[Intel-wired-lan] [PATCH iwl-next] e1000e: Avoid DMA re-mapping on RX copybreak

[Matt Vollrath <tactii@gmail.com>]

This patch factors out DMA re-mapping for skbs which were recycled in
the RX path due to copybreak or errors. There is only one path out of
the e1000_clean_rx_irq() loop where the skb is consumed and DMA needs
to be re-mapped, so don't unmap it before checking the conditions.

The buffer allocation loop is adjusted to not assume that DMA is
unmapped, handling mapping errors gracefully.

On systems with IOMMU enabled, the cost of re-mapping DMA is greater
than the cost of copying data out of the ring buffer. When I use this
patch and configure e1000e with copybreak=2048, my system with IOMMU
completes RX twice as fast under load.

The kludge of unconditional unmapping has existed since this driver was
introduced in 2007, inherited from the e