Subject: Re: [PATCH RFC net-next 2/6] cadence: macb/gem: handle multi-descriptor frame reception

From: Paolo Valerio <[email-masked]>
Date: Tue, 02 Dec 2025 18:32:10 +0100

On 27 Nov 2025 at 02:38:45 PM, Théo Lebrun <[email-masked]> wrote:

> On Wed Nov 19, 2025 at 2:53 PM CET, Paolo Valerio wrote:
>> 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. It also uses
>> page pool fragments allocation instead of full page for saving
>> memory.
>>
>> Signed-off-by: Paolo Valerio <[email-masked]>
>> ---
>>  drivers/net/ethernet/cadence/macb.h      |   4 +-
>>  drivers/net/ethernet/cadence/macb_main.c | 180 ++++++++++++++---------
>>  2 files changed, 111 insertions(+), 73 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/cadence/macb.h b/drivers/net/ethernet/cadence/macb.h
>> index dcf768bd1bc1..e2f397b7a27f 100644
>> --- a/drivers/net/ethernet/cadence/macb.h
>> +++ b/drivers/net/ethernet/cadence/macb.h
>> @@ -960,8 +960,7 @@ struct macb_dma_desc_ptp {
>>  #define PPM_FRACTION	16
>>  
>>  /* The buf includes headroom compatible with both skb and xdpf */
>> -#define MACB_PP_HEADROOM		XDP_PACKET_HEADROOM
>> -#define MACB_PP_MAX_BUF_SIZE(num)	(((num) * PAGE_SIZE) - MACB_PP_HEADROOM)
>> +#define MACB_PP_HEADROOM	XDP_PACKET_HEADROOM
>>  
>>  /* struct macb_tx_skb - data about an skb which is being transmitted
>>   * @skb: skb currently being transmitted, only set for the last buffer
>> @@ -1273,6 +1272,7 @@ struct macb_queue {
>>  	struct napi_struct	napi_rx;
>>  	struct queue_stats stats;
>>  	struct page_pool	*page_pool;
>> +	struct sk_buff		*skb;
>>  };
>>  
>>  struct ethtool_rx_fs_item {
>> diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
>> index 985c81913ba6..be0c8e101639 100644
>> --- a/drivers/net/ethernet/cadence/macb_main.c
>> +++ b/drivers/net/ethernet/cadence/macb_main.c
>> @@ -1250,21 +1250,25 @@ static int macb_tx_complete(struct macb_queue *queue, int budget)
>>  	return packets;
>>  }
>>  
>> -static void *gem_page_pool_get_buff(struct page_pool *pool,
>> +static void *gem_page_pool_get_buff(struct  macb_queue *queue,
>>  				    dma_addr_t *dma_addr, gfp_t gfp_mask)
>>  {
>> +	struct macb *bp = queue->bp;
>>  	struct page *page;
>> +	int offset;
>>  
>> -	if (!pool)
>> +	if (!queue->page_pool)
>>  		return NULL;
>>  
>> -	page = page_pool_alloc_pages(pool, gfp_mask | __GFP_NOWARN);
>> +	page = page_pool_alloc_frag(queue->page_pool, &offset,
>> +				    bp->rx_buffer_size,
>> +				    gfp_mask | __GFP_NOWARN);
>>  	if (!page)
>>  		return NULL;
>>  
>> -	*dma_addr = page_pool_get_dma_addr(page) + MACB_PP_HEADROOM;
>> +	*dma_addr = page_pool_get_dma_addr(page) + MACB_PP_HEADROOM + offset;
>>  
>> -	return page_address(page);
>> +	return page_address(page) + offset;
>>  }
>>  
