linux-kernel - Re: [PATCH v9] tilegx network driver: initial support

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Date:	Wed, 06 Jun 2012 19:40:59 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	bhutchings@...arflare.com, arnd@...db.de,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v9] tilegx network driver: initial support

On Mon, 2012-06-04 at 16:12 -0400, Chris Metcalf wrote:

> +/* Allocate and push a buffer. */
> +static bool tile_net_provide_buffer(bool small)
> +{
> +	int stack = small ? small_buffer_stack : large_buffer_stack;
> +	const unsigned long buffer_alignment = 128;
> +	struct sk_buff *skb;
> +	int len;
> +
> +	len = sizeof(struct sk_buff **) + buffer_alignment;
> +	len += (small ? 128 : 1664);

1664 is a magic number, it should be a nice define

#define ..... ( ETH_DATA_LEN + .... )

> +	skb = dev_alloc_skb(len);
> +	if (skb == NULL)
> +		return false;
> +
> +	/* Make room for a back-pointer to 'skb' and guarantee alignment. */
> +	skb_reserve(skb, sizeof(struct sk_buff **));
> +	skb_reserve(skb, -(long)skb->data & (buffer_alignment - 1));
> +
> +	/* Save a back-pointer to 'skb'. */
> +	*(struct sk_buff **)(skb->data - sizeof(struct sk_buff **)) = skb;
> +
> +	/* Make sure "skb" and the back-pointer have been flushed. */
> +	wmb();

Interesting, have you considered using build_skb() instead of this
convoluted thing ?

This could save some cache misses...



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