Subject: Re: [PATCH net-next v8 3/3] net: ethernet: ti: am65-cpsw: Add minimal XDP support

From: Jakub Kicinski <[email-masked]>
Date: Tue, 9 Apr 2024 17:49:28 -0700

On Mon, 08 Apr 2024 11:38:04 +0200 Julien Panis wrote:
> +static struct sk_buff *am65_cpsw_alloc_skb(struct am65_cpsw_rx_chn *rx_chn,
> +					   struct net_device *ndev,
> +					   unsigned int len,
> +					   int desc_idx,
> +					   bool allow_direct)
> +{
> +	struct sk_buff *skb;
> +	struct page *page;
> +
> +	page = page_pool_dev_alloc_pages(rx_chn->page_pool);
> +	if (unlikely(!page))
> +		return NULL;
> +
> +	len += AM65_CPSW_HEADROOM;
> +
> +	skb = build_skb(page_address(page), len);

You shouldn't build the skb upfront any more. Give the page to the HW,
once HW sends you a completion - build the skbs. If build fails
(allocation failure) just give the page back to HW. If it succeeds,
however, you'll get a skb which is far more likely to be cache hot.

> +	if (unlikely(!skb)) {
> +		page_pool_put_full_page(rx_chn->page_pool, page, allow_direct);
> +		rx_chn->pages[desc_idx] = NULL;
> +		return NULL;
> +	}