netdev - Re: [PATCH net-next 1/2] net: bcmgenet: update ring producer index and buffer count in xmit

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Date:	Mon, 9 Mar 2015 15:49:16 -0700
From:	Petri Gynther <pgynther@...gle.com>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
	Jaedon Shin <jaedon.shin@...il.com>
Subject: Re: [PATCH net-next 1/2] net: bcmgenet: update ring producer index
 and buffer count in xmit

On Mon, Mar 9, 2015 at 2:52 PM, Florian Fainelli <f.fainelli@...il.com> wrote:
> There is no need to have both bcmgenet_xmit_single() and
> bcmgenet_xmit_frag() perform a free_bds decrement and a prod_index
> increment by one. In case one of these functions fails to map a SKB or
> fragment for transmit, we will return and exit bcmgenet_xmit() with an
> error.
>
> We can therefore safely use our local copy of nr_frags to know by how
> much we should decrement the number of free buffers available, and by
> how much the producer count must be incremented and do this in the tail
> of bcmgenet_xmit().
>
> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> Acked-by: Petri Gynther <pgynther@...gle.com>

Reviewed-by: Petri Gynther <pgynther@...gle.com>

> ---
>  drivers/net/ethernet/broadcom/genet/bcmgenet.c | 18 +++++-------------
>  1 file changed, 5 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> index 83c0cb323e0c..9b3b8a3db59b 100644
> --- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> +++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> @@ -1130,11 +1130,6 @@ static int bcmgenet_xmit_single(struct net_device *dev,
>
>         dmadesc_set(priv, tx_cb_ptr->bd_addr, mapping, length_status);
>
> -       /* Decrement total BD count and advance our write pointer */
> -       ring->free_bds -= 1;
> -       ring->prod_index += 1;
> -       ring->prod_index &= DMA_P_INDEX_MASK;
> -
>         return 0;
>  }
>
> @@ -1173,11 +1168,6 @@ static int bcmgenet_xmit_frag(struct net_device *dev,
>                     (frag->size << DMA_BUFLENGTH_SHIFT) | dma_desc_flags |
>                     (priv->hw_params->qtag_mask << DMA_TX_QTAG_SHIFT));
>
> -
> -       ring->free_bds -= 1;
> -       ring->prod_index += 1;
> -       ring->prod_index &= DMA_P_INDEX_MASK;
> -
>         return 0;
>  }
>
> @@ -1321,9 +1311,11 @@ static netdev_tx_t bcmgenet_xmit(struct sk_buff *skb, struct net_device *dev)
>
>         skb_tx_timestamp(skb);
>
> -       /* we kept a software copy of how much we should advance the TDMA
> -        * producer index, now write it down to the hardware
> -        */
> +       /* Decrement total BD count and advance our write pointer */
> +       ring->free_bds -= nr_frags + 1;
> +       ring->prod_index += nr_frags + 1;
> +       ring->prod_index &= DMA_P_INDEX_MASK;
> +
>         bcmgenet_tdma_ring_writel(priv, ring->index,
>                                   ring->prod_index, TDMA_PROD_INDEX);
>
> --
> 2.1.0
>

This change looks good.

However, I just noticed another issue in bcmgenet_xmit_single() and
bcmgenet_xmit_frag(). If the DMA mapping fails, we return early from
those functions, but bcmgenet_get_txcb() had already advanced
ring->write_ptr. That needs to be fixed.

Similarly, in bcmgenet_xmit(), imagine that a call to
bcmgenet_xmit_single() is OK, but then a subsequent call to
bcmgenet_xmit_frag() fails. The driver should restore the Tx ring
pointers back to the values before bcmgenet_xmit_single() was called.

-- Petri
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