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

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Message-ID:
 <PAXPR04MB9185AAF36A7FB42C4D00CBDE893CA@PAXPR04MB9185.eurprd04.prod.outlook.com>
Date: Sat, 23 Aug 2025 19:53:37 +0000
From: Shenwei Wang <shenwei.wang@....com>
To: Andrew Lunn <andrew@...n.ch>
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



> -----Original Message-----
> From: Andrew Lunn <andrew@...n.ch>
> Sent: Saturday, August 23, 2025 2:26 PM
> 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; netdev@...r.kernel.org; linux-kernel@...r.kernel.org; dl-
> linux-imx <linux-imx@....com>
> Subject: [EXT] 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. 

> 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.

Thanks,
Shenwei

>         Andrew