Subject: Re: [PATCH RFC net-next v2 3/8] cadence: macb: Add page pool support handle multi-descriptor frame rx

From: Paolo Valerio <[email-masked]>
Date: Mon, 12 Jan 2026 19:43:14 +0100

On 12 Jan 2026 at 03:16:24 PM, Paolo Valerio <[email-masked]> wrote:

> On 08 Jan 2026 at 04:43:43 PM, Théo Lebrun <[email-masked]> wrote:
>
>> On Sun Dec 21, 2025 at 12:51 AM CET, Paolo Valerio wrote:
>>> Use the page pool allocator for the data buffers and enable skb recycling
>>> support, instead of relying on netdev_alloc_skb allocating the entire skb
>>> during the refill.
>>>
>>> The patch also add support for receiving network frames that span multiple
>>> DMA descriptors in the Cadence MACB/GEM Ethernet driver.
>>>
>>> The patch removes the requirement that limited frame reception to
>>> a single descriptor (RX_SOF && RX_EOF), also avoiding potential
>>> contiguous multi-page allocation for large frames.
>>>
>>> Signed-off-by: Paolo Valerio <[email-masked]>
>>> ---
>>>  drivers/net/ethernet/cadence/Kconfig     |   1 +
>>>  drivers/net/ethernet/cadence/macb.h      |   5 +
>>>  drivers/net/ethernet/cadence/macb_main.c | 345 +++++++++++++++--------
>>>  3 files changed, 235 insertions(+), 116 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/cadence/Kconfig b/drivers/net/ethernet/cadence/Kconfig
>>> index 5b2a461dfd28..ae500f717433 100644
>>> --- a/drivers/net/ethernet/cadence/Kconfig
>>> +++ b/drivers/net/ethernet/cadence/Kconfig
>>> @@ -25,6 +25,7 @@ config MACB
>>>  	depends on PTP_1588_CLOCK_OPTIONAL
>>>  	select PHYLINK
>>>  	select CRC32
>>> +	select PAGE_POOL
>>>  	help
>>>  	  The Cadence MACB ethernet interface is found on many Atmel AT32 and
>>>  	  AT91 parts.  This driver also supports the Cadence GEM (Gigabit
>>> diff --git a/drivers/net/ethernet/cadence/macb.h b/drivers/net/ethernet/cadence/macb.h
>>> index 3b184e9ac771..45c04157f153 100644
>>> --- a/drivers/net/ethernet/cadence/macb.h
>>> +++ b/drivers/net/ethernet/cadence/macb.h
>>> @@ -14,6 +14,8 @@
>>>  #include <linux/interrupt.h>
>>>  #include <linux/phy/phy.h>
>>>  #include <linux/workqueue.h>
>>> +#include <net/page_pool/helpers.h>
>>> +#include <net/xdp.h>
>>
>> nit: `#include <net/xdp.h>` is not needed yet.
>>
>
> ack
>
>>>  
>>>  #define MACB_GREGS_NBR 16
>>>  #define MACB_GREGS_VERSION 2
>>> @@ -1266,6 +1268,8 @@ struct macb_queue {
>>>  	void			*rx_buffers;
>>>  	struct napi_struct	napi_rx;
>>>  	struct queue_stats stats;
>>> +	struct page_pool	*page_pool;
>>> +	struct sk_buff		*skb;
>>>  };
>>>  
>>>  struct ethtool_rx_fs_item {
>>> @@ -1289,6 +1293,7 @@ struct macb {
>>>  	struct macb_dma_desc	*rx_ring_tieoff;
>>>  	dma_addr_t		rx_ring_tieoff_dma;
>>>  	size_t			rx_buffer_size;
>>> +	size_t			rx_headroom;
>>>  
>>>  	unsigned int		rx_ring_size;
>>>  	unsigned int		tx_ring_size;
>>> diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
>>> index b4e2444b2e95..9e1efc1f56d8 100644
>>> --- a/drivers/net/ethernet/cadence/macb_main.c
>>> +++ b/drivers/net/ethernet/cadence/macb_main.c
>>> @@ -1249,14 +1249,22 @@ static int macb_tx_complete(struct macb_queue *queue, int budget)
>>>  	return packets;
>>>  }
>>>  
>>> -static int gem_rx_refill(struct macb_queue *queue)
>>> +static int gem_total_rx_buffer_size(struct macb *bp)
>>> +{
>>> +	return SKB_HEAD_ALIGN(bp->rx_buffer_size + bp->rx_headroom);
>>> +}
>>
>> nit: something closer to a buffer size, either `unsigned int` or
>> `size_t`, sounds better than an int return type.