>>  static void gem_rx_refill(struct macb_queue *queue, bool napi)
>> @@ -1286,7 +1290,7 @@ static void gem_rx_refill(struct macb_queue *queue, bool napi)
>>  
>>  		if (!queue->rx_buff[entry]) {
>>  			/* allocate sk_buff for this free entry in ring */
>> -			data = gem_page_pool_get_buff(queue->page_pool, &paddr,
>> +			data = gem_page_pool_get_buff(queue, &paddr,
>>  						      napi ? GFP_ATOMIC : GFP_KERNEL);
>>  			if (unlikely(!data)) {
>>  				netdev_err(bp->dev,
>> @@ -1344,20 +1348,17 @@ static int gem_rx(struct macb_queue *queue, struct napi_struct *napi,
>>  		  int budget)
>>  {
>>  	struct macb *bp = queue->bp;
>> -	int			buffer_size;
>>  	unsigned int		len;
>>  	unsigned int		entry;
>>  	void			*data;
>> -	struct sk_buff		*skb;
>>  	struct macb_dma_desc	*desc;
>> +	int			data_len;
>>  	int			count = 0;
>>  
>> -	buffer_size = DIV_ROUND_UP(bp->rx_buffer_size, PAGE_SIZE) * PAGE_SIZE;
>> -
>>  	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);
>> @@ -1368,6 +1369,9 @@ static int gem_rx(struct macb_queue *queue, struct napi_struct *napi,
>>  		rxused = (desc->addr & MACB_BIT(RX_USED)) ? true : false;
>>  		addr = macb_get_addr(bp, desc);
>>  
>> +		dma_sync_single_for_cpu(&bp->pdev->dev,
>> +					addr, bp->rx_buffer_size - bp->rx_offset,
>> +					page_pool_get_dma_dir(queue->page_pool));
>
> page_pool_get_dma_dir() will always return the same value, we could
> hardcode it.
>
>>  		if (!rxused)
>>  			break;
>>  
>> @@ -1379,13 +1383,6 @@ 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;
>> -		}
>>  		data = queue->rx_buff[entry];
>>  		if (unlikely(!data)) {
>>  			netdev_err(bp->dev,
>> @@ -1395,64 +1392,102 @@ static int gem_rx(struct macb_queue *queue, struct napi_struct *napi,
>>  			break;
>>  		}
>>  
>> -		skb = napi_build_skb(data, buffer_size);
>> -		if (unlikely(!skb)) {
>> -			netdev_err(bp->dev,
>> -				   "Unable to allocate sk_buff\n");
>> -			page_pool_put_full_page(queue->page_pool,
>> -						virt_to_head_page(data),
>> -						false);
>> -			break;
>> +		first_frame = ctrl & MACB_BIT(RX_SOF);
>> +		len = ctrl & bp->rx_frm_len_mask;
>> +
>> +		if (len) {
>> +			data_len = len;
>> +			if (!first_frame)
>> +				data_len -= queue->skb->len;
>> +		} else {
>> +			data_len = SKB_WITH_OVERHEAD(bp->rx_buffer_size) - bp->rx_offset;
>>  		}
>>  
>> -		/* 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(skb, bp->rx_offset);
>> -		skb_mark_for_recycle(skb);
>> +		if (first_frame) {
>> +			queue->skb = napi_build_skb(data, bp->rx_buffer_size);
>> +			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_offset);
>> +			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;
>> +			}
>
> Here as well we want `queue->rx_buff[entry] = NULL;` no?
> Maybe put it in the free_frags codepath to ensure it isn't forgotten?
>

