[fdt] Add the concept of an FDT image

Define the concept of an "FDT" image, representing a Flattened Device
Tree blob that has been downloaded in order to be provided to a kernel
or other executable image.  FDT images are represented using an image
tag (as with other special-purpose images such as the UEFI shim), and
are similarly marked as hidden so that they will not be included in a
generated magic initrd or show up in a virtual filesystem directory
listing.

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

src/core/fdt.c

@@ -28,6 +28,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <assert.h>
 #include <byteswap.h>
 #include <ipxe/netdevice.h>
+#include <ipxe/image.h>
 #include <ipxe/fdt.h>
 
 /** @file
@@ -39,6 +40,11 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 /** The system flattened device tree (if present) */
 static struct fdt fdt;
 
+/** The downloaded flattened device tree tag */
+struct image_tag fdt_image __image_tag = {
+	.name = "FDT",
+};
+
 /** A position within a device tree */
 struct fdt_cursor {
 	/** Offset within structure block */

src/include/ipxe/fdt.h

@@ -10,6 +10,7 @@
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <stdint.h>
+#include <ipxe/image.h>
 
 struct net_device;
 
@@ -93,6 +94,8 @@ struct fdt {
 	size_t strings_len;
 };
 
+extern struct image_tag fdt_image __image_tag;
+
 extern int fdt_path ( const char *path, unsigned int *offset );
 extern int fdt_alias ( const char *name, unsigned int *offset );
 extern const char * fdt_string ( unsigned int offset, const char *name );

src/include/usr/fdtmgmt.h

@@ -0,0 +1,16 @@
+#ifndef _USR_FDTMGMT_H
+#define _USR_FDTMGMT_H
+
+/** @file
+ *
+ * Flattened Device Tree management
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <ipxe/image.h>
+
+extern int imgfdt ( struct image *image );
+
+#endif /* _USR_FDTMGMT_H */

src/usr/fdtmgmt.c

@@ -0,0 +1,51 @@
+/*
+ * 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 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 <ipxe/fdt.h>
+#include <usr/fdtmgmt.h>
+
+/** @file
+ *
+ * Flattened Device Tree management
+ *
+ */
+
+/**
+ * Apply flattened device tree image
+ *
+ * @v image		FDT image, or NULL to clear FDT
+ * @ret rc		Return status code
+ */
+int imgfdt ( struct image *image ) {
+
+	/* Record (or clear) FDT image */
+	image_tag ( image, &fdt_image );
+
+	/* Avoid including image in constructed initrd */
+	if ( image )
+		image_hide ( image );
+
+	return 0;
+}