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

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Date:	Sat, 29 Sep 2007 01:51:16 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	akpm@...ux-foundation.org, shemminger@...ux-foundation.org
CC:	netdev <netdev@...r.kernel.org>
Subject: Re: [patch 08/17] sky2: avoid divide in receive path

akpm@...ux-foundation.org wrote:
> From: Stephen Hemminger <shemminger@...ux-foundation.org>
> 
> Avoid divide (modulus) in receive buffer handling path.
> 
> Signed-off-by: Stephen Hemminger <shemminger@...ux-foundation.org>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
> 
>  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.

-
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