netdev - Re: [PATCH v3 net-next 3/5] net: fec: add rx_frame

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Message-ID:
 <PAXPR04MB918577F27FD6521B23601219893CA@PAXPR04MB9185.eurprd04.prod.outlook.com>
Date: Sat, 23 Aug 2025 20:09:54 +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 3/5] net: fec: add rx_frame_size to support
 configurable RX length



> -----Original Message-----
> From: Andrew Lunn <andrew@...n.ch>
> Sent: Saturday, August 23, 2025 2:11 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 3/5] net: fec: add rx_frame_size to
> support configurable RX length
> On Sat, Aug 23, 2025 at 02:01:08PM -0500, Shenwei Wang wrote:
> > Add a new rx_frame_size member in the fec_enet_private structure to
> > decouple frame size configuration from max_buf_size. This allows more
> > precise control over RX frame length settings. It is particularly
> > useful for Jumbo frame support because the RX frame size may possible
> > larger than the allocated RX buffer.
> 
> Please could you extend that a little. What happens if the received frame is bigger
> than the buffer? Does the hardware fragment it over two buffers?
> 

The hardware doesn't have the capability to fragment received frames that exceed the MAX_FL 
value. Instead, it flags an overrun error in the status register when such frames are encountered.

> >
> > Configure TRUNC_FL (Frame Truncation Length) based on the RX buffer size.
> > Frames exceeding this limit will be treated as error packets and dropped.
> 
> This bit confuses me. You want to allow rx_frame_size to be bigger than the
> buffer size, but you also want to discard frames bigger than the buffer size?
> 

MAX_FL defines the maximum allowable frame length, while TRUNC_FL specifies the 
threshold beyond which frames are truncated.  

Here, TRUNC_FL is configured based on the RX buffer size, allowing the hardware to 
handle oversized frame errors automatically without requiring software intervention.

Regards,
Shenwei

>     Andrew
> 
> ---
> pw-bot: cr