netdev - RE: [PATCH net] net: fec: add wmb to ensure correct descriptor values

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Message-ID:
 <AM5PR04MB31393AF1641234ECD2918761887E9@AM5PR04MB3139.eurprd04.prod.outlook.com>
Date: Wed, 17 May 2023 04:18:16 +0000
From: Wei Fang <wei.fang@....com>
To: Shenwei Wang <shenwei.wang@....com>, "David S. Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman
	<horms@...nel.org>, Horatiu Vultur <horatiu.vultur@...rochip.com>
CC: Clark Wang <xiaoning.wang@....com>, dl-linux-imx <linux-imx@....com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, "imx@...ts.linux.dev"
	<imx@...ts.linux.dev>
Subject: RE: [PATCH net] net: fec: add wmb to ensure correct descriptor values

> -----Original Message-----
> From: Shenwei Wang <shenwei.wang@....com>
> Sent: 2023年5月17日 5:21
> To: Wei Fang <wei.fang@....com>; David S. Miller <davem@...emloft.net>;
> Eric Dumazet <edumazet@...gle.com>; Jakub Kicinski <kuba@...nel.org>;
> Paolo Abeni <pabeni@...hat.com>; Simon Horman <horms@...nel.org>;
> Horatiu Vultur <horatiu.vultur@...rochip.com>
> Cc: Shenwei Wang <shenwei.wang@....com>; Clark Wang
> <xiaoning.wang@....com>; dl-linux-imx <linux-imx@....com>;
> netdev@...r.kernel.org; imx@...ts.linux.dev
> Subject: [PATCH net] net: fec: add wmb to ensure correct descriptor values
> 
> Two wmb() are added in the XDP TX path to ensure proper ordering of
> descriptor and buffer updates:
> 1. A wmb() is added after updating the last BD to make sure
>    the updates to rest of the descriptor are visible before
>    transferring ownership to FEC.
> 2. A wmb() is also added after updating the tx_skbuff and bdp
>    to ensure these updates are visible before updating txq->bd.cur.
> 3. Start the xmit of the frame immediately right after configuring the
>    tx descriptor.
> 
> Fixes: 6d6b39f180b8 ("net: fec: add initial XDP support")
> Signed-off-by: Shenwei Wang <shenwei.wang@....com>
> ---
>  drivers/net/ethernet/freescale/fec_main.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/freescale/fec_main.c
> b/drivers/net/ethernet/freescale/fec_main.c
> index 6d0b46c76924..ba4335d5ddc3 100644
> --- a/drivers/net/ethernet/freescale/fec_main.c
> +++ b/drivers/net/ethernet/freescale/fec_main.c
> @@ -3834,6 +3834,11 @@ static int fec_enet_txq_xmit_frame(struct
> fec_enet_private *fep,
>  	index = fec_enet_get_bd_index(last_bdp, &txq->bd);
>  	txq->tx_skbuff[index] = NULL;
> 
> +	/* Make sure the updates to rest of the descriptor are performed before
> +	 * transferring ownership.
> +	 */
> +	wmb();
> +
>  	/* Send it on its way.  Tell FEC it's ready, interrupt when done,
>  	 * it's the last BD of the frame, and to put the CRC on the end.
>  	 */
> @@ -3843,8 +3848,16 @@ static int fec_enet_txq_xmit_frame(struct
> fec_enet_private *fep,
>  	/* If this was the last BD in the ring, start at the beginning again. */
>  	bdp = fec_enet_get_nextdesc(last_bdp, &txq->bd);
> 
> +	/* Make sure the update to bdp and tx_skbuff are performed before
> +	 * txq->bd.cur.
> +	 */
I think this description should be modify appropriately, because tx_skbuff is
updated at the first wmb(), not here.

> +	wmb();
> +
>  	txq->bd.cur = bdp;
> 
> +	/* Trigger transmission start */
> +	writel(0, txq->bd.reg_desc_active);
> +
>  	return 0;
>  }
> 
> @@ -3873,12 +3886,6 @@ static int fec_enet_xdp_xmit(struct net_device
> *dev,
>  		sent_frames++;
>  	}
> 
> -	/* Make sure the update to bdp and tx_skbuff are performed. */
> -	wmb();
> -
> -	/* Trigger transmission start */
> -	writel(0, txq->bd.reg_desc_active);
> -
>  	__netif_tx_unlock(nq);
> 
>  	return sent_frames;
> --
> 2.34.1