Re: [net-next v18 5/7] net: mtip: Add mtip_switch_{rx|tx} functions to the L2 switch driver

Hi Jakub,

On Wed, 13 Aug 2025 09:07:53 +0200 Lukasz Majewski wrote:

quoted

+		page = fep->page[bdp - fep->rx_bd_base];
+		/* Process the incoming frame */
+		pkt_len = bdp->cbd_datlen;
+
+		dma_sync_single_for_cpu(&fep->pdev->dev,
bdp->cbd_bufaddr,
+					pkt_len, DMA_FROM_DEVICE);
+		net_prefetch(page_address(page));
+		data = page_address(page);
+
+		if (fep->quirks & FEC_QUIRK_SWAP_FRAME)
+			swap_buffer(data, pkt_len);
+
+		eth_hdr = (struct ethhdr *)data;
+		mtip_atable_get_entry_port_number(fep,
eth_hdr->h_source,
+						  &rx_port);
+		if (rx_port == MTIP_PORT_FORWARDING_INIT)
+			mtip_atable_dynamicms_learn_migration(fep,
+
mtip_get_time(),
+
eth_hdr->h_source,
+
&rx_port); +
+		if ((rx_port == 1 || rx_port == 2) &&
fep->ndev[rx_port - 1])
+			pndev = fep->ndev[rx_port - 1];
+		else
+			pndev = dev;
+
+		*port = rx_port;
+
+		/* This does 16 byte alignment, exactly what we
need.
+		 * The packet length includes FCS, but we don't
want to
+		 * include that when passing upstream as it messes
up
+		 * bridging applications.
+		 */
+		skb = netdev_alloc_skb(pndev, pkt_len +
NET_IP_ALIGN);
+		if (unlikely(!skb)) {
+			dev_dbg(&fep->pdev->dev,
+				"%s: Memory squeeze, dropping
packet.\n",
+				pndev->name);
+			page_pool_recycle_direct(fep->page_pool,
page);
+			pndev->stats.rx_dropped++;
+			return -ENOMEM;
+		}
+
+		skb_reserve(skb, NET_IP_ALIGN);
+		skb_put(skb, pkt_len);      /* Make room */
+		skb_copy_to_linear_data(skb, data, pkt_len);
+		skb->protocol = eth_type_trans(skb, pndev);
+		skb->offload_fwd_mark = fep->br_offload;
+		napi_gro_receive(&fep->napi, skb);  

The rx buffer circulation is very odd.

The fec_main.c uses page_pool_alloc_pages() to allocate RX page from
the pool.

At the RX function the __build_skb(data, ...) is called to create skb.

Last step with the RX function is to call skb_mark_for_recycle(skb),
which sets skb->pp_recycle = 1.

And yes, in the MTIP I do copy the data to the newly created skb in RX
function (anyway, I need to swap bytes in the buffer). 

It seems like extra copy is performed in the RX function.

You seem to pre-allocate
buffers for the full ring from a page_pool. And then copy the data
out of those pages.

Yes, correct.

The normal process is that after packet is
received a new page is allocated to give to HW, and old is attached
to an skb, and sent up the stack.

Ok.

Also you are releasing the page to be recycled without clearing it
from the ring. I think you'd free it again on shutdown, so it's a
double-free.

No, the page is persistent. It will be removed when the driver is
closed and memory for pages and descriptors is released.

-- 
Best regards,

Łukasz Majewski