[PATCH net-next v4 6/8] gve: Add rx hardware timestamp expansion

[Harshitha Ramamurthy <hramamurthy@google.com>]

From: John Fraker <jfraker@google.com>

Allow the rx path to recover the high 32 bits of the full 64 bit rx
timestamp.

Use the low 32 bits of the last synced nic time and the 32 bits of the
timestamp provided in the rx descriptor to generate a difference, which
is then applied to the last synced nic time to reconstruct the complete
64-bit timestamp.

This scheme remains accurate as long as no more than ~2 seconds have
passed between the last read of the nic clock and the timestamping
application of the received packet.

Signed-off-by: John Fraker <jfraker@google.com>
Signed-off-by: Ziwei Xiao <ziweixiao@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
---
 Changes in v3:
 - Change the last_read to be u64 (Vadim Fedorenko)

 Changes in v2:
 - Add the missing READ_ONCE (Joe Damato)
---
 drivers/net/ethernet/google/gve/gve_rx_dqo.c | 23 ++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/drivers/net/ethernet/google/gve/gve_rx_dqo.c b/drivers/net/ethernet/google/gve/gve_rx_dqo.c
index dcb0545baa50..9aadf8435f8b 100644
--- a/drivers/net/ethernet/google/gve/gve_rx_dqo.c
+++ b/drivers/net/ethernet/google/gve/gve_rx_dqo.c
@@ -437,6 +437,29 @@ static void gve_rx_skb_hash(struct sk_buff *skb,
 	skb_set_hash(skb, le32_to_cpu(compl_desc->hash), hash_type);
 }
 
+/* Expand the hardware timestamp to the full 64 bits of width, and add it to the
+ * skb.
+ *
+ * This algorithm works by using the passed hardware timestamp to generate a
+ * diff relative to the last read of the nic clock. This diff can be positive or
+ * negative, as it is possible that we have read the clock more recently than
+ * the hardware has received this packet. To detect this, we use the high bit of
+ * the diff, and assume that the read is more recent if the high bit is set. In
+ * this case we invert the process.
+ *
+ * Note that this means if the time delta between packet reception and the last
+ * clock read is greater than ~2 seconds, this will provide invalid results.
+ */
+static void __maybe_unused gve_rx_skb_hwtstamp(struct gve_rx_ring *rx, u32 hwts)
+{
+	u64 last_read = READ_ONCE(rx->gve->last_sync_nic_counter);
+	struct sk_buff *skb = rx->ctx.skb_head;
+	u32 low = (u32)last_read;
+	s32 diff = hwts - low;
+
+	skb_hwtstamps(skb)->hwtstamp = ns_to_ktime(last_read + diff);
+}
+
 static void gve_rx_free_skb(struct napi_struct *napi, struct gve_rx_ring *rx)
 {
 	if (!rx->ctx.skb_head)
-- 
2.50.0.rc0.604.gd4ff7b7c86-goog