RE: [PATCH v2 net-next 4/5] net: fec: add change_mtu to support dynamic buffer allocation

[Shenwei Wang]

> -----Original Message-----
> From: Wei Fang <wei.fang@xxxxxxx>
> Sent: Friday, August 22, 2025 4:49 AM
> 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 4/5] net: fec: add change_mtu to support
> dynamic buffer allocation
> 
> > +static int fec_change_mtu(struct net_device *ndev, int new_mtu) {
> > +	struct fec_enet_private *fep = netdev_priv(ndev);
> > +	int order, done;
> > +	bool running;
> > +
> > +	order = get_order(new_mtu + ETH_HLEN + ETH_FCS_LEN);
> > +	if (fep->pagepool_order == order) {
> > +		WRITE_ONCE(ndev->mtu, new_mtu);
> > +		return 0;
> > +	}
> > +
> > +	fep->pagepool_order = order;
> > +	fep->rx_frame_size = (PAGE_SIZE << order) -
> FEC_ENET_XDP_HEADROOM
> > +			     - SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> > +
> 
> I think we need to add a check for rx_frame_size, as FEC_R_CNTRL[MAX_FL] and
> FEC_FTRL[TRUNC_FL] only have 14 bits.

That would be redundant, since rx_frame_size cannot exceed max_buf_size which value
would either be PKT_MAXBUF_SIZE or MAX_JUMBO_BUF_SIZE.

Regards,
Shenwei