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

[Alexander Lobakin <alexandr.lobakin@intel.com>]

From: Horatiu Vultur <horatiu.vultur@microchip.com>
Date: Tue, 15 Nov 2022 22:44:55 +0100

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()).
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.
For %XDP_REDIRECT, as you don't know the source of the XDP frame,
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.

+
 		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.

+
+	/* 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