net: dsa: sja1105: bring deferred xmit implementation in line with ocelot-8021q

When the ocelot-8021q driver was converted to deferred xmit as part of
commit 8d5f7954b7c8 ("net: dsa: felix: break at first CPU port during
init and teardown"), the deferred implementation was deliberately made
subtly different from what sja1105 has.

The implementation differences lied on the following observations:

- There might be a race between these two lines in tag_sja1105.c:

       skb_queue_tail(&sp->xmit_queue, skb_get(skb));
       kthread_queue_work(sp->xmit_worker, &sp->xmit_work);

  and the skb dequeue logic in sja1105_port_deferred_xmit(). For
  example, the xmit_work might be already queued, however the work item
  has just finished walking through the skb queue. Because we don't
  check the return code from kthread_queue_work, we don't do anything if
  the work item is already queued.

  However, nobody will take that skb and send it, at least until the
  next timestampable skb is sent. This creates additional (and
  avoidable) TX timestamping latency.

  To close that race, what the ocelot-8021q driver does is it doesn't
  keep a single work item per port, and a skb timestamping queue, but
  rather dynamically allocates a work item per packet.

- It is also unnecessary to have more than one kthread that does the
  work. So delete the per-port kthread allocations and replace them with
  a single kthread which is global to the switch.

This change brings the two implementations in line by applying those
observations to the sja1105 driver as well.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 3 deletions

diff --git a/include/linux/dsa/sja1105.h b/include/linux/dsa/sja1105.h
index e6c78be40bde..acd9d2afccab 100644
--- a/include/linux/dsa/sja1105.h
+++ b/include/linux/dsa/sja1105.h
@@ -37,6 +37,12 @@
 
 #define SJA1105_HWTS_RX_EN			0
 
+struct sja1105_deferred_xmit_work {
+	struct dsa_port *dp;
+	struct sk_buff *skb;
+	struct kthread_work work;
+};
+
 /* Global tagger data: each struct sja1105_port has a reference to
  * the structure defined in struct sja1105_private.
  */
@@ -52,6 +58,8 @@ struct sja1105_tagger_data {
 	 * 2-step TX timestamps
 	 */
 	struct sk_buff_head skb_txtstamp_queue;
+	struct kthread_worker *xmit_worker;
+	void (*xmit_work_fn)(struct kthread_work *work);
 };
 
 struct sja1105_skb_cb {
@@ -65,9 +73,6 @@ struct sja1105_skb_cb {
 	((struct sja1105_skb_cb *)((skb)->cb))
 
 struct sja1105_port {
-	struct kthread_worker *xmit_worker;
-	struct kthread_work xmit_work;
-	struct sk_buff_head xmit_queue;
 	struct sja1105_tagger_data *data;
 	bool hwts_tx_en;
 };