RE: [PATCH v2 net-next 3/5] et: fec: add rx_frame_size to support configurable RX length

[Shenwei Wang]

> -----Original Message-----
> From: Wei Fang <wei.fang@xxxxxxx>
> Sent: Sunday, August 24, 2025 9:26 PM
> To: Shenwei Wang <shenwei.wang@xxxxxxx>
> Cc: Clark Wang <xiaoning.wang@xxxxxxx>; Stanislav Fomichev
> <sdf@xxxxxxxxxxx>; imx@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; dl-linux-imx <linux-imx@xxxxxxx>; Andrew Lunn
> <andrew+netdev@xxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>; Eric
> Dumazet <edumazet@xxxxxxxxxx>; Jakub Kicinski <kuba@xxxxxxxxxx>; Paolo
> Abeni <pabeni@xxxxxxxxxx>; Alexei Starovoitov <ast@xxxxxxxxxx>; Daniel
> Borkmann <daniel@xxxxxxxxxxxxx>; Jesper Dangaard Brouer
> <hawk@xxxxxxxxxx>; John Fastabend <john.fastabend@xxxxxxxxx>
> Subject: RE: [PATCH v2 net-next 3/5] et: fec: add rx_frame_size to support
> configurable RX length
> 
> > > > @@ -4563,6 +4563,7 @@ fec_probe(struct platform_device *pdev)
> > > >  	pinctrl_pm_select_sleep_state(&pdev->dev);
> > > >
> > > >  	fep->pagepool_order = 0;
> > > > +	fep->rx_frame_size = FEC_ENET_RX_FRSIZE;
> > >
> > > According to the RM, to allow one maximum size frame per buffer,
> > > FEC_R_BUFF_SIZE must be set to FEC_R_CNTRL [MAX_FL] or larger.
> > > FEC_ENET_RX_FRSIZE is greater than PKT_MAXBUF_SIZE, I'm not sure
> > whether it
> > > will cause some unknown issues.
> >
> > MAX_FL defines the maximum allowable frame length, while TRUNC_FL
> > specifies the threshold beyond which frames are truncated. Based on
> > this logic, the documentation appears to be incorrect-TRUNC_FL should
> > never exceed MAX_FL, as doing so would make the truncation mechanism
> > ineffective.
> >
> > This has been confirmed through testing data.
> >
> 
> One obvious issue I can see is that the Rx error statistic would be doubled the
> actual number when the FEC receives jumbo frames.
> 
> For example, the sender sends 1000 jumbo frames (8000 bytes) to the FEC port,
> without this patch set, the Rx error statistic of FEC should be 1000, however, after
> applying this patch set (rx_frame_size is 3520, max_buf_size is 1984), I can see
> the Rx error statistic is 2000.

I don't think there is a case that rx_frame_size is 3520 while the max_buf_size is 1984.
With the patch, the rx_frame_size should always less than the max_buf_size. Otherwise,
the implementation might have a logic bug.

Regards,
Shenwei