netdev - Re: CLOCK_MONOTONIC datagram timestamps by the kernel

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Date:	Wed, 28 Feb 2007 10:22:31 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	John <linux.kernel@...e.fr>
Cc:	linux-net@...r.kernel.org, netdev@...r.kernel.org,
	linux.kernel@...e.fr
Subject: Re: CLOCK_MONOTONIC datagram timestamps by the kernel

On Wed, 28 Feb 2007 14:37:49 +0100
John <linux.kernel@...e.fr> wrote:

> John wrote:
> 
> > I know it's possible to have Linux timestamp incoming datagrams as soon
> > as they are received, then for one to retrieve this timestamp later with
> > an ioctl command or a recvmsg call.
> 
> Has it ever been proposed to modify struct skb_timeval to hold 
> nanosecond stamps instead of just microsecond stamps? Then make the 
> improved precision somehow available to user space.
> 

I am playing with a couple of possible future changes.
1. Change skb timestamp to be a timespec instead of timeval, for ABI
   compatiablity the existing SO_TIMESTAMP has to stay microseconds,
   but add a new SO_TIMESPEC to get the nanosecond version.

   The change gets non trivial because of other uses of timestamp (like vegas)
   so I gave up for now.

2. Use hardware receive timestamp in Yukon2 to put actual receive time
   into skb timestamp.  Works, but still figuring out how to manage
   clock skew/resync.
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