Re: [PATCH net-next v2 1/2] net: ethernet: mtk_eth_soc: use prefetch methods

[Joe Damato <jdamato@fastly.com>]

On Mon, Jul 29, 2024 at 09:29:54PM +0300, Elad Yifee wrote:
> Utilize kernel prefetch methods for faster cache line access.
> This change boosts driver performance,
> allowing the CPU to handle about 5% more packets/sec.
> 
> Signed-off-by: Elad Yifee <eladwf@gmail.com>
> ---
> Changes in v2:
> 	- use net_prefetchw as suggested by Joe Damato
> 	- add (NET_SKB_PAD + eth->ip_align) offset to prefetched data
> 	- use eth->ip_align instead of NET_IP_ALIGN as it could be 0,
> 	depending on the platform 
> ---
>  drivers/net/ethernet/mediatek/mtk_eth_soc.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.c b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
> index 16ca427cf4c3..4d0052dbe3f4 100644
> --- a/drivers/net/ethernet/mediatek/mtk_eth_soc.c
> +++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.c

[...]

> @@ -2143,6 +2147,7 @@ static int mtk_poll_rx(struct napi_struct *napi, int budget,
>  			dma_unmap_single(eth->dma_dev, ((u64)trxd.rxd1 | addr64),
>  					 ring->buf_size, DMA_FROM_DEVICE);
>  
> +			net_prefetch(data + NET_SKB_PAD + eth->ip_align);
>  			skb = build_skb(data, ring->frag_size);
>  			if (unlikely(!skb)) {
>  				netdev->stats.rx_dropped++;
> @@ -2150,7 +2155,8 @@ static int mtk_poll_rx(struct napi_struct *napi, int budget,
>  				goto skip_rx;
>  			}
>  
> -			skb_reserve(skb, NET_SKB_PAD + NET_IP_ALIGN);
> +			net_prefetchw(skb->data);
> +			skb_reserve(skb, NET_SKB_PAD + eth->ip_align);

Based on the code in mtk_probe, I am guessing that only
MTK_SOC_MT7628 can DMA to unaligned addresses, because for
everything else eth->ip_align would be 0.

Is that right?

I am asking because the documentation in
Documentation/core-api/unaligned-memory-access.rst refers to the
case you mention, NET_IP_ALIGN = 0, suggesting that this is
intentional for performance reasons on powerpc:

  One notable exception here is powerpc which defines NET_IP_ALIGN to
  0 because DMA to unaligned addresses can be very expensive and dwarf
  the cost of unaligned loads.

It goes on to explain that some devices cannot DMA to unaligned
addresses and I assume that for your driver that is everything which
is not MTK_SOC_MT7628 ?