lib: serial: fix RX path in litex-uart

When used to read characters from the terminal (e.g., when the SBI
console is used via ecall from linux with `console=hvc0`), we must
acknowledge receipt of each character to "pop" it off the LiteUART
hardware queue, and allow the next character to be made available.

Fixes: 52af6e4b ("lib: utils: Add LiteX UART support")
Suggested-by: Dolu1990 <charles.papon.90@gmail.com>
Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

lib/utils/serial/litex-uart.c

@@ -20,6 +20,8 @@
 #define UART_REG_EV_PENDING	4
 #define UART_REG_EV_ENABLE	5
 
+#define UART_EV_RX		0x2
+
 /* clang-format on */
 
 static volatile u32 *uart_base;
@@ -42,10 +44,14 @@ static void litex_uart_putc(char ch)
 
 static int litex_uart_getc(void)
 {
+	int ret;
+
 	if (get_reg(UART_REG_RXEMPTY))
 		return -1;
-	else
-		return get_reg(UART_REG_RXTX);
+
+	ret = get_reg(UART_REG_RXTX);
+	set_reg(UART_REG_EV_PENDING, UART_EV_RX); /* ack. char read */
+	return ret;
 }
 
 static struct sbi_console_device litex_console = {