netdev - trying to set skb->tstamp before netif_rx did not work in tun/tap driver, 2.6.27

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Message-ID: <loom.20110708T042440-631@post.gmane.org>
Date:	Fri, 8 Jul 2011 02:33:59 +0000 (UTC)
From:	Bill F <falcigno@...o.com>
To:	netdev@...r.kernel.org
Subject: trying to set skb->tstamp before netif_rx did not work in tun/tap driver, 2.6.27

Hi all,

I am trying to set the receive timestamp for a packet in the tun/tap device
driver before sending it into the network stack (by calling netif_rx_ni). On the 
application side I'm using recvmsg() to receive the timestamp along with the 
received packet.

I expected that any non-zero skb->tstamp I set would be the value my 
application would see when it used recvmsg(), but a corrupted value is 
coming out.
* If I hardcode skb->tstamp.tv64 to x7777777766666666 
        then recvmsg() always gets x011afd371a82fcc0.
* If I hardcode x4de684b100000000, 
        then recvmsg always gets x4f5f96640f2d1608.
* If I set a timestamp that a different process gave to the tun/tap driver
        (this is the goal), then recvmsg gets a corrupted version of that 
        timestamp.
* If I hardcode a zero tstamp, 
        then an accurate timestamp comes out (but not precise enough for my 
        project).

The kernel build is 2.6.27.7, the architecture is MIPS, 64bit. Shouldn't this 
work? Do I need to apply any known patches?

This arrangement worked last month in a similar setup: an e1000 driver 
setting tstamp before calling netif_receive_skb() in a 2.6.24 kernel and a 
686 architecture, 32 bit.

Thanks in advance.
Bill

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