Subject: Re: [patch 08/17] sky2: avoid divide in receive path

From: Jeff Garzik <[email-masked]>
Date: Sat, 29 Sep 2007 01:51:16 -0400

[email-masked] wrote:
> From: Stephen Hemminger <[email-masked]>
> 
> Avoid divide (modulus) in receive buffer handling path.
> 
> Signed-off-by: Stephen Hemminger <[email-masked]>
> Signed-off-by: Andrew Morton <[email-masked]>
> ---
> 
>  drivers/net/sky2.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff -puN drivers/net/sky2.c~sky2-avoid-divide-in-receive-path drivers/net/sky2.c
> --- a/drivers/net/sky2.c~sky2-avoid-divide-in-receive-path
> +++ a/drivers/net/sky2.c
> @@ -2155,7 +2155,8 @@ static struct sk_buff *sky2_receive(stru
>  		printk(KERN_DEBUG PFX "%s: rx slot %u status 0x%x len %d\n",
>  		       dev->name, sky2->rx_next, status, length);
>  
> -	sky2->rx_next = (sky2->rx_next + 1) % sky2->rx_pending;
> +	if (++sky2->rx_next >= sky2->rx_pending)
> +		sky2->rx_next = 0;

it is silly to introduce a branch just for this.

require that the ring size is a power-of-2, and then replace the divide 
with a mask.