[serial] Add general abstraction of a 16550-compatible UART

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

src/arch/x86/core/x86_uart.c

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2014 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * 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; either version 2 of the
+ * License, or any later version.
+ *
+ * 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 Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/** @file
+ *
+ * 16550-compatible UART
+ *
+ */
+
+#include <errno.h>
+#include <ipxe/uart.h>
+
+/** UART port bases */
+static uint16_t uart_base[] = {
+	[COM1] = 0x3f8,
+	[COM2] = 0x2f8,
+	[COM3] = 0x3e8,
+	[COM4] = 0x2e8,
+};
+
+/**
+ * Select UART port
+ *
+ * @v uart		UART
+ * @v port		Port number, or 0 to disable
+ * @ret rc		Return status code
+ */
+int uart_select ( struct uart *uart, unsigned int port ) {
+
+	/* Clear UART base */
+	uart->base = NULL;
+
+	/* Set new UART base */
+	if ( port < ( sizeof ( uart_base ) / sizeof ( uart_base[0] ) ) ) {
+		uart->base = ( ( void * ) ( intptr_t ) uart_base[port] );
+		return 0;
+	} else {
+		return -ENODEV;
+	}
+}