>>
>
> will do
>
>>> +
>>> +static int gem_rx_refill(struct macb_queue *queue, bool napi)
>>>  {
>>>  	unsigned int		entry;
>>> -	struct sk_buff		*skb;
>>>  	dma_addr_t		paddr;
>>> +	void			*data;
>>>  	struct macb *bp = queue->bp;
>>>  	struct macb_dma_desc *desc;
>>> +	struct page *page;
>>> +	gfp_t gfp_alloc;
>>>  	int err = 0;
>>> +	int offset;
>>>  
>>>  	while (CIRC_SPACE(queue->rx_prepared_head, queue->rx_tail,
>>>  			bp->rx_ring_size) > 0) {
>>> @@ -1268,25 +1276,20 @@ static int gem_rx_refill(struct macb_queue *queue)
>>>  		desc = macb_rx_desc(queue, entry);
>>>  
>>>  		if (!queue->rx_buff[entry]) {
>>> -			/* allocate sk_buff for this free entry in ring */
>>> -			skb = netdev_alloc_skb(bp->dev, bp->rx_buffer_size);
>>> -			if (unlikely(!skb)) {
>>> +			gfp_alloc = napi ? GFP_ATOMIC : GFP_KERNEL;
>>> +			page = page_pool_alloc_frag(queue->page_pool, &offset,
>>> +						    gem_total_rx_buffer_size(bp),
>>> +						    gfp_alloc | __GFP_NOWARN);
>>> +			if (!page) {
>>>  				netdev_err(bp->dev,
>>> -					   "Unable to allocate sk_buff\n");
>>> +					   "Unable to allocate page\n");
>>>  				err = -ENOMEM;
>>>  				break;
>>>  			}
>>>  
>>> -			/* now fill corresponding descriptor entry */
>>> -			paddr = dma_map_single(&bp->pdev->dev, skb->data,
>>> -					       bp->rx_buffer_size,
>>> -					       DMA_FROM_DEVICE);
>>> -			if (dma_mapping_error(&bp->pdev->dev, paddr)) {
>>> -				dev_kfree_skb(skb);
>>> -				break;
>>> -			}
>>> -
>>> -			queue->rx_buff[entry] = skb;
>>> +			paddr = page_pool_get_dma_addr(page) + XDP_PACKET_HEADROOM + offset;
>>> +			data = page_address(page) + offset;
>>> +			queue->rx_buff[entry] = data;
>>>  
>>>  			if (entry == bp->rx_ring_size - 1)
>>>  				paddr |= MACB_BIT(RX_WRAP);
>>> @@ -1296,20 +1299,6 @@ static int gem_rx_refill(struct macb_queue *queue)
>>>  			 */
>>>  			dma_wmb();
>>>  			macb_set_addr(bp, desc, paddr);
>>> -
>>> -			/* Properly align Ethernet header.
>>> -			 *
>>> -			 * Hardware can add dummy bytes if asked using the RBOF
>>> -			 * field inside the NCFGR register. That feature isn't
>>> -			 * available if hardware is RSC capable.
>>> -			 *
>>> -			 * We cannot fallback to doing the 2-byte shift before
>>> -			 * DMA mapping because the address field does not allow
>>> -			 * setting the low 2/3 bits.
>>> -			 * It is 3 bits if HW_DMA_CAP_PTP, else 2 bits.
>>> -			 */
>>> -			if (!(bp->caps & MACB_CAPS_RSC))
>>> -				skb_reserve(skb, NET_IP_ALIGN);
>>>  		} else {
>>>  			desc->ctrl = 0;
>>>  			dma_wmb();
>>> @@ -1353,14 +1342,19 @@ static int gem_rx(struct macb_queue *queue, struct napi_struct *napi,
>>>  	struct macb *bp = queue->bp;
>>>  	unsigned int		len;
>>>  	unsigned int		entry;
>>> -	struct sk_buff		*skb;
>>>  	struct macb_dma_desc	*desc;
>>> +	int			data_len;
>>>  	int			count = 0;
>>> +	void			*buff_head;
>>> +	struct skb_shared_info	*shinfo;
>>> +	struct page		*page;
>>> +	int			nr_frags;
>>
>> nit: you add 5 new stack variables, maybe you could apply reverse xmas
>> tree while at it. You do it for the loop body in [5/8].
