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

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Date: Wed, 10 Apr 2024 08:02:25 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Julien Panis <jpanis@...libre.com>
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 Wed, 10 Apr 2024 16:02:00 +0200 Julien Panis wrote:
> > 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() ?

Yes, I think so. I think that's what you effectively do now anyway,
you just limit the len and wrap it in an skb. But
am65_cpsw_nuss_rx_push() will effectively get that page back from
skb->data and map it.