netdev - Re: TX timestamping for ICMP (sfc and core)?

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Message-ID: <1394384857.2861.159.camel@deadeye.wl.decadent.org.uk>
Date:	Sun, 09 Mar 2014 17:07:37 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Network Development <netdev@...r.kernel.org>,
	Solarflare linux maintainers <linux-net-drivers@...arflare.com>
Subject: Re: TX timestamping for ICMP (sfc and core)?

On Tue, 2014-03-04 at 12:43 -0800, Andy Lutomirski wrote:
> I'm trying to get tx timestamping working on ping sockets.  My program
> works fine (with hardware TX timestamps) if I send my "pings" on the
> PTP UDP ports.  I want to send real ICMP pings, though.
> 
> It looks like there are two problems:
> 
>  - Something in the core code seems to eat SKBTX_HW_STAMP on ping
> sockets.  At least, this code in efx_hard_start_xmit triggers for
> IPPROTO_UDP on PTP ports but not for IPPROTO_ICMP:
> 
>         if (unlikely(efx_xmit_with_hwtstamp(skb)))
>                 printk(KERN_DEBUG "sfc: trying to do a TX timestamp\n");
>
>  - The sfc driver refuses to timestamp non-PTP packets.  There's a
> check in efx_hard_start_xmit and another in ptp.c.  I suspect that the
> hardware can support timestamping any outgoing packet, but I'm not
> really sure.  (I'm using an SFN7322F.)

I think that the SFC9100 family can do TX timestamping for arbitrary
packets, but they still have to be sent via the MC and that will greatly
limit the packet rate.

(It may or may not be possible on the SFN[56]322F, as the addition of
timestamping in a peripheral to the SFC9020 is quite a delicate
arrangement.  The MCDI transport there also limits TX timestamped
packets to 252 bytes.)

If you want to send more than a very low rate of packets with TX
timestamping, the driver should probably tell the kernel to allocate an
extra TX queue for them and should implement ndo_select_queue.

Ben.

-- 
Ben Hutchings
I say we take off; nuke the site from orbit.  It's the only way to be sure.

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