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

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On 4/10/24 02:49, Jakub Kicinski wrote:
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.

Not sure I get this point.

"Give the page to the HW" = Do you mean that I should dma_map_single()
a full page (|PAGE_SIZE|) in am65_cpsw_nuss_rx_push() ?



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




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