Re: [PATCH v2 net] net: fec: add wmb to ensure correct descriptor values

[Heiner Kallweit <hkallweit1@gmail.com>]

On 17.05.2023 18:15, Shenwei Wang wrote:
> 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@nxp.com>
> ---
>  v2:
>   - update the inline comments for 2nd wmb per Wei Fang's review.
> 
>  drivers/net/ethernet/freescale/fec_main.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
> index 6d0b46c76924..d210e67a188b 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 +3848,14 @@ 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 are performed before txq->bd.cur. */
> +	wmb();
> +

Do you really need wmb() here or would a more light-weight smp_wmb() or
dma_wmb() be sufficient?

>  	txq->bd.cur = bdp;
> 
> +	/* Trigger transmission start */
> +	writel(0, txq->bd.reg_desc_active);
> +
>  	return 0;
>  }
> 
> @@ -3873,12 +3884,6 @@ 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
> 
>