switchdev: update documentation on FDB ageing_time

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/switchdev.txt

@@ -239,20 +239,20 @@ The driver should initialize the attributes to the hardware defaults.
 FDB Ageing
 ^^^^^^^^^^
 
-There are two FDB ageing models supported: 1) ageing by the device, and 2)
-ageing by the kernel.  Ageing by the device is preferred if many FDB entries
-are supported.  The driver calls call_switchdev_notifiers(SWITCHDEV_FDB_DEL,
-...) to age out the FDB entry.  In this model, ageing by the kernel should be
-turned off.  XXX: how to turn off ageing in kernel on a per-port basis or
-otherwise prevent the kernel from ageing out the FDB entry?
-
-In the kernel ageing model, the standard bridge ageing mechanism is used to age
-out stale FDB entries.  To keep an FDB entry "alive", the driver should refresh
-the FDB entry by calling call_switchdev_notifiers(SWITCHDEV_FDB_ADD, ...).  The
+The bridge will skip ageing FDB entries marked with NTF_EXT_LEARNED and it is
+the responsibility of the port driver/device to age out these entries.  If the
+port device supports ageing, when the FDB entry expires, it will notify the
+driver which in turn will notify the bridge with SWITCHDEV_FDB_DEL.  If the
+device does not support ageing, the driver can simulate ageing using a
+garbage collection timer to monitor FBD entries.  Expired entries will be
+notified to the bridge using SWITCHDEV_FDB_DEL.  See rocker driver for
+example of driver running ageing timer.
+
+To keep an NTF_EXT_LEARNED entry "alive", the driver should refresh the FDB
+entry by calling call_switchdev_notifiers(SWITCHDEV_FDB_ADD, ...).  The
 notification will reset the FDB entry's last-used time to now.  The driver
 should rate limit refresh notifications, for example, no more than once a
-second.  If the FDB entry expires, fdb_delete is called to remove entry from
-the device.
+second.  (The last-used time is visible using the bridge -s fdb option).
 
 STP State Change on Port
 ^^^^^^^^^^^^^^^^^^^^^^^^