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

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Date:	Fri, 5 Feb 2016 17:03:47 -0800
From:	Joshua Clayton <stillcompiling@...il.com>
To:	Troy Kisky <troy.kisky@...ndarydevices.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, B38611@...escale.com,
	fabio.estevam@...escale.com, andrew@...n.ch,
	linux@....linux.org.uk, arnd@...db.de, laci@...ndarydevices.com,
	johannes@...solutions.net, l.stach@...gutronix.de,
	shawnguo@...nel.org, linux-arm-kernel@...ts.infradead.org,
	tremyfr@...il.com
Subject: Re: [PATCH net-next V2 7/8] net: fec: don't transfer ownership
 until descriptor write is complete

On Fri,  5 Feb 2016 14:52:49 -0700
Troy Kisky <troy.kisky@...ndarydevices.com> 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);
You use almost exactly the same code in each place.
I'd prefer to have wmb(); bdp->cbd_sc = cpu_to_fec16(status) wrapped in
a function or function like macro, and put the comment in one place.

e.g.

/* Make sure the updates to rest of the descriptor
 * performed before transferring ownership.
 */
#define fec_enet_xfr_ownership(status) \
	do { \
		wmb();
		bdp->cbd_sc = cpu_to_fec16(status);
	} while (0)

(I don't know if thats the right macro name, I'm just basing it off
your comment)

>  	}
>  
> @@ -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. @@ -1475,7 +1483,6 @@ rx_processing_done:
>  
>  		/* Mark the buffer empty */
>  		status |= BD_ENET_RX_EMPTY;
> -		bdp->cbd_sc = cpu_to_fec16(status);
>  
>  		if (fep->bufdesc_ex) {
>  			struct bufdesc_ex *ebdp = (struct bufdesc_ex
> *)bdp; @@ -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);
>  
>  		/* Update BD pointer to next entry */
>  		bdp = fec_enet_get_nextdesc(bdp, &rxq->bd);