[riscv] Inhibit SBI console when a serial console is active

When a native serial driver is enabled for the system console device
specified via "/chosen/stdout-path", it is very likely that this will
correspond to the same physical serial port used for the SBI debug
console.

Inhibit input and output via the SBI console whenever a serial console
is active, to avoid duplicated output characters and unpredictable
input behaviour.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/riscv/interface/sbi/sbi_console.c

@@ -32,6 +32,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <ipxe/sbi.h>
 #include <ipxe/io.h>
 #include <ipxe/keys.h>
+#include <ipxe/serial.h>
 #include <ipxe/console.h>
 #include <config/console.h>
 
@@ -43,6 +44,9 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 extern void early_uart_putchar ( int character );
 
+/** Dummy serial console (if not present in build) */
+struct uart *serial_console __attribute__ (( weak ));
+
 /** Buffered input character (if any) */
 static unsigned char sbi_console_input;
 
@@ -54,6 +58,10 @@ static unsigned char sbi_console_input;
 static void sbi_putchar ( int character ) {
 	struct sbi_return ret;
 
+	/* Do nothing if a real serial console has been enabled */
+	if ( serial_console )
+		return;
+
 	/* Write byte to early UART, if enabled */
 	early_uart_putchar ( character );
 
@@ -99,6 +107,10 @@ static int sbi_iskey ( void ) {
 	if ( sbi_console_input )
 		return sbi_console_input;
 
+	/* Do nothing if a real serial console has been enabled */
+	if ( serial_console )
+		return 0;
+
 	/* Read and buffer byte from console, if any */
 	ret = sbi_ecall_3 ( SBI_DBCN, SBI_DBCN_READ,
 			    sizeof ( sbi_console_input ),