netdev - Re: TX time stamping

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Date:	Wed, 25 Mar 2009 08:08:44 +0100
From:	Patrick Ohly <patrick.ohly@...el.com>
To:	David Miller <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
Subject: Re: TX time stamping

Hello Dave, Herbert!

On Fri, 2009-03-20 at 02:10 +0000, Herbert Xu wrote: 
> On Thu, Mar 19, 2009 at 02:05:09PM -0700, David Miller wrote:
> >
> > Instead, we have a half-working turd in a tree, and I'm removing it.
> 
> Yes, we need to spend a bit more time pondering the semantics
> of all this.

Agreed. It never was the goal to somehow force this into the kernel
unless you are happy with it - not that this would have worked
anyway ;-) I'd be happy to discuss better ways of solving these issues;
the current patches work, but they have their shortcomings. There's no
point in including them when you don't deem them sufficient.

As I said in my initial email in this thread, I don't know a better
solution and depend on some guidance by experts in this area. When I
didn't get a reply to that email I thought that the current solution had
been accepted, but clearly that wasn't the case. I have had a patch
ready for the TX software time stamping for a while (the third point of
my email), but that doesn't address the main reason why you are unhappy
about the patches.

> First of all, if a packet bifurcates and is transmitted through
> two interfaces both capable of timestamping, which event do we
> take as the timestamp of the original packet?

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.

> 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 still think we need to ensure that only the sender is told about the
send time stamp and the associated packet data. Is there perhaps a
unique integer ID for each socket, or is adding it acceptable (a running
count basically)?

In that case we could do something like this: 
      * when sending a packet with TX request, add the socket ID 
      * clear skb->sk 
      * after generating the TX time stamp, try to find the socket by
        ID 
              * if it is found, send packet back with additional info
                like it is done now
              * if not, discard information because the sender is gone 

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.

-- 
Best Regards, Patrick Ohly

The content of this message is my personal opinion only and although
I am an employee of Intel, the statements I make here in no way
represent Intel's position on the issue, nor am I authorized to speak
on behalf of Intel on this matter.

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