linux-kernel - RE: [PATCH v2 net-next 4/5] net: fec: add change

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Message-ID:
 <PAXPR04MB9185A682B7B3D37E157F4184893EA@PAXPR04MB9185.eurprd04.prod.outlook.com>
Date: Mon, 25 Aug 2025 12:34:10 +0000
From: Shenwei Wang <shenwei.wang@....com>
To: Wei Fang <wei.fang@....com>
CC: 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>, 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>
Subject: RE: [PATCH v2 net-next 4/5] net: fec: add change_mtu to support
 dynamic buffer allocation



> -----Original Message-----
> From: Wei Fang <wei.fang@....com>
> Sent: Sunday, August 24, 2025 9:13 PM
> To: Shenwei Wang <shenwei.wang@....com>
> Cc: 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>; 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>
> 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.
> >
> 
> Looked at the entire patch set, the rx_frame_size is set to FEC_FTRL[TRUNC_FL]
> and FEC_R_CNTRL[MAX_FL], and both TRUNC_FL and MAX_FL are 14 bits, if the
> value set exceeds the hardware capability, the driver should return an error.
> 
> For example, the order is 3, so rx_frame_size is 0x7dc0, but MAX_FL will be set to
> 0x3dc0, that is not correct.

How can you get the order of 3? If the PAGE_SIZE is 4k, the order is no greater than 2.

Regards,
Shenwei