linux-kernel - Re: [PATCH net] net: enetc: do not transmit redirected XDP frames when the link is down

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Message-ID: <20251209083531.2yk2lv2rahouytv2@skbuf>
Date: Tue, 9 Dec 2025 10:35:31 +0200
From: Vladimir Oltean <vladimir.oltean@....com>
To: Wei Fang <wei.fang@....com>
Cc: claudiu.manoil@....com, xiaoning.wang@....com, andrew+netdev@...n.ch,
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, ast@...nel.org, daniel@...earbox.net,
	hawk@...nel.org, john.fastabend@...il.com, sdf@...ichev.me,
	imx@...ts.linux.dev, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net] net: enetc: do not transmit redirected XDP frames
 when the link is down

On Fri, Dec 05, 2025 at 06:53:07PM +0800, Wei Fang wrote:
> In the current implementation, the enetc_xdp_xmit() always transmits
> redirected XDP frames even if the link is down, but the frames cannot
> be transmitted from TX BD rings when the link is down, so the frames
> are still kept in the TX BD rings. If the XDP program is uninstalled,
> users will see the following warning logs.
> 
> fsl_enetc 0000:00:00.0 eno0: timeout for tx ring #6 clear
> 
> More worse, the TX BD ring cannot work properly anymore, because the
> HW PIR and CIR are not the same after the re-initialization of the TX
> BD ring.

I understand and I don't disagree that the TX BD ring doesn't work
anymore if we disable it while it has pending frames (the TB0MR[EN]
documentation says that this is unsafe too), but:
- I don't understand why the hardware PIR and CIR are not the same after
  the TX ring reinitialization
- I don't understand how the effect and the claimed cause are connected

Could you please give more details what you mean here?

> And I see no reasons to transmit the redirected XDP frames
> when the link is down, so add a link status check to quickly fix this
> issue. However, this solution does not completely solve the problem,
> for example, if the link is broken during transmission and the TX BD
> ring still has unsent frames. I think this requires another patch to
> address this situation, but it will not conflict with the current
> solution and can coexist.
> 
> Fixes: 9d2b68cc108d ("net: enetc: add support for XDP_REDIRECT")
> Signed-off-by: Wei Fang <wei.fang@....com>
> ---
>  drivers/net/ethernet/freescale/enetc/enetc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
> index 0535e92404e3..f410c245ea91 100644
> --- a/drivers/net/ethernet/freescale/enetc/enetc.c
> +++ b/drivers/net/ethernet/freescale/enetc/enetc.c
> @@ -1778,7 +1778,8 @@ int enetc_xdp_xmit(struct net_device *ndev, int num_frames,
>  	int xdp_tx_bd_cnt, i, k;
>  	int xdp_tx_frm_cnt = 0;
>  
> -	if (unlikely(test_bit(ENETC_TX_DOWN, &priv->flags)))
> +	if (unlikely(test_bit(ENETC_TX_DOWN, &priv->flags) ||
> +		     !netif_carrier_ok(ndev)))
>  		return -ENETDOWN;
>  
>  	enetc_lock_mdio();
> -- 
> 2.34.1
>