[fdtcon] Add basic support for FDT-based system serial console

Add support for probing a device based on the path or alias found in
the "/chosen/stdout-path" node, and using a consequently instantiated
UART as the default serial console.

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

src/config/defaults/sbi.h

@@ -22,10 +22,11 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #endif
 
 #define CONSOLE_SBI
+#define CONSOLE_SERIAL
 #define REBOOT_SBI
 #define UMALLOC_UHEAP
 #define MEMMAP_FDT
-#define SERIAL_NULL
+#define SERIAL_FDT
 
 #define ACPI_NULL
 #define MPAPI_NULL

src/core/fdtcon.c

@@ -0,0 +1,108 @@
+/*
+ * Copyright (C) 2025 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 (at your option) 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 );
+
+#include <string.h>
+#include <ipxe/serial.h>
+#include <ipxe/devtree.h>
+#include <ipxe/fdt.h>
+
+/** @file
+ *
+ * Flattened Device Tree serial console
+ *
+ */
+
+#ifdef SERIAL_FDT
+#define SERIAL_PREFIX_fdt
+#else
+#define SERIAL_PREFIX_fdt __fdt_
+#endif
+
+/** FDT console parent device */
+static struct device fdtcon_parent = {
+	.name = "fdtcon",
+	.siblings = LIST_HEAD_INIT ( fdtcon_parent.siblings ),
+	.children = LIST_HEAD_INIT ( fdtcon_parent.children ),
+};
+
+/** Colour for debug messages */
+#define colour &fdtcon_parent
+
+/**
+ * Identify default serial console
+ *
+ * @ret uart		Default serial console UART, or NULL
+ */
+static struct uart * fdtcon_default ( void ) {
+	unsigned int chosen;
+	unsigned int stdout;
+	const char *path;
+	struct uart *prev;
+	struct uart *uart;
+	int rc;
+
+	/* Record existing UART, if any */
+	prev = list_last_entry ( &uarts, struct uart, list );
+
+	/* Locate "/chosen" node */
+	if ( ( rc = fdt_path ( &sysfdt, "/chosen", &chosen ) ) != 0 ) {
+		DBGC ( colour, "FDTCON could not locate \"/chosen\": %s\n",
+		       strerror ( rc ) );
+		return NULL;
+	}
+
+	/* Get console device path (or alias) */
+	path = fdt_string ( &sysfdt, chosen, "stdout-path" );
+	if ( ! path ) {
+		DBGC ( colour, "FDTCON has no console device\n" );
+		return NULL;
+	}
+	DBGC ( colour, "FDTCON console device is \"%s\"\n", path );
+
+	/* Locate device */
+	if ( ( ( rc = fdt_path ( &sysfdt, path, &stdout ) ) != 0 ) &&
+	     ( ( rc = fdt_alias ( &sysfdt, path, &stdout ) ) != 0 ) ) {
+		DBGC ( colour, "FDTCON could not locate \"/%s\": %s\n",
+		       path, strerror ( rc ) );
+		return NULL;
+	}
+
+	/* Probe device */
+	if ( ( rc = dt_probe_node ( &fdtcon_parent, stdout ) ) != 0 ) {
+		DBGC ( colour, "FDTCON could not probe \"%s\": %s\n",
+		       path, strerror ( rc ) );
+		return NULL;
+	}
+
+	/* Use newly added UART, if any */
+	uart = list_last_entry ( &uarts, struct uart, list );
+	if ( uart == prev )
+		return NULL;
+
+	DBGC ( colour, "FDTCON using UART %s\n", uart->name );
+	return uart;
+}
+
+PROVIDE_SERIAL ( fdt, default_serial_console, fdtcon_default );