netdev - Re: [net-next-2.6 PATCH v2 3/3] fs_enet: add FEC TX buffer alignment workaround for MPC5121

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Date:	Wed, 17 Feb 2010 16:17:16 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Anatolij Gustschin <agust@...x.de>
Cc:	netdev@...r.kernel.org, linuxppc-dev@...abs.org,
	"David S. Miller" <davem@...emloft.net>,
	Grant Likely <grant.likely@...retlab.ca>,
	Detlev Zundel <dzu@...x.de>, Wolfgang Denk <wd@...x.de>,
	John Rigby <jcrigby@...il.com>,
	Piotr Ziecik <kosmo@...ihalf.com>
Subject: Re: [net-next-2.6 PATCH v2 3/3] fs_enet: add FEC TX buffer
 alignment workaround for MPC5121

Le mercredi 17 février 2010 à 15:55 +0100, Anatolij Gustschin a écrit :
> MPC5121 FEC requeries 4-byte alignmnent for TX data buffers.
> This patch is a work around that copies misaligned tx packets
> to an aligned skb before sending.
> 
> Signed-off-by: John Rigby <jcrigby@...il.com>
> Signed-off-by: Piotr Ziecik <kosmo@...ihalf.com>
> Signed-off-by: Wolfgang Denk <wd@...x.de>
> Signed-off-by: Anatolij Gustschin <agust@...x.de>
> ---
>  drivers/net/fs_enet/fs_enet-main.c |   44 ++++++++++++++++++++++++++++++++++++
>  1 files changed, 44 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/net/fs_enet/fs_enet-main.c b/drivers/net/fs_enet/fs_enet-main.c
> index 4297021..166a89d 100644
> --- a/drivers/net/fs_enet/fs_enet-main.c
> +++ b/drivers/net/fs_enet/fs_enet-main.c
> @@ -580,6 +580,37 @@ void fs_cleanup_bds(struct net_device *dev)
>  
>  /**********************************************************************************/
>  
> +#ifdef CONFIG_FS_ENET_MPC5121_FEC
> +/*
> + * MPC5121 FEC requeries 4-byte alignment for TX data buffer!
> + */
> +static struct sk_buff *tx_skb_align_workaround(struct net_device *dev,
> +					       struct sk_buff *skb)
> +{
> +	struct sk_buff *new_skb;
> +	struct fs_enet_private *fep = netdev_priv(dev);
> +
> +	/* Alloc new skb */
> +	new_skb = dev_alloc_skb(ENET_RX_FRSIZE + 4);


ENET_RX_FRSIZE looks strange in TX path

Why not using skb->len + 4 instead of ENET_RX_FRSIZE + 4 ?


> +	if (!new_skb) {
> +		dev_warn(fep->dev, "Memory squeeze, dropping tx packet.\n");

I am just wondering if this is ratelimited ?

> +		return NULL;
> +	}
> +
> +	/* Make sure new skb is properly aligned */
> +	skb_align(new_skb, 4);

> +
> +	/* Copy data to new skb ... */
> +	skb_copy_from_linear_data(skb, new_skb->data, skb->len);
> +	skb_put(new_skb, skb->len);
> +
> +	/* ... and free an old one */
> +	dev_kfree_skb_any(skb);
> +
> +	return new_skb;
> +}
> +#endif
> +
>  static int fs_enet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>  	struct fs_enet_private *fep = netdev_priv(dev);
> @@ -588,6 +619,19 @@ static int fs_enet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	u16 sc;
>  	unsigned long flags;
>  
> +#ifdef CONFIG_FS_ENET_MPC5121_FEC
> +	if (((unsigned long)skb->data) & 0x3) {
> +		skb = tx_skb_align_workaround(dev, skb);
> +		if (!skb) {
> +			/*
> +			 * We have lost packet due to memory allocation error
> +			 * in tx_skb_align_workaround(). Hopefully original
> +			 * skb is still valid, so try transmit it later.
> +			 */

Could you define 'try to transmit later' ?
Who is responsible to trigger this event ?


> +			return NETDEV_TX_BUSY;
> +		}
> +	}
> +#endif
>  	spin_lock_irqsave(&fep->tx_lock, flags);
>  
>  	/*


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