>>
>
> sure
>
>>> +
>>>  
>>>  	while (count < budget) {
>>>  		u32 ctrl;
>>>  		dma_addr_t addr;
>>> -		bool rxused;
>>> +		bool rxused, first_frame;
>>>  
>>>  		entry = macb_rx_ring_wrap(bp, queue->rx_tail);
>>>  		desc = macb_rx_desc(queue, entry);
>>> @@ -1374,6 +1368,12 @@ static int gem_rx(struct macb_queue *queue, struct napi_struct *napi,
>>>  		if (!rxused)
>>>  			break;
>>>  
>>> +		if (!(bp->caps & MACB_CAPS_RSC))
>>> +			addr += NET_IP_ALIGN;
>>> +
>>> +		dma_sync_single_for_cpu(&bp->pdev->dev,
>>> +					addr, bp->rx_buffer_size,
>>> +					page_pool_get_dma_dir(queue->page_pool));
>>>  		/* Ensure ctrl is at least as up-to-date as rxused */
>>>  		dma_rmb();
>>>  
>>> @@ -1382,58 +1382,118 @@ static int gem_rx(struct macb_queue *queue, struct napi_struct *napi,
>>>  		queue->rx_tail++;
>>>  		count++;
>>>  
>>> -		if (!(ctrl & MACB_BIT(RX_SOF) && ctrl & MACB_BIT(RX_EOF))) {
>>> -			netdev_err(bp->dev,
>>> -				   "not whole frame pointed by descriptor\n");
>>> -			bp->dev->stats.rx_dropped++;
>>> -			queue->stats.rx_dropped++;
>>> -			break;
>>> -		}
>>> -		skb = queue->rx_buff[entry];
>>> -		if (unlikely(!skb)) {
>>> +		buff_head = queue->rx_buff[entry];
>>> +		if (unlikely(!buff_head)) {
>>>  			netdev_err(bp->dev,
>>>  				   "inconsistent Rx descriptor chain\n");
>>>  			bp->dev->stats.rx_dropped++;
>>>  			queue->stats.rx_dropped++;
>>>  			break;
>>>  		}
>>> -		/* now everything is ready for receiving packet */
>>> -		queue->rx_buff[entry] = NULL;
>>> +
>>> +		first_frame = ctrl & MACB_BIT(RX_SOF);
>>>  		len = ctrl & bp->rx_frm_len_mask;
>>>  
>>> -		netdev_vdbg(bp->dev, "gem_rx %u (len %u)\n", entry, len);
>>> +		if (len) {
>>> +			data_len = len;
>>> +			if (!first_frame)
>>> +				data_len -= queue->skb->len;
>>> +		} else {
>>> +			data_len = bp->rx_buffer_size;
>>> +		}
>>
>> Why deal with the `!len` case? How can it occur? User guide doesn't hint
>> that. It would mean we would grab uninitialised bytes as we assume len
>> is the max buffer size.
>>
>
> Good point. After taking a second look, !len may not be the most reliable
> way to check this.
> From the datasheet, status signals are only valid (with some exceptions)
> when MACB_BIT(RX_EOF) is set. As a side effect, len is always zero on my
> hw for frames without the EOF bit, but it's probably better to just rely
> on MACB_BIT(RX_EOF) instead of reading something that may end up being
> unreliable.
>
>>> +
>>> +		if (first_frame) {
>>> +			queue->skb = napi_build_skb(buff_head, gem_total_rx_buffer_size(bp));
>>> +			if (unlikely(!queue->skb)) {
>>> +				netdev_err(bp->dev,
>>> +					   "Unable to allocate sk_buff\n");
>>> +				goto free_frags;
>>> +			}
>>> +
>>> +			/* Properly align Ethernet header.
>>> +			 *
>>> +			 * Hardware can add dummy bytes if asked using the RBOF
>>> +			 * field inside the NCFGR register. That feature isn't
>>> +			 * available if hardware is RSC capable.
>>> +			 *
>>> +			 * We cannot fallback to doing the 2-byte shift before
>>> +			 * DMA mapping because the address field does not allow
>>> +			 * setting the low 2/3 bits.
>>> +			 * It is 3 bits if HW_DMA_CAP_PTP, else 2 bits.
