netdev - Re: [PATCH 6/6] sis190: account for Tx errors

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Message-Id: <20080501161651.0a754572.akpm@linux-foundation.org>
Date:	Thu, 1 May 2008 16:16:51 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Francois Romieu <romieu@...zoreil.com>
Cc:	jeff@...zik.org, netdev@...r.kernel.org
Subject: Re: [PATCH 6/6] sis190: account for Tx errors

On Sun, 27 Apr 2008 19:06:04 +0200
Francois Romieu <romieu@...zoreil.com> wrote:

> Update the collision counter as well.
> 
> Signed-off-by: Francois Romieu <romieu@...zoreil.com>
> ---
>  drivers/net/sis190.c |   38 +++++++++++++++++++++++++++++++++++---
>  1 files changed, 35 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/sis190.c b/drivers/net/sis190.c
> index 20f4829..abc63b0 100644
> --- a/drivers/net/sis190.c
> +++ b/drivers/net/sis190.c
> @@ -212,6 +212,12 @@ enum _DescStatusBit {
>  	THOL2		= 0x20000000,
>  	THOL1		= 0x10000000,
>  	THOL0		= 0x00000000,
> +
> +	WND		= 0x00080000,
> +	TABRT		= 0x00040000,
> +	FIFO		= 0x00020000,
> +	LINK		= 0x00010000,
> +	ColCountMask	= 0x0000ffff,
>  	/* RxDesc.status */
>  	IPON		= 0x20000000,
>  	TCPON		= 0x10000000,
> @@ -653,9 +659,31 @@ static void sis190_unmap_tx_skb(struct pci_dev *pdev, struct sk_buff *skb,
>  	memset(desc, 0x00, sizeof(*desc));
>  }
>  
> +static inline int sis190_tx_pkt_err(u32 status, struct net_device_stats *stats)
> +{
> +#define TxErrMask	(WND | TABRT | FIFO | LINK)
> +
> +	if (!unlikely(status & TxErrMask))
> +		return 0;
> +
> +	if (status & WND)
> +		stats->tx_window_errors++;
> +	if (status & TABRT)
> +		stats->tx_aborted_errors++;
> +	if (status & FIFO)
> +		stats->tx_fifo_errors++;
> +	if (status & LINK)
> +		stats->tx_carrier_errors++;
> +
> +	stats->tx_errors++;
> +
> +	return -1;
> +}

Does !unlikely(...) actually do what we want?

>  static void sis190_tx_interrupt(struct net_device *dev,
>  				struct sis190_private *tp, void __iomem *ioaddr)
>  {
> +	struct net_device_stats *stats = &dev->stats;
>  	u32 pending, dirty_tx = tp->dirty_tx;
>  	/*
>  	 * It would not be needed if queueing was allowed to be enabled
> @@ -670,15 +698,19 @@ static void sis190_tx_interrupt(struct net_device *dev,
>  	for (; pending; pending--, dirty_tx++) {
>  		unsigned int entry = dirty_tx % NUM_TX_DESC;
>  		struct TxDesc *txd = tp->TxDescRing + entry;
> +		u32 status = le32_to_cpu(txd->status);
>  		struct sk_buff *skb;
>  
> -		if (le32_to_cpu(txd->status) & OWNbit)
> +		if (status & OWNbit)
>  			break;
>  
>  		skb = tp->Tx_skbuff[entry];
>  
> -		dev->stats.tx_packets++;
> -		dev->stats.tx_bytes += skb->len;
> +		if (likely(sis190_tx_pkt_err(status, stats) == 0)) {
> +			stats->tx_packets++;
> +			stats->tx_bytes += skb->len;
> +			stats->collisions += ((status & ColCountMask) - 1);
> +		}

Because it kinda matters here.  We would like to prevent the unlikely
error-handling code from gumming up the interrupt handler's cache
footprint.

To be confident, one could do:

static noinline sis190_tx_pkt_err(...)
{
	if (status & WND)
	...
}


	if (unlikely(status & TxErrMask))
		sis190_tx_pkt_err(...);



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