Re: [PATCH net-next v2 4/5] net: lan966x: Add support for XDP_TX

Horatiu Vultur
2022-11-16T20:51:22Z

The 11/16/2022 16:34, Alexander Lobakin wrote:
>
> From: Horatiu Vultur <horatiu.vultur@microchip.com>
> Date: Tue, 15 Nov 2022 22:44:55 +0100

Hi Olek,

>
> Extend lan966x XDP support with the action XDP_TX. In this case when the
> received buffer needs to execute XDP_TX, the buffer will be moved to the
> TX buffers. So a new RX buffer will be allocated.
> When the TX finish with the frame, it would release completely this
> buffer.
>
> Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
> ---
> .../ethernet/microchip/lan966x/lan966x_fdma.c | 78 +++++++++++++++++--
> .../ethernet/microchip/lan966x/lan966x_main.c | 4 +-
> .../ethernet/microchip/lan966x/lan966x_main.h | 8 ++
> .../ethernet/microchip/lan966x/lan966x_xdp.c | 8 ++
> 4 files changed, 91 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> index 384ed34197d58..c2e56233a8da5 100644
> --- a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> +++ b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> @@ -394,13 +394,21 @@ static void lan966x_fdma_tx_clear_buf(struct lan966x *lan966x, int weight)
> dcb_buf->dev->stats.tx_bytes += dcb_buf->len;
>
> dcb_buf->used = false;
> - dma_unmap_single(lan966x->dev,
> - dcb_buf->dma_addr,
> - dcb_buf->len,
> - DMA_TO_DEVICE);
> - if (!dcb_buf->ptp)
> + if (dcb_buf->skb)
> + dma_unmap_single(lan966x->dev,
> + dcb_buf->dma_addr,
> + dcb_buf->len,
> + DMA_TO_DEVICE);
> +
> + if (dcb_buf->skb && !dcb_buf->ptp)
> dev_kfree_skb_any(dcb_buf->skb);
>
> + if (dcb_buf->page) {
> + page_pool_release_page(lan966x->rx.page_pool,
> + dcb_buf->page);
> + put_page(dcb_buf->page);
> + }
>
> Hmm, that's not really correct.
>
> For skb, you need to unmap + free, true (BPW, just use
> napi_consume_skb()).

What does BPW stand for?
Yes, I can use napi_consume_skb instead of dev_kfree_skb_any();

> For %XDP_TX, as you use Page Pool, you don't need to unmap, but you
> need to do xdp_return_frame{,_bulk}. Plus, as Tx is being done here
> directly from an Rx NAPI polling cycle, xdp_return_frame_rx_napi()
> is usually used. Anyway, each of xdp_return_frame()'s variants will
> call page_pool_put_full_page() for you.

If I understand correctly this part that you describe, the page will
be added back in the page_pool cache. While in my case, I am giving
back the page to the page allocator. In this way the page_pool needs
to allocate more pages every time when the action XDP_TX is happening.

BTW, this shows that there is a missing xdp_rxq_info_reg_mem_model call,
because when calling xdp_return_frame_rx_napi, the frame was not going
to page_pool but the was simply freed because xdp_mem_info was the wrong
type.

> For %XDP_REDIRECT, as you don't know the source of the XDP frame,

Why I don't know the source?
Will it not be from an RX page that is allocated by Page Pool?

> you need to unmap it (as it was previously mapped in
> ::ndo_xdp_xmit()), plus call xdp_return_frame{,_bulk} to free the
> XDP frame. Note that _rx_napi() variant is not applicable here.
>
> That description might be confusing, so you can take a look at the
> already existing code[0] to get the idea. I think this piece shows
> the expected logics rather well.

I think you forgot to write the link to the code.
I looked also at different drivers but I didn't figure it out why the
frame needed to be mapped and where is happening that.

