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

Nicolai Buchwitz
2026-03-13T12:45:40Z

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

Hi Sundeep

>
> On 2026-03-13 at 14:50:59, Nicolai Buchwitz (nb@tipi-net.de) 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@tipi-net.de>
>> ---
>> .../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

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