memory-device: document MemoryDeviceClass

Document the functions. Don't document get_region_size(), as we will be
dropping/replacing that one soon.

Use same documentation style as in include/exec/memory.h, but don't
document the parameters, as they are self-explanatory.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20181005092024.14344-9-david@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

include/hw/mem/memory-device.h

@@ -29,12 +29,48 @@ typedef struct MemoryDeviceState {
     Object parent_obj;
 } MemoryDeviceState;
 
+/**
+ * MemoryDeviceClass:
+ *
+ * All memory devices need to implement TYPE_MEMORY_DEVICE as an interface.
+ *
+ * A memory device is a device that owns a memory region which is
+ * mapped into guest physical address space at a certain address. The
+ * address in guest physical memory can either be specified explicitly
+ * or get assigned automatically.
+ */
 typedef struct MemoryDeviceClass {
+    /* private */
     InterfaceClass parent_class;
 
+    /*
+     * Return the address of the memory device in guest physical memory.
+     *
+     * Called when (un)plugging a memory device or when iterating over
+     * all memory devices mapped into guest physical address space.
+     *
+     * If "0" is returned, no address has been specified by the user and
+     * no address has been assigned to this memory device yet.
+     */
     uint64_t (*get_addr)(const MemoryDeviceState *md);
+
+    /*
+     * Return the amount of memory provided by the memory device currently
+     * usable ("plugged") by the VM.
+     *
+     * Called when calculating the total amount of ram available to the
+     * VM (e.g. to report memory stats to the user).
+     *
+     * This is helpful for devices that dynamically manage the amount of
+     * memory accessible by the guest via the reserved memory region. For
+     * most devices, this corresponds to the size of the memory region.
+     */
     uint64_t (*get_plugged_size)(const MemoryDeviceState *md, Error **errp);
     uint64_t (*get_region_size)(const MemoryDeviceState *md, Error **errp);
+
+    /*
+     * Translate the memory device into #MemoryDeviceInfo.
+     */
     void (*fill_device_info)(const MemoryDeviceState *md,
                              MemoryDeviceInfo *info);
 } MemoryDeviceClass;