>>> +			 */
>>> +			skb_reserve(queue->skb, bp->rx_headroom);
>>> +			skb_mark_for_recycle(queue->skb);
>>> +			skb_put(queue->skb, data_len);
>>> +			queue->skb->protocol = eth_type_trans(queue->skb, bp->dev);
>>> +
>>> +			skb_checksum_none_assert(queue->skb);
>>> +			if (bp->dev->features & NETIF_F_RXCSUM &&
>>> +			    !(bp->dev->flags & IFF_PROMISC) &&
>>> +			    GEM_BFEXT(RX_CSUM, ctrl) & GEM_RX_CSUM_CHECKED_MASK)
>>> +				queue->skb->ip_summed = CHECKSUM_UNNECESSARY;
>>> +		} else {
>>> +			if (!queue->skb) {
>>> +				netdev_err(bp->dev,
>>> +					   "Received non-starting frame while expecting it\n");
>>> +				goto free_frags;
>>> +			}
>>> +
>>> +			shinfo = skb_shinfo(queue->skb);
>>> +			page = virt_to_head_page(buff_head);
>>> +			nr_frags = shinfo->nr_frags;
>>> +
>>> +			if (nr_frags >= ARRAY_SIZE(shinfo->frags))
>>> +				goto free_frags;
>>>  
>>> -		skb_put(skb, len);
>>> -		dma_unmap_single(&bp->pdev->dev, addr,
>>> -				 bp->rx_buffer_size, DMA_FROM_DEVICE);
>>> +			skb_add_rx_frag(queue->skb, nr_frags, page,
>>> +					buff_head - page_address(page) + bp->rx_headroom,
>>> +					data_len, gem_total_rx_buffer_size(bp));
>>> +		}
>>> +
>>> +		/* now everything is ready for receiving packet */
>>> +		queue->rx_buff[entry] = NULL;
>>>  
>>> -		skb->protocol = eth_type_trans(skb, bp->dev);
>>> -		skb_checksum_none_assert(skb);
>>> -		if (bp->dev->features & NETIF_F_RXCSUM &&
>>> -		    !(bp->dev->flags & IFF_PROMISC) &&
>>> -		    GEM_BFEXT(RX_CSUM, ctrl) & GEM_RX_CSUM_CHECKED_MASK)
>>> -			skb->ip_summed = CHECKSUM_UNNECESSARY;
>>> +		netdev_vdbg(bp->dev, "%s %u (len %u)\n", __func__, entry, data_len);
>>>  
>>> -		bp->dev->stats.rx_packets++;
>>> -		queue->stats.rx_packets++;
>>> -		bp->dev->stats.rx_bytes += skb->len;
>>> -		queue->stats.rx_bytes += skb->len;
>>> +		if (ctrl & MACB_BIT(RX_EOF)) {
>>> +			bp->dev->stats.rx_packets++;
>>> +			queue->stats.rx_packets++;
>>> +			bp->dev->stats.rx_bytes += queue->skb->len;
>>> +			queue->stats.rx_bytes += queue->skb->len;
>>>  
>>> -		gem_ptp_do_rxstamp(bp, skb, desc);
>>> +			gem_ptp_do_rxstamp(bp, queue->skb, desc);
>>>  
>>>  #if defined(DEBUG) && defined(VERBOSE_DEBUG)
>>> -		netdev_vdbg(bp->dev, "received skb of length %u, csum: %08x\n",
>>> -			    skb->len, skb->csum);
>>> -		print_hex_dump(KERN_DEBUG, " mac: ", DUMP_PREFIX_ADDRESS, 16, 1,
>>> -			       skb_mac_header(skb), 16, true);
>>> -		print_hex_dump(KERN_DEBUG, "data: ", DUMP_PREFIX_ADDRESS, 16, 1,
>>> -			       skb->data, 32, true);
>>> +			netdev_vdbg(bp->dev, "received skb of length %u, csum: %08x\n",
>>> +				    queue->skb->len, queue->skb->csum);
>>> +			print_hex_dump(KERN_DEBUG, " mac: ", DUMP_PREFIX_ADDRESS, 16, 1,
>>> +				       skb_mac_header(queue->skb), 16, true);
>>> +			print_hex_dump(KERN_DEBUG, "buff_head: ", DUMP_PREFIX_ADDRESS, 16, 1,
>>> +				       queue->skb->buff_head, 32, true);
>>>  #endif
>>
>> nit: while you are at it, maybe replace with print_hex_dump_debug()?
