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

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Date:	Mon, 14 Jul 2008 20:58:00 +0200
From:	Oliver Hartkopp <oliver@...tkopp.net>
To:	Octavian Purdila <opurdila@...acom.com>
CC:	Andi Kleen <andi@...stfloor.org>, netdev@...r.kernel.org
Subject: Re: [rfc] new sk_buff member: hwstamp

Octavian Purdila wrote:
>> 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] ?
> [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
>
>   

Yes - exactly :-)

> 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. 
>   

Is it necessary to have syncronized clocks on the different CPUs or is
it feasible to calculate a per-cpu-clock offset in your driver, so that
you can build a complete timing overview of your setup?

Regards,
Oliver

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