct sifive_uart_registers {
 #define IP_TXWM			BIT(0)
 #define IP_RXWM			BIT(1)
 
-void uart_init(int idx)
+static void sifive_uart_init(struct sifive_uart_registers *regs, int div)
 {
-	struct sifive_uart_registers *regs = uart_platform_baseptr(idx);
-
-	/* TODO: Configure the divisor */
+	/* Configure the divisor */
+	write32(&regs->div, div);
 
 	/* Enable transmission, one stop bit, transmit watermark at 1 */
 	write32(&regs->txctrl, TXCTRL_TXEN|TXCTRL_NSTOP(1)|TXCTRL_TXCNT(1));
@@ -59,6 +58,14 @@ void uart_init(int idx)
 	write32(&regs->rxctrl, RXCTRL_RXEN|RXCTRL_RXCNT(0));
 }
 
+void uart_init(int idx)
+{
+	unsigned int div;
+	div = uart_baudrate_divisor(get_uart_baudrate(),
+		uart_platform_refclk(), uart_input_clock_divider());
+	sifive_uart_init(uart_platform_baseptr(idx), div);
+}
+
 static bool uart_can_tx(struct sifive_uart_registers *regs)
 {
 	return !(read32(&regs->txdata) & TXDATA_FULL);

src/soc/sifive/fu540/Kconfig

@@ -20,7 +20,7 @@ config SOC_SIFIVE_FU540
 	select ARCH_RAMSTAGE_RISCV
 	select BOOTBLOCK_CONSOLE
 	select DRIVERS_UART_SIFIVE
-
+	select UART_OVERRIDE_REFCLK
 if SOC_SIFIVE_FU540
 
 config RISCV_ARCH

src/soc/sifive/fu540/uart.c

@@ -23,3 +23,11 @@ uintptr_t uart_platform_base(int idx)
 	else
 		return 0;
 }
+
+unsigned int uart_platform_refclk(void)
+{
+	/*
+	 * The SiFive UART uses tlclk, which is coreclk/2 as input
+	 */
+	return 33330000 / 2;
+}