netdev - Re: [rfc] new sk_buff member: hwstamp

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Date:	Mon, 14 Jul 2008 21:30:12 +0300
From:	Octavian Purdila <opurdila@...acom.com>
To:	Oliver Hartkopp <oliver@...tkopp.net>
Cc:	Andi Kleen <andi@...stfloor.org>, netdev@...r.kernel.org
Subject: Re: [rfc] new sk_buff member: hwstamp

>
> However i feel, that *one* nanosec resolution timestamp (as it already
> exists inside the skbuff) is enough. AFAIK the timestamp is only set in
> the netif_rx(), when it is not already set by the driver itself. For
> that reason i would suggest to create some semi-intelligent offset
> calculation inside the driver that makes the skb->tstamp value
> correspond with the hw timestamp and therefore transports the high
> resolution timestamp requirement into the userspace.
>

You mean something like here [1] ?

The problem for this approach in our usecase (one way delay measurement) is 
that the real time clock of the CPUs (of the RX and TX ports) is not 
synchronized, but the hw timestamp (implemented in the NIC/FPGA) is. 

[1] 
http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.23-rc4/2.6.23-rc4-mm1/broken-out/sky2-hardware-receive-timestamp-counter.patch

Thanks,
tavi

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