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

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Message-ID: <9cb903df-3e83-409a-aa4b-218742804cc6@baylibre.com>
Date: Wed, 10 Apr 2024 16:02:00 +0200
From: Julien Panis <jpanis@...libre.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 Russell King <linux@...linux.org.uk>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>,
 Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 Simon Horman <horms@...nel.org>, Andrew Lunn <andrew@...n.ch>,
 Ratheesh Kannoth <rkannoth@...vell.com>,
 Naveen Mamindlapalli <naveenm@...vell.com>, danishanwar@...com,
 yuehaibing@...wei.com, rogerq@...nel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH net-next v8 3/3] net: ethernet: ti: am65-cpsw: Add minimal
 XDP support

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;
>> +	}