netdev - Re: TX time stamping

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Message-ID: <20090326144835.GA19592@gondor.apana.org.au>
Date:	Thu, 26 Mar 2009 22:48:35 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Patrick Ohly <patrick.ohly@...el.com>
Cc:	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
Subject: Re: TX time stamping

On Wed, Mar 25, 2009 at 08:08:44AM +0100, Patrick Ohly wrote:
>
> I suggest to make it so that the sender gets the packet back once per
> interface, with different time stamps and information about the
> interface.

That could be awkward.  What if the second or subsequent instance
of the packet is held up indefinitely?

> > As to the problem of skb->sk, I don't think that's even needed
> > as we can simply use the skb shared area as the communication
> > medium.
> 
> Can you elaborate on that?

I was thinking of the sender keeping hold of the sent skb and
using its shared area to retrieve the time stamp(s).

> The drawback is the more costly socket lookup. For PTP this isn't an
> issue due to the low packet rate, so a very simple solution would be
> good enough. But for other use cases it might be problematic. I also
> have no idea how the locking for the socket lookup can be done safely.

Could you give the main usage scenarios for timestamps, including
what the user-space app does with the info?

Thanks,
-- 
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