netdev - Re: [PATCH 03/12] fs_enet: Add FEC TX Alignment workaround for MPC5121.

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Date:	Wed, 6 May 2009 14:37:41 -0600
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Wolfgang Denk <wd@...x.de>
Cc:	linuxppc-dev@...abs.org, John Rigby <jrigby@...escale.com>,
	Piotr Ziecik <kosmo@...ihalf.com>, netdev@...r.kernel.org,
	John Rigby <jcrigby@...il.com>
Subject: Re: [PATCH 03/12] fs_enet: Add FEC TX Alignment workaround for 
	MPC5121.

On Wed, May 6, 2009 at 2:15 PM, Wolfgang Denk <wd@...x.de> wrote:
> From: John Rigby <jrigby@...escale.com>
>
> The FEC on 5121 has problems with misaligned tx buffers.
> The RM says any alignment is ok but empirical results
> show that packet buffers ending in 0x1E will sometimes
> hang the FEC.  Other bad alignment does not hang but will
> cause silent TX failures resulting in about a 1% packet
> loss as tested by ping -f from a remote host.
>
> This patch is a work around that copies every tx packet
> to an aligned skb before sending.

OUCH!

> diff --git a/drivers/net/fs_enet/fs_enet-main.c b/drivers/net/fs_enet/fs_enet-main.c
> index 4170d33..c83ffc3 100644
> --- a/drivers/net/fs_enet/fs_enet-main.c
> +++ b/drivers/net/fs_enet/fs_enet-main.c
> @@ -594,6 +594,37 @@ void fs_cleanup_bds(struct net_device *dev)
>
>  /**********************************************************************************/
>
> +#ifdef CONFIG_FS_ENET_FEC_TX_ALIGN_WORKAROUND
> +static struct sk_buff *tx_skb_align_workaround(struct net_device *dev,
> +                                                       struct sk_buff *skb)
> +{
> +       struct sk_buff *new_skb;
> +
> +       /* Alloc new skb */
> +       new_skb = dev_alloc_skb(ENET_RX_FRSIZE + 32);
> +       if (!new_skb) {
> +               printk(KERN_WARNING DRV_MODULE_NAME
> +                               ": %s Memory squeeze, dropping tx packet.\n",
> +                                                               dev->name);
> +               return NULL;
> +       }
> +
> +       /* Make sure new skb is properly aligned */
> +       skb_align(new_skb, 32);
> +
> +       /* 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;
> +}
> +#else
> +#define tx_skb_align_workaround(dev, skb) (skb)
> +#endif

Another use of #ifdef blocks.  What is the multiplatform impact?

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.
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