Re: [PATCH net-next v6 1/7] net: bcmgenet: convert RX path to page_pool

[Jakub Kicinski <kuba@kernel.org>]

On Mon,  6 Apr 2026 10:35:25 +0200 Nicolai Buchwitz wrote:
> Replace the per-packet __netdev_alloc_skb() + dma_map_single() in the
> RX path with page_pool, which provides efficient page recycling and
> DMA mapping management. This is a prerequisite for XDP support (which
> requires stable page-backed buffers rather than SKB linear data).
> 
> Key changes:
> - Create a page_pool per RX ring (PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV)
> - bcmgenet_rx_refill() allocates pages via page_pool_alloc_pages()
> - bcmgenet_desc_rx() builds SKBs from pages via napi_build_skb() with
>   skb_mark_for_recycle() for automatic page_pool return
> - Buffer layout reserves XDP_PACKET_HEADROOM (256 bytes) before the HW
>   RSB (64 bytes) + alignment pad (2 bytes) for future XDP headroom

some nits here, since I have more "real" comments on later patches

> +/* Page pool RX buffer layout:
> + * XDP_PACKET_HEADROOM | RSB(64) + pad(2) | frame data | skb_shared_info
> + * The HW writes the 64B RSB + 2B alignment padding before the frame.
> + */
> +#define GENET_XDP_HEADROOM	XDP_PACKET_HEADROOM

subjective but IDK what value this define adds vs using
XDP_PACKET_HEADROOM directly.

> +#define GENET_RSB_PAD		(sizeof(struct status_64) + 2)
> +#define GENET_RX_HEADROOM	(GENET_XDP_HEADROOM + GENET_RSB_PAD)

> +static int bcmgenet_rx_refill(struct bcmgenet_rx_ring *ring,
> +			      struct enet_cb *cb)
>  {
> -	struct device *kdev = &priv->pdev->dev;
> -	struct sk_buff *skb;
> -	struct sk_buff *rx_skb;
> +	struct bcmgenet_priv *priv = ring->priv;
>  	dma_addr_t mapping;
> +	struct page *page;
>  
> -	/* Allocate a new Rx skb */
> -	skb = __netdev_alloc_skb(priv->dev, priv->rx_buf_len + SKB_ALIGNMENT,
> -				 GFP_ATOMIC | __GFP_NOWARN);

page pool adds __GFP_NOWARN automatically, you can drop it now

> -	if (!skb) {
> +	page = page_pool_alloc_pages(ring->page_pool,
> +				     GFP_ATOMIC | __GFP_NOWARN);
> +	if (!page) {
>  		priv->mib.alloc_rx_buff_failed++;
>  		netif_err(priv, rx_err, priv->dev,
> -			  "%s: Rx skb allocation failed\n", __func__);
> -		return NULL;
> -	}