sfc: document TC-to-EF100-MAE action translation concepts

Includes an explanation of the lifetime of the 'cursor' action-set `act`.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/ethernet/sfc/tc.c b/drivers/net/ethernet/sfc/tc.c
index 2b07bb2..34c1ff8 100644 (file)
--- a/drivers/net/ethernet/sfc/tc.c
+++ b/drivers/net/ethernet/sfc/tc.c
@@ -66,7 +66,7 @@ static const struct rhashtable_params efx_tc_match_action_ht_params = {
 static void efx_tc_free_action_set(struct efx_nic *efx,
                                   struct efx_tc_action_set *act, bool in_hw)
 {
-       /* Failure paths calling this on the 'running action' set in_hw=false,
+       /* Failure paths calling this on the 'cursor' action set in_hw=false,
         * because if the alloc had succeeded we'd've put it in acts.list and
         * not still have it in act.
         */
@@ -407,6 +407,30 @@ static int efx_tc_flower_replace(struct efx_nic *efx,
                goto release;
        }
 
+       /**
+        * DOC: TC action translation
+        *
+        * Actions in TC are sequential and cumulative, with delivery actions
+        * potentially anywhere in the order.  The EF100 MAE, however, takes
+        * an 'action set list' consisting of 'action sets', each of which is
+        * applied to the _original_ packet, and consists of a set of optional
+        * actions in a fixed order with delivery at the end.
+        * To translate between these two models, we maintain a 'cursor', @act,
+        * which describes the cumulative effect of all the packet-mutating
+        * actions encountered so far; on handling a delivery (mirred or drop)
+        * action, once the action-set has been inserted into hardware, we
+        * append @act to the action-set list (@rule->acts); if this is a pipe
+        * action (mirred mirror) we then allocate a new @act with a copy of
+        * the cursor state _before_ the delivery action, otherwise we set @act
+        * to %NULL.
+        * This ensures that every allocated action-set is either attached to
+        * @rule->acts or pointed to by @act (and never both), and that only
+        * those action-sets in @rule->acts exist in hardware.  Consequently,
+        * in the failure path, @act only needs to be freed in memory, whereas
+        * for @rule->acts we remove each action-set from hardware before
+        * freeing it (efx_tc_free_action_set_list()), even if the action-set
+        * list itself is not in hardware.
+        */
        flow_action_for_each(i, fa, &fr->action) {
                struct efx_tc_action_set save;
                u16 tci;