Re: [Linuxptp-users] One-step sync and P2P mode issue

[Sriharsha B. <Sri...@Em...>]

-----Original Message-----
From: Keller, Jacob E [mailto:jac...@in...]
Sent: Wednesday, September 03, 2014 5:59 AM
To: Sriharsha Basavapatna; Richard Cochran
Cc: lin...@li...
Subject: RE: [Linuxptp-users] One-step sync and P2P mode issue



Hi,



> -----Original Message-----

> From: Sriharsha Basavapatna

> [mailto:Sri...@Em...]

> Sent: Tuesday, September 02, 2014 3:00 AM

> To: Richard Cochran

> Cc: Keller, Jacob E; lin...@li...<mailto:lin...@li...>

> Subject: RE: [Linuxptp-users] One-step sync and P2P mode issue

>

> Thanks for the pointer to the flag. But this is still an issue for the

> driver, since it'd have to parse the packet to find that it's a Sync

> pkt; please correct me if I'm missing something here.

>

> The driver saves this flag when it gets the ioctl request. Then at

> transmit completion time the driver would still have to check that the

> flag HWTSTAMP_TX_ONESTEP_SYNC has been set and it should not queue the

> packet in the error queue if it's a Sync pkt. For other packets say

> Delay Request it should still continue to queue the pkt in error

> queue.

>

> For example, here's the pseudo code. Before checking the msg type

> below, the driver would have to parse the packet to find that it's a

> PTP Sync msg(in a raw Ethernet frame or UDP/IPv4/IPV6 pkt).

>



I do not know if one-step mode is specific to SYNC packets or should handle any transmit timestamps. Richard, do you know?



>> I believe it is specific to SYNC packets only in this context; at least

>> w.r.t the flag HWTSTAMP_TX_ONESTEP_SYNC.



You may have to check these. At any rate, I believe we already implement packet filters which will do this for you very efficiently.



>> It'd be better if we can avoid this condition itself, which requires packet

>> filtering.



In the one-step case, you will configure hardware to directly insert the timestamp into the packet buffer as it transmits it



>> HW inserts the timestamp (see my earlier email with details). And provides a

>> copy of timestamp to driver. The driver has been asked by the stack to give

>> a copy of the transmitted packet via the error queue, as clearly indicated

>> by skb->tx_flags: SKBTX_HW_TSTAMP. But the driver has to make an exception

>> just for the 1-step Sync packet and to know that it's a 1-step Sync, it has

>> to parse the packet.



Thanks,

-Harsha



In the other case, you configure hardware to notify driver of the timestamp, and then return it via skb_tstamp_tx



I do actually believe that one-step works for any PTP transmitted timestamp, and if you enable TX_ONESTEP_SYNC that you actually have to do this for all packets, but I honestly cannot remember.



Regards,

Jake



>         if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP &&

>             (ptp_msg->type != MSG_SYNC ||

>             (dev-inst->ptp_flags & HWTSTAMP_TX_ONESTEP_SYNC) == 0)) {

>

>                 /* add skb to error queue */

>

>         }

>

> Thanks,

> -Harsha