qapi: Improve qobject visitor documentation

Backports commit aa3a982e674b09ae32502940f93ba98b3a8ad50e from qemu

qemu/include/qapi/qobject-input-visitor.h

@@ -1,6 +1,7 @@
 /*
  * Input Visitor
  *
+ * Copyright (C) 2017 Red Hat, Inc.
  * Copyright IBM, Corp. 2011
  *
  * Authors:
@@ -20,7 +21,41 @@
 typedef struct QObjectInputVisitor QObjectInputVisitor;
 
 /*
- * Return a new input visitor that converts QMP to QAPI.
+ * Create a QObject input visitor for @obj
+ *
+ * A QObject input visitor visit builds a QAPI object from a QObject.
+ * This simultaneously walks the QAPI object being built and the
+ * QObject.  The latter walk starts at @obj.
+ *
+ * visit_type_FOO() creates an instance of QAPI type FOO.  The visited
+ * QObject must match FOO.  QDict matches struct/union types, QList
+ * matches list types, QString matches type 'str' and enumeration
+ * types, QInt matches integer types, QFloat matches type 'number',
+ * QBool matches type 'bool'.  Type 'any' is matched by QObject.  A
+ * QAPI alternate type is matched when one of its member types is.
+ *
+ * visit_start_struct() ... visit_end_struct() visits a QDict and
+ * creates a QAPI struct/union.  Visits in between visit the
+ * dictionary members.  visit_optional() is true when the QDict has
+ * this member.  visit_check_struct() fails if unvisited members
+ * remain.
+ *
+ * visit_start_list() ... visit_end_list() visits a QList and creates
+ * a QAPI list.  Visits in between visit list members, one after the
+ * other.  visit_next_list() returns NULL when all QList members have
+ * been visited.  visit_check_list() fails if unvisited members
+ * remain.
+ *
+ * visit_start_alternate() ... visit_end_alternate() visits a QObject
+ * and creates a QAPI alternate.  The visit in between visits the same
+ * QObject and initializes the alternate member that is in use.
+ *
+ * Error messages refer to parts of @obj in JavaScript/Python syntax.
+ * For example, 'a.b[2]' refers to the second member of the QList
+ * member 'b' of the QDict member 'a' of QDict @obj.
+ *
+ * The caller is responsible for freeing the visitor with
+ * visit_free().
  */
 Visitor *qobject_input_visitor_new(QObject *obj);
 

qemu/include/qapi/qobject-output-visitor.h

@@ -19,11 +19,38 @@
 
 typedef struct QObjectOutputVisitor QObjectOutputVisitor;
 
-/*
+/**
- * Create a new QMP output visitor.
+ * Create a QObject output visitor for @obj
  *
- * If everything else succeeds, pass @result to visit_complete() to
+ * A QObject output visitor visit builds a QObject from QAPI Object.
- * collect the result of the visit.
+ * This simultaneously walks the QAPI object and the QObject being
+ * built.  The latter walk starts at @obj.
+ *
+ * visit_type_FOO() creates a QObject for QAPI type FOO.  It creates a
+ * QDict for struct/union types, a QList for list types, QString for
+ * type 'str' and enumeration types, QInt for integer types, QFloat
+ * for type 'number', QBool for type 'bool'.  For type 'any', it
+ * increments the QObject's reference count.  For QAPI alternate
+ * types, it creates the QObject for the member that is in use.
+ *
+ * visit_start_struct() ... visit_end_struct() visits a QAPI
+ * struct/union and creates a QDict.  Visits in between visit the
+ * members.  visit_optional() is true when the struct/union has this
+ * member.  visit_check_struct() does nothing.
+ *
+ * visit_start_list() ... visit_end_list() visits a QAPI list and
+ * creates a QList.  Visits in between visit list members, one after
+ * the other.  visit_next_list() returns NULL when all QAPI list
+ * members have been visited.  visit_check_list() does nothing.
+ *
+ * visit_start_alternate() ... visit_end_alternate() visits a QAPI
+ * alternate.  The visit in between creates the QObject for the
+ * alternate member that is in use.
+ *
+ * Errors are not expected to happen.
+ *
+ * The caller is responsible for freeing the visitor with
+ * visit_free().
  */
 Visitor *qobject_output_visitor_new(QObject **result);