>>
>
> sure
>
>>>  
>>> -		napi_gro_receive(napi, skb);
>>> +			napi_gro_receive(napi, queue->skb);
>>> +			queue->skb = NULL;
>>> +		}
>>> +
>>> +		continue;
>>> +
>>> +free_frags:
>>> +		if (queue->skb) {
>>> +			dev_kfree_skb(queue->skb);
>>> +			queue->skb = NULL;
>>> +		} else {
>>> +			page_pool_put_full_page(queue->page_pool,
>>> +						virt_to_head_page(buff_head),
>>> +						false);
>>> +		}
>>> +
>>> +		bp->dev->stats.rx_dropped++;
>>> +		queue->stats.rx_dropped++;
>>> +		queue->rx_buff[entry] = NULL;
>>>  	}
>>>  
>>> -	gem_rx_refill(queue);
>>> +	gem_rx_refill(queue, true);
>>>  
>>>  	return count;
>>>  }
>>> @@ -2367,12 +2427,25 @@ static netdev_tx_t macb_start_xmit(struct sk_buff *skb, struct net_device *dev)
>>>  	return ret;
>>>  }
>>>  
>>> -static void macb_init_rx_buffer_size(struct macb *bp, size_t size)
>>> +static void macb_init_rx_buffer_size(struct macb *bp, unsigned int mtu)
>>>  {
>>> +	int overhead;
>>
>> nit: Maybe `unsigned int` or `size_t` rather than `int`?
>>
>
> ack
>
>>> +	size_t size;
>>> +
>>>  	if (!macb_is_gem(bp)) {
>>>  		bp->rx_buffer_size = MACB_RX_BUFFER_SIZE;
>>>  	} else {
>>> -		bp->rx_buffer_size = size;
>>> +		size = mtu + ETH_HLEN + ETH_FCS_LEN;
>>> +		if (!(bp->caps & MACB_CAPS_RSC))
>>> +			size += NET_IP_ALIGN;
>>
>> NET_IP_ALIGN looks like it is accounted for twice, once in
>> bp->rx_headroom and once in bp->rx_buffer_size. This gets fixed in
>> [5/8] where gem_max_rx_data_size() gets introduced.
>>
>
> ah, right
>
>>> +
>>> +		bp->rx_buffer_size = SKB_DATA_ALIGN(size);
>>> +		if (gem_total_rx_buffer_size(bp) > PAGE_SIZE) {
>>> +			overhead = bp->rx_headroom +
>>> +				SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>>> +			bp->rx_buffer_size = rounddown(PAGE_SIZE - overhead,
>>> +						       RX_BUFFER_MULTIPLE);
>>> +		}
>>
>> I've seen your comment in [0/8]. Do you have any advice on how to test
>> this clamping? All I can think of is to either configure a massive MTU
>> or, more easily, cheat with the headroom.
>>
>
> I normally test the set with 4k PAGE_SIZE and, as you said, setting the
> mtu to something bigger than that. This is still possible with 8k pages
> (given .jumbo_max_len = 10240).
>
>
>> Also, should we warn? It means MTU-sized packets will be received in
>> fragments. It will work but is probably unexpected by users and a
>> slowdown reason that users might want to know about.
>>
>
> I'm not sure about the warning as I don't see this as a user level detail.
> For debugging purpose, I guess we should be fine the last print out (even
> better once extended with your suggestion). Of course, feel free to disagree.
>
>> --
>>
>> nit: while in macb_init_rx_buffer_size(), can you tweak the debug line
>> from mtu & rx_buffer_size to also have rx_headroom and total? So that
>> we have everything available to understand what is going on buffer size
>> wise. Something like:
>>
>> -       netdev_dbg(bp->dev, "mtu [%u] rx_buffer_size [%zu]\n",
>> -                  bp->dev->mtu, bp->rx_buffer_size);
>> +       netdev_info(bp->dev, "mtu [%u] rx_buffer_size [%zu] rx_headroom [%zu] total [%u]\n",
>> +                   bp->dev->mtu, bp->rx_buffer_size, bp->rx_headroom,
>> +                   gem_total_rx_buffer_size(bp));
>>

I missed this before:
I assume so, but just checking, is the promotion from dbg to info also
wanted?

>> Thanks,
>>
>> --
>> Théo Lebrun, Bootlin
>> Embedded Linux and Kernel engineering
>> https://bootlin.com