netdev - Re: [PATCH net-next V2 7/8] net: fec: don't transfer ownership until descriptor write is complete

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Date:	Sat, 6 Feb 2016 14:52:12 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Troy Kisky <troy.kisky@...ndarydevices.com>,
	netdev@...r.kernel.org, davem@...emloft.net, B38611@...escale.com
Cc:	fabio.estevam@...escale.com, l.stach@...gutronix.de,
	andrew@...n.ch, tremyfr@...il.com, linux@....linux.org.uk,
	linux-arm-kernel@...ts.infradead.org, laci@...ndarydevices.com,
	shawnguo@...nel.org, johannes@...solutions.net, arnd@...db.de
Subject: Re: [PATCH net-next V2 7/8] net: fec: don't transfer ownership until
 descriptor write is complete

Hello.

On 2/6/2016 12:52 AM, Troy Kisky wrote:

> If you don't own it, you shouldn't write to it.
>
> Signed-off-by: Troy Kisky <troy.kisky@...ndarydevices.com>
> ---
>   drivers/net/ethernet/freescale/fec_main.c | 14 +++++++++++++-
>   1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
> index ca2708d..97ca72a 100644
> --- a/drivers/net/ethernet/freescale/fec_main.c
> +++ b/drivers/net/ethernet/freescale/fec_main.c
> @@ -390,6 +390,10 @@ fec_enet_txq_submit_frag_skb(struct fec_enet_priv_tx_q *txq,
>
>   		bdp->cbd_bufaddr = cpu_to_fec32(addr);
>   		bdp->cbd_datlen = cpu_to_fec16(frag_len);
> +		/* Make sure the updates to rest of the descriptor are
> +		 * performed before transferring ownership.
> +		 */
> +		wmb();
>   		bdp->cbd_sc = cpu_to_fec16(status);
>   	}
>
> @@ -499,6 +503,10 @@ static int fec_enet_txq_submit_skb(struct fec_enet_priv_tx_q *txq,
>
>   	bdp->cbd_datlen = cpu_to_fec16(buflen);
>   	bdp->cbd_bufaddr = cpu_to_fec32(addr);
> +	/* 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.
[...]
> @@ -1484,6 +1491,11 @@ rx_processing_done:
>   			ebdp->cbd_prot = 0;
>   			ebdp->cbd_bdu = 0;
>   		}
> +		/* Make sure the updates to rest of the descriptor are
> +		 * performed before transferring ownership.
> +		 */
> +		wmb();
> +		bdp->cbd_sc = cpu_to_fec16(status);

    I think you can use "ligter" dma_wmb() in this case.

[...]

MBR, Sergei