>
> +
> clear = true;
> }
>
> @@ -532,6 +540,9 @@ static int lan966x_fdma_napi_poll(struct napi_struct *napi, int weight)
> lan966x_fdma_rx_free_page(rx);
> lan966x_fdma_rx_advance_dcb(rx);
> goto allocate_new;
> + case FDMA_TX:
> + lan966x_fdma_rx_advance_dcb(rx);
> + continue;
> case FDMA_DROP:
> lan966x_fdma_rx_free_page(rx);
> lan966x_fdma_rx_advance_dcb(rx);
> @@ -653,6 +664,62 @@ static void lan966x_fdma_tx_start(struct lan966x_tx *tx, int next_to_use)
> tx->last_in_use = next_to_use;
> }
>
> +int lan966x_fdma_xmit_xdpf(struct lan966x_port *port,
> + struct xdp_frame *xdpf,
> + struct page *page)
> +{
> + struct lan966x *lan966x = port->lan966x;
> + struct lan966x_tx_dcb_buf *next_dcb_buf;
> + struct lan966x_tx *tx = &lan966x->tx;
> + dma_addr_t dma_addr;
> + int next_to_use;
> + __be32 *ifh;
> + int ret = 0;
> +
> + spin_lock(&lan966x->tx_lock);
> +
> + /* Get next index */
> + next_to_use = lan966x_fdma_get_next_dcb(tx);
> + if (next_to_use < 0) {
> + netif_stop_queue(port->dev);
> + ret = NETDEV_TX_BUSY;
> + goto out;
> + }
> +
> + /* Generate new IFH */
> + ifh = page_address(page) + XDP_PACKET_HEADROOM;
> + memset(ifh, 0x0, sizeof(__be32) * IFH_LEN);
> + lan966x_ifh_set_bypass(ifh, 1);
> + lan966x_ifh_set_port(ifh, BIT_ULL(port->chip_port));
> +
> + dma_addr = page_pool_get_dma_addr(page);
> + dma_sync_single_for_device(lan966x->dev, dma_addr + XDP_PACKET_HEADROOM,
> + xdpf->len + IFH_LEN_BYTES,
> + DMA_TO_DEVICE);
>
> Also not correct. This page was mapped with %DMA_FROM_DEVICE in the
> Rx code, now you sync it for the opposite.
> Most drivers in case of XDP enabled create Page Pools with ::dma_dir
> set to %DMA_BIDIRECTIONAL. Now you would need only to sync it here
> with the same direction (bidir) and that's it.

That is a really good catch!
I was wondering why the things were working when I tested this. Because
definitely, I can see the right behaviour.

>
> +
> + /* Setup next dcb */
> + lan966x_fdma_tx_setup_dcb(tx, next_to_use, xdpf->len + IFH_LEN_BYTES,
> + dma_addr + XDP_PACKET_HEADROOM);
> +
> + /* Fill up the buffer */
> + next_dcb_buf = &tx->dcbs_buf[next_to_use];
> + next_dcb_buf->skb = NULL;
> + next_dcb_buf->page = page;
> + next_dcb_buf->len = xdpf->len + IFH_LEN_BYTES;
> + next_dcb_buf->dma_addr = dma_addr;
> + next_dcb_buf->used = true;
> + next_dcb_buf->ptp = false;
> + next_dcb_buf->dev = port->dev;
> +
> + /* Start the transmission */
> + lan966x_fdma_tx_start(tx, next_to_use);
> +
> +out:
> + spin_unlock(&lan966x->tx_lock);
> +
> + return ret;
> +}
> +
> int lan966x_fdma_xmit(struct sk_buff *skb, __be32 *ifh, struct net_device *dev)
> {
> struct lan966x_port *port = netdev_priv(dev);
> @@ -709,6 +776,7 @@ int lan966x_fdma_xmit(struct sk_buff *skb, __be32 *ifh, struct net_device *dev)
> /* Fill up the buffer */
> next_dcb_buf = &tx->dcbs_buf[next_to_use];
> next_dcb_buf->skb = skb;
> + next_dcb_buf->page = NULL;
> next_dcb_buf->len = skb->len;
> next_dcb_buf->dma_addr = dma_addr;
> next_dcb_buf->used = true;
>
> [...]
>
> --
> 2.38.0
>
> Thanks,
> Olek

--
/Horatiu