[efi] Define an interface operation to describe using an EFI device path

Allow arbitrary objects to support describing themselves using an EFI
device path.

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

src/include/ipxe/efi/efi_path.h

@@ -9,11 +9,16 @@
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
+#include <ipxe/interface.h>
 #include <ipxe/efi/efi.h>
 #include <ipxe/efi/Protocol/DevicePath.h>
 
 extern EFI_DEVICE_PATH_PROTOCOL *
 efi_path_end ( EFI_DEVICE_PATH_PROTOCOL *path );
 extern size_t efi_path_len ( EFI_DEVICE_PATH_PROTOCOL *path );
 
+extern EFI_DEVICE_PATH_PROTOCOL * efi_describe ( struct interface *interface );
+#define efi_describe_TYPE( object_type ) \
+	typeof ( EFI_DEVICE_PATH_PROTOCOL * ( object_type ) )
+
 #endif /* _IPXE_EFI_PATH_H */

src/interface/efi/efi_path.c

@@ -55,3 +55,29 @@ size_t efi_path_len ( EFI_DEVICE_PATH_PROTOCOL *path ) {
 
 	return ( ( ( void * ) end ) - ( ( void * ) path ) );
 }
+
+/**
+ * Describe object as an EFI device path
+ *
+ * @v intf		Interface
+ * @ret path		EFI device path, or NULL
+ *
+ * The caller is responsible for eventually calling free() on the
+ * allocated device path.
+ */
+EFI_DEVICE_PATH_PROTOCOL * efi_describe ( struct interface *intf ) {
+	struct interface *dest;
+	efi_describe_TYPE ( void * ) *op =
+		intf_get_dest_op ( intf, efi_describe, &dest );
+	void *object = intf_object ( dest );
+	EFI_DEVICE_PATH_PROTOCOL *path;
+
+	if ( op ) {
+		path = op ( object );
+	} else {
+		path = NULL;
+	}
+
+	intf_put ( dest );
+	return path;
+}