linux-kernel - Re: [net-next PATCH 2/2] net: ravb: Fix R-Car RX frame size limit

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Date: Mon, 17 Jun 2024 16:29:38 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Paul Barker <paul.barker.ct@...renesas.com>
Cc: Sergey Shtylyov <s.shtylyov@....ru>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Niklas Söderlund <niklas.soderlund+renesas@...natech.se>,
	Biju Das <biju.das.jz@...renesas.com>,
	Claudiu Beznea <claudiu.beznea.uj@...renesas.com>,
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
	Mitsuhiro Kimura <mitsuhiro.kimura.kc@...esas.com>,
	netdev@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [net-next PATCH 2/2] net: ravb: Fix R-Car RX frame size limit

On Mon, Jun 17, 2024 at 03:03:21PM +0100, Paul Barker wrote:
> On 16/06/2024 02:23, Andrew Lunn wrote:
> > On Sat, Jun 15, 2024 at 11:30:38AM +0100, Paul Barker wrote:
> >> The RX frame size limit should not be based on the current MTU setting.
> >> Instead it should be based on the hardware capabilities.
> > 
> > This is a bit odd. MTU is Maximum Transmission Unit, so clearly is
> > about Tx. MRU does not really exist. Does TCP allow for asymmetric
> > MTU/MRU? Does MTU discovery work correctly for this?
> > 
> > In general, it seems like drivers implement min(MTU, MRU) and nothing
> > more. Do you have a real use case for this asymmetry?
> > 
> >       Andrew
> 
> Hi Andrew,
> 
> This change is based on my understanding of MTU/MRU, on the specs of the
> RZ SoCs I'm working with (primarily RZ/G2L family, RZ/G3S and RZ/G2H)
> and on some testing. My goal here is just to make the capabilities of
> the hardware available to users.
> 
> For the RZ/G2L family and RZ/G3S, we can only support an MTU of up to
> 1500 bytes, but we can receive frames of up to (IIRC) 8192 bytes. I have
> tested sending jumbo frames to an RZ/G2L device using both iperf3 and
> ping and I see no errors.
> 
> * For iperf3 RX testing, the RZ/G2L is only responding with acks. These
>   are small regardless of the size of the received packets, so the
>   mis-match in MTU between the two hosts causes no issue.
> 
> * For ping testing, the RZ/G2L will give a fragmented response to the
>   ping packet which the other host can reassemble.
> 
> For the RZ/G2H, we support sending frames of up to 2047 bytes but we can
> receive frames of up to 4092 bytes. The driver will need a few more
> changes to handle reception of packets >2kB in size, but this is
> something we can do in the near future.
> 
> Is there any reason why we shouldn't support this? I am by no means an
> expert in the Linux networking internals so there may be things I'm
> missing.

I've no really experience what the Linux stack will do. I have seen a
number of devices which do not limit received packets to the MTU,
i.e. they are happy to receive bigger packets. And i don't think this
has caused an issue. But is it ever actually used? Do users setup
asymmetric jumbo systems? I've no idea.

One thing i suggest you test is you put this in the middle of two
systems which do support jumbo.

     4K       1.5K       4K
A <-----> B <-----> C <------> D.

Make your device B and C, setting the MTU as supported. But set A and
D with a jumbo MTU which your systems should be able to receive. Do IP
routing along the chain. And then do some iperf transfers. I would
test both IPv4 and IPv6, since MTU path discovery in IPv6 is used a
lot, and gateways are not supposed to fragment.

I'm assuming here your device does actually have two interfaces?  If
it is only ever an end system, that simplifies it a lot, and these
tests are not needed.

    Andrew