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Date:      Thu, 27 Feb 2003 16:03:09 -0300
From:      "Daniel C. Sobral" <dcs@tcoip.com.br>
To:        Bill Paul <wpaul@FreeBSD.ORG>
Cc:        jlemon@FreeBSD.ORG, ru@FreeBSD.ORG, src-committers@FreeBSD.ORG, cvs-src@FreeBSD.ORG, cvs-all@FreeBSD.ORG
Subject:   Re: cvs commit: src/sys/dev/fxp if_fxp.c if_fxpreg.h if_fxpvar.h
Message-ID:  <3E5E60ED.70406@tcoip.com.br>
In-Reply-To: <20030227183346.657C937B401@hub.freebsd.org>
References:  <20030227183346.657C937B401@hub.freebsd.org>

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Bill Paul wrote:

> Er... look. The checksum offload capabilities are turned on by default,
> if your chip supports them, which it appears it does. Also, please try
> and remember just how bent I can get at someone who says "it doesn't
> work" without defining in precise detail just what "it" is and what
> "doesn't work" implies.
> 
> Do you mean that using ifconfig to twiddle the capabilities doesn't
> seem to make any difference?
> Do you mean that the cards aren't working with the updated driver (i.e.
> you can't talk to the network)?
> Do you mean they work but you don't notice any significant performance
> difference?
> Do you mean the machine halts and catches fire?

Well, whenever I use that txcsum/rxcsum thingy, the machine acquires a 
kind of life-like appearance, and sends some strange pulses through the 
TP cables and start gobbling all other switches it is connected to in a 
similar fashion, in a manner very much like the BOOMERs of BubbleGum 
Crisis and AD Police. I'm still wondering if that's an advanced feature 
added by the .jp crew or not, though. :-)


What I mean by "doesn't work" was what I cut&pasted. If the messages 
didn't show it, sorry, I must have made a mistake when preparing them.


I asked how I could see if the thing was in use or not. Someone told me 
to look in the "options" field displayed by ifconfig. Well:

root@piratinga:/root# ifconfig fxp2 | grep options
         options=8<VLAN_MTU>

dcs@pajeus:/home/dcs$ ifconfig xl0 | grep options
         options=3<RXCSUM,TXCSUM>

 From where I concluded the option was not active. So I asked how I 
activated it, and was told to use the undocumented options rxcsum and 
txcsum. I did that and:

root@piratinga:/root# ifconfig fxp2 rxcsum txcsum
root@piratinga:/root# ifconfig fxp2 | grep options
         options=8<VLAN_MTU>

Nothing happened wrt to what ifconfig displayed.

Now, someone, maybe on irc, also told me -m would show the available 
options. The result of this was:

root@piratinga:/root# ifconfig -m fxp2
fxp2: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
         options=8<VLAN_MTU>
         capability list:
                 =b<RXCSUM,TXCSUM,VLAN_MTU>
[...]

So, -m in fact shows rxcsum and txcsum being supported capabilities, but 
they are never listed in options, no matter what I do.

> Like I said, you don't have to do anything to use the checksum offload
> capabilities: they're just on, and that's that. In terms of peformance,
> the fact that the chip doesn't do checksums over fragmented packets
> means that the biggest improvement will be with packets that fit into
> a single frame.

I suspect not even that, as all traffic going through these interfaces 
is 802.1q, and you mentioned IP checksum not being on for now.


So, ignoring all that, my questions are:

1) How do I know if an interface has actually been identified as 
supporting the feature?

2) How do I know if the feature is in use?

3) If it can be turned on/off, how do I do it?

4) If the answer to any of these questions is the stuff I did above, why 
didn't it result in, well, the expected results?

-- 
Daniel C. Sobral                   (8-DCS)
Gerencia de Operacoes
Divisao de Comunicacao de Dados
Coordenacao de Seguranca
TCO
Fones: 55-61-313-7654/Cel: 55-61-9618-0904
E-mail: Daniel.Capo@tco.net.br
         Daniel.Sobral@tcoip.com.br
         dcs@tcoip.com.br

Outros:
	dcs@newsguy.com
	dcs@freebsd.org
	capo@notorious.bsdconspiracy.net

"Don't tell me I'm burning the candle at both ends -- tell me where to
get more wax!!"


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