That's correct, that slipped in this RFC.

>> +			struct skb_shared_info *shinfo = skb_shinfo(queue->skb);
>> +			struct page *page = virt_to_head_page(data);
>> +			int nr_frags = shinfo->nr_frags;
>
> Variable definitions in the middle of a scope? I think it is acceptable
> when using __attribute__((cleanup)) but otherwise not so much (yet).
>

I guess I simply forgot to move them.
Ack for this and for the previous ones.

>> +			if (nr_frags >= ARRAY_SIZE(shinfo->frags))
>> +				goto free_frags;
>> +
>> +			skb_add_rx_frag(queue->skb, nr_frags, page,
>> +					data - page_address(page) + bp->rx_offset,
>> +					data_len, bp->rx_buffer_size);
>> +		}
>>  
>>  		/* now everything is ready for receiving packet */
>>  		queue->rx_buff[entry] = NULL;
>> -		len = ctrl & bp->rx_frm_len_mask;
>> -
>> -		netdev_vdbg(bp->dev, "gem_rx %u (len %u)\n", entry, len);
>>  
>> -		dma_sync_single_for_cpu(&bp->pdev->dev,
>> -					addr, len,
>> -					page_pool_get_dma_dir(queue->page_pool));
>> -		skb_put(skb, len);
>> -		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, "data: ", DUMP_PREFIX_ADDRESS, 16, 1,
>> +				       queue->skb->data, 32, true);
>>  #endif
>>  
>> -		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(data),
>> +						false);
>> +		}
>>  	}
>>  
>>  	gem_rx_refill(queue, true);
>> @@ -2394,7 +2429,10 @@ static void macb_init_rx_buffer_size(struct macb *bp, size_t size)
>>  	if (!macb_is_gem(bp)) {
>>  		bp->rx_buffer_size = MACB_RX_BUFFER_SIZE;
>>  	} else {
>> -		bp->rx_buffer_size = size;
>> +		bp->rx_buffer_size = size + SKB_DATA_ALIGN(sizeof(struct skb_shared_info))
>> +			+ MACB_PP_HEADROOM;
>> +		if (bp->rx_buffer_size > PAGE_SIZE)
>> +			bp->rx_buffer_size = PAGE_SIZE;
>>  
>>  		if (bp->rx_buffer_size % RX_BUFFER_MULTIPLE) {
>>  			netdev_dbg(bp->dev,
>> @@ -2589,18 +2627,15 @@ static int macb_alloc_consistent(struct macb *bp)
>>  
>>  static void gem_create_page_pool(struct macb_queue *queue)
>>  {
>> -	unsigned int num_pages = DIV_ROUND_UP(queue->bp->rx_buffer_size, PAGE_SIZE);
>> -	struct macb *bp = queue->bp;
>>  	struct page_pool_params pp_params = {
>> -		.order = order_base_2(num_pages),
>> +		.order = 0,
>>  		.flags = PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV,
>>  		.pool_size = queue->bp->rx_ring_size,
>>  		.nid = NUMA_NO_NODE,
>>  		.dma_dir = DMA_FROM_DEVICE,
>>  		.dev = &queue->bp->pdev->dev,
>>  		.napi = &queue->napi_rx,
>> -		.offset = bp->rx_offset,
>> -		.max_len = MACB_PP_MAX_BUF_SIZE(num_pages),
>> +		.max_len = PAGE_SIZE,
>>  	};
>>  	struct page_pool *pool;
>
> I forgot about it in [PATCH 1/6], but the error handling if
> gem_create_page_pool() fails is odd. We set queue->page_pool to NULL
> and keep on going. Then once opened we'll fail allocating any buffer
> but still be open. Shouldn't we fail the link up operation?
>
> If we want to support this codepath (page pool not allocated), then we
> should unmask Rx interrupts only if alloc succeeded. I don't know if
> we'd want that though.
>
> 	queue_writel(queue, IER, bp->rx_intr_mask | ...);
>

Makes sense to fail the link up.
Doesn't this imply to move the page pool creation and refill into
macb_open()?

I didn't look into it, I'm not sure if this can potentially become a
bigger change.

>> @@ -2784,8 +2819,9 @@ static void macb_configure_dma(struct macb *bp)
>>  	unsigned int q;
>>  	u32 dmacfg;
>>  
>> -	buffer_size = bp->rx_buffer_size / RX_BUFFER_MULTIPLE;
>>  	if (macb_is_gem(bp)) {
>> +		buffer_size = SKB_WITH_OVERHEAD(bp->rx_buffer_size) - bp->rx_offset;
>> +		buffer_size /= RX_BUFFER_MULTIPLE;
>>  		dmacfg = gem_readl(bp, DMACFG) & ~GEM_BF(RXBS, -1L);
>>  		for (q = 0, queue = bp->queues; q < bp->num_queues; ++q, ++queue) {
>>  			if (q)
>> @@ -2816,6 +2852,8 @@ static void macb_configure_dma(struct macb *bp)
>>  		netdev_dbg(bp->dev, "Cadence configure DMA with 0x%08x\n",
>>  			   dmacfg);
>>  		gem_writel(bp, DMACFG, dmacfg);
>> +	} else {
>> +		buffer_size = bp->rx_buffer_size / RX_BUFFER_MULTIPLE;
>>  	}
>>  }
>
> You do:
>
> static void macb_configure_dma(struct macb *bp)
> {
> 	u32 buffer_size;
>
> 	if (macb_is_gem(bp)) {
> 		buffer_size = SKB_WITH_OVERHEAD(bp->rx_buffer_size) - bp->rx_offset;
> 		buffer_size /= RX_BUFFER_MULTIPLE;
> 		// ... use buffer_size ...
> 	} else {
> 		buffer_size = bp->rx_buffer_size / RX_BUFFER_MULTIPLE;
> 	}
> }
>
> Instead I'd vote for:
>
> static void macb_configure_dma(struct macb *bp)
> {
> 	u32 buffer_size;
>
> 	if (!macb_is_gem(bp))
> 		return;
>
> 	buffer_size = SKB_WITH_OVERHEAD(bp->rx_buffer_size) - bp->rx_offset;
> 	buffer_size /= RX_BUFFER_MULTIPLE;
> 	// ... use buffer_size ...
> }
>
> Thanks,
>

I vote for this as well :)

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