Re: [PATCH net-next 4/6] net: bcmgenet: add XDP_TX support

[Nicolai Buchwitz]

On 13.3.2026 12:37, Subbaraya Sundeep wrote:
> Hi,

Hi Sundeep

> On 2026-03-13 at 14:50:59, Nicolai Buchwitz (nb@xxxxxxxxxxx) wrote:
> > Implement XDP_TX by submitting XDP frames through the default TX ring
> > (DESC_INDEX). The frame is DMA-mapped and placed into a single TX
> > descriptor with SOP|EOP|APPEND_CRC flags.
> >
> > The xdp_frame pointer is stored in the TX control block so that
> > bcmgenet_free_tx_cb() can call xdp_return_frame() on TX completion,
> > returning the page to the originating page_pool.
> >
> > The page_pool DMA direction is changed from DMA_FROM_DEVICE to
> > DMA_BIDIRECTIONAL to support the TX DMA mapping of received pages.
> >
> > Signed-off-by: Nicolai Buchwitz <nb@xxxxxxxxxxx>
> > ---
> >  .../net/ethernet/broadcom/genet/bcmgenet.c    | 73 
> > ++++++++++++++++++-
> >  .../net/ethernet/broadcom/genet/bcmgenet.h    |  1 +
> >  2 files changed, 71 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c 
> > b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> > index d43729fc2b1b..373ba5878ca1 100644
> > --- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> > +++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> > @@ -1893,6 +1893,12 @@ static struct sk_buff 
> > *bcmgenet_free_tx_cb(struct device *dev,
> >  		if (cb == GENET_CB(skb)->last_cb)
> >  			return skb;
> >
> > +	} else if (cb->xdpf) {
> > +		dma_unmap_single(dev, dma_unmap_addr(cb, dma_addr),
> > +				 dma_unmap_len(cb, dma_len), DMA_TO_DEVICE);
> > +		dma_unmap_addr_set(cb, dma_addr, 0);
> > +		xdp_return_frame(cb->xdpf);
> > +		cb->xdpf = NULL;
> >  	} else if (dma_unmap_addr(cb, dma_addr)) {
> >  		dma_unmap_page(dev,
> >  			       dma_unmap_addr(cb, dma_addr),
> > @@ -2299,10 +2305,62 @@ static struct sk_buff 
> > *bcmgenet_xdp_build_skb(struct bcmgenet_rx_ring *ring,
> >  	return skb;
> >  }
> >
> > +static bool bcmgenet_xdp_xmit_frame(struct bcmgenet_priv *priv,
> > +				     struct xdp_frame *xdpf)
> > +{
> > +	struct bcmgenet_tx_ring *ring = &priv->tx_rings[DESC_INDEX];
> > +	struct device *kdev = &priv->pdev->dev;
> > +	struct enet_cb *tx_cb_ptr;
> > +	dma_addr_t mapping;
> > +	u32 len_stat;
> > +
> > +	spin_lock(&ring->lock);
> > +
> > +	if (ring->free_bds < 1) {
> > +		spin_unlock(&ring->lock);
> > +		return false;
> > +	}
> > +
> > +	tx_cb_ptr = bcmgenet_get_txcb(priv, ring);
> > +
> > +	mapping = dma_map_single(kdev, xdpf->data, xdpf->len, 
> > DMA_TO_DEVICE);
>
> AFAIU you are transmitting the frame received on a RQ which is from the 
> page pool
> and already dma mapped. Do you have to do dma_map again?
>
> Thanks,
> Sundeep

You're right. Since the page_pool is configured with DMA_BIDIRECTIONAL,
the pages are already mapped and we can reuse the existing mapping for
XDP_TX frames. The initial implementation took the simple route of
mapping everything uniformly, but that's unnecessary overhead for the
local XDP_TX case.

In v2 I'll add a bool dma_map parameter to bcmgenet_xdp_xmit_frame()
(following the mvneta/stmmac pattern): XDP_TX will reuse the page_pool
mapping via page_pool_get_dma_addr() + dma_sync_single_for_device(),
while ndo_xdp_xmit will keep dma_map_single() for foreign frames. The
cleanup path will be split accordingly.

Regards
Nicolai

> > [...]