Subject: Re: [PATCH net 1/4] sh_eth: Ensure proper ordering of descriptor active bit write/read

From: Sergei Shtylyov <[email-masked]>
Date: Fri, 10 Apr 2015 01:21:14 +0300

Hello.

On 03/03/2015 03:52 AM, Ben Hutchings wrote:

> When submitting a DMA descriptor, the active bit must be written last.
> When reading a completed DMA descriptor, the active bit must be read
> first.

> Add memory barriers to ensure that this ordering is maintained.

    Looks like those should have been lighter dma_rmb() and dma_wmb() instead.
Correct, Alexander?

> Signed-off-by: Ben Hutchings <[email-masked]>
> ---
>   drivers/net/ethernet/renesas/sh_eth.c |    6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/drivers/net/ethernet/renesas/sh_eth.c b/drivers/net/ethernet/renesas/sh_eth.c
> index 654b48d1e61a..5c212a833bcf 100644
> --- a/drivers/net/ethernet/renesas/sh_eth.c
> +++ b/drivers/net/ethernet/renesas/sh_eth.c
> @@ -1410,6 +1410,8 @@ static int sh_eth_txfree(struct net_device *ndev)
>   		txdesc = &mdp->tx_ring[entry];
>   		if (txdesc->status & cpu_to_edmac(mdp, TD_TACT))
>   			break;
> +		/* TACT bit must be checked before all the following reads */
> +		rmb();
>   		/* Free the original skb. */
>   		if (mdp->tx_skbuff[entry]) {
>   			dma_unmap_single(&ndev->dev, txdesc->addr,
> @@ -1447,6 +1449,8 @@ static int sh_eth_rx(struct net_device *ndev, u32 intr_status, int *quota)
>   	limit = boguscnt;
>   	rxdesc = &mdp->rx_ring[entry];
>   	while (!(rxdesc->status & cpu_to_edmac(mdp, RD_RACT))) {
> +		/* RACT bit must be checked before all the following reads */
> +		rmb();
>   		desc_status = edmac_to_cpu(mdp, rxdesc->status);
>   		pkt_len = rxdesc->frame_length;
>
> @@ -1526,6 +1530,7 @@ static int sh_eth_rx(struct net_device *ndev, u32 intr_status, int *quota)
>   			skb_checksum_none_assert(skb);
>   			rxdesc->addr = dma_addr;
>   		}
> +		wmb(); /* RACT bit must be set after all the above writes */
>   		if (entry >= mdp->num_rx_ring - 1)
>   			rxdesc->status |=
>   				cpu_to_edmac(mdp, RD_RACT | RD_RFP | RD_RDEL);
> @@ -2195,6 +2200,7 @@ static int sh_eth_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>   	}
>   	txdesc->buffer_length = skb->len;
>
> +	wmb(); /* TACT bit must be set after all the above writes */
>   	if (entry >= mdp->num_tx_ring - 1)
>   		txdesc->status |= cpu_to_edmac(mdp, TD_TACT | TD_TDLE);
>   	else

WBR, Sergei