RE: [net-next 06/12] ixgbe: Hardware Timestamping + PTP Hardware Clock (PHC)

-----Original Message-----
From: Richard Cochran [mailto:richardcochran@gmail.com]
Sent: Thursday, May 10, 2012 7:11 AM
To: Kirsher, Jeffrey T
Cc: davem@davemloft.net; Keller, Jacob E; netdev@vger.kernel.org;
gospo@redhat.com; sassmann@redhat.com
Subject: Re: [net-next 06/12] ixgbe: Hardware Timestamping + PTP Hardware
Clock (PHC)

Mostly, this looks very good. I do have one concern and a nit, though.

On Wed, May 09, 2012 at 11:46:47PM -0700, Jeff Kirsher wrote:

quoted

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index 1693ec3..9a83c40 100644

--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -789,6 +789,13 @@ static bool ixgbe_clean_tx_irq(struct ixgbe_q_vector

*q_vector,

quoted

 		total_bytes += tx_buffer->bytecount;
 		total_packets += tx_buffer->gso_segs;

+#ifdef CONFIG_IXGBE_PTP
+		if (unlikely(tx_buffer->tx_flags &
+			     IXGBE_TX_FLAGS_TSTAMP))
+			ixgbe_ptp_tx_hwtstamp(q_vector,
+					      tx_buffer->skb);

This looks strangely wrapped.

quoted

+
+#endif
 		/* free the skb */
 		dev_kfree_skb_any(tx_buffer->skb);

...

quoted

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c

b/drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c
new file mode 100644
index 0000000..0b6553e

--- /dev/null
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c

...

quoted

+/**
+ * ixgbe_ptp_rx_hwtstamp - utility function which checks for RX time
+stamp
+ * @q_vector: structure containing interrupt and ring information
+ * @skb: particular skb to send timestamp with
+ *
+ * if the timestamp is valid, we convert it into the timecounter ns
+ * value, then store that result into the shhwtstamps structure which
+ * is passed up the network stack
+ */
+void ixgbe_ptp_rx_hwtstamp(struct ixgbe_q_vector *q_vector,
+			   struct sk_buff *skb)
+{
+	struct ixgbe_adapter *adapter;
+	struct ixgbe_hw *hw;
+	struct skb_shared_hwtstamps *shhwtstamps;
+	u64 regval = 0, ns;
+	u32 tsyncrxctl;
+	unsigned long flags;
+
+	/* we cannot process timestamps on a ring without a q_vector */
+	if (!q_vector || !q_vector->adapter)
+		return;
+
+	adapter = q_vector->adapter;
+	hw = &adapter->hw;
+
+	tsyncrxctl = IXGBE_READ_REG(hw, IXGBE_TSYNCRXCTL);
+	regval |= (u64)IXGBE_READ_REG(hw, IXGBE_RXSTMPL);
+	regval |= (u64)IXGBE_READ_REG(hw, IXGBE_RXSTMPH) << 32;
+
+	/*
+	 * If this bit is set, then the RX registers contain the time stamp. No
+	 * other packet will be time stamped until we read these registers, so
+	 * read the registers to make them available again. Because only one
+	 * packet can be time stamped at a time, we know that the register
+	 * values must belong to this one here and therefore we don't need to
+	 * compare any of the additional attributes stored for it.

I suspect that this assumption is wrong. What happens if the time stamping
logic locks a value but the packet is lost because the ring is full?

BTW, the IGB driver also has this defect.

Note how I read the rx registers first? So it will always clear the value.
That should unlock the value for the next rx stamp packet.
 

Thanks,
Richard