netdev - Re: Problems obtaining software TX timestamps

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Date:	Thu, 9 Sep 2010 08:35:41 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Ingo Kofler <ikofler@....uni-klu.ac.at>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: Problems obtaining software TX timestamps

On Wed, Sep 08, 2010 at 11:21:36PM +0200, Ingo Kofler wrote:
> Ok, for hardware timestamps it seems logically for me that the
> driver has to support this. But what about the software timestamps -
> do they also require support from the driver? Is there no generic
> implementation in the networking stack?

There was once a completely generic implementation, but the Tx path
had issues. The generic Tx SW code was reverted in

cd4d8fdad1f13205c769266dfa99015e226b6e07

Now, there is a way to support this again, but it requires adding one
line into each and every MAC driver. It is not very hard to do:

linux/skbuff.h:1992
/**
 * skb_tx_timestamp() - Driver hook for transmit timestamping
 *
 * Ethernet MAC Drivers should call this function in their hard_xmit()
 * function as soon as possible after giving the sk_buff to the MAC
 * hardware, but before freeing the sk_buff.
 *
 * @skb: A socket buffer.
 */
static inline void skb_tx_timestamp(struct sk_buff *skb);

I am already carrying patches for this privately. If you post a patch
for your MAC, then you can be the first one to support this feature! ;^)

Richard

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