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

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Date:	Sat, 12 May 2012 07:34:59 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	"Keller, Jacob E" <jacob.e.keller@...el.com>
Cc:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"gospo@...hat.com" <gospo@...hat.com>,
	"sassmann@...hat.com" <sassmann@...hat.com>
Subject: Re: [net-next 06/12] ixgbe: Hardware Timestamping + PTP Hardware
 Clock (PHC)

On Fri, May 11, 2012 at 07:23:44PM +0000, Keller, Jacob E wrote:
> 
>
> I believe this very rare case might be possible, but I don't think
> that checking the ptp seqid will fix anything. In normal cases,
> hardware latches Rx packet timestamp, then the ptp packet goes into
> the queue and we process it shortly after. Before we process that
> packet there will never be another packet in the queue that needs a
> timestamp. We know this because the hardware stops timestamping
> until we unlatch the RX registers. This should mean we don't need to
> check the sequence ID, and spending time doing it would never fix
> the issue you are talking about.
>
> The issue is for when a packet is timestamped and then never reaches
> the queue. Then the rx stamp registers are locked for good, because
> we never clear them, and hardware would never timestamp another
> receive packet. I don't know a good solution to this, except to
> clear the registers periodically. Do you have any suggestions?

Well, one solution would be to check every received packet with the
BPF in ptp_classify.h (whenever Rx time stamping is enabled).

When the driver finds an event packet in the Rx queue, and
TSYNCRXCTL[RXTT] is set, it reads out the time stamp along with
RXSATRL/H. If the fields match, then add the time stamp to the skb.

[ Or perhaps instead of using RXSATRL/H, just use the descriptor bit.
  If *not* set, then the time stamp does not belong to this packet. ]

HTH,
Richard
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