linux-kernel - Re: [PATCH v3 net-next 5/5] net: fec: enable the Jumbo frame support for i.MX8QM

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Message-ID: <6c40ae63-fc9a-4738-9091-fa459771591d@lunn.ch>
Date: Sat, 23 Aug 2025 22:44:48 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Shenwei Wang <shenwei.wang@....com>
Cc: Wei Fang <wei.fang@....com>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Clark Wang <xiaoning.wang@....com>,
	Stanislav Fomichev <sdf@...ichev.me>,
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH v3 net-next 5/5] net: fec: enable the Jumbo frame support
 for i.MX8QM

> > > -             /* enable ENET store and forward mode */
> > > -             writel(FEC_TXWMRK_STRFWD, fep->hwp + FEC_X_WMRK);
> > > +
> > > +             /* When Jumbo Frame is enabled, the FIFO may not be large enough
> > > +              * to hold an entire frame. In this case, configure the interface
> > > +              * to operate in cut-through mode, triggered by the FIFO threshold.
> > > +              * Otherwise, enable the ENET store-and-forward mode.
> > > +              */
> > > +             if (fep->quirks & FEC_QUIRK_JUMBO_FRAME)
> > > +                     writel(0xF, fep->hwp + FEC_X_WMRK);
> > 
> > The quirk indicates the hardware is capable of jumbo frames, not that jumbo
> > frames are enabled. Don't you need to compare the mtu with ETH_FRAME_LEN +
> > ETH_FCS_LEN to say jumbo is enabled?
> > 
> 
> The comments here do have some confusion. The goal is to enable cut-through mode 
> when the hardware supports Jumbo frames.  But we can limit the scope to enable it
> only when MTU is less than 2k bytes. 

That would be good. I've no idea how many embedded systems actually
use jumbo packets. Maybe your marketing people can tell you. 1%? 5%?
But you are changing this for 100% of your customers, which might
cause regressions. So it would be better to only enable this when
needed.

> > Is there a counter or other indication that the FIFO experienced an underflow?
> > 
> 
> There is a Underrun bit in the status field in the TX buffer descriptor. The hardware 
> supports retransmit frames if high memory latency is encountered due to other 
> high-priority bus masters.

So this:
                        if (status & BD_ENET_TX_UN)  /* Underrun */
                                ndev->stats.tx_fifo_errors++;

Good to see there is a counter for it.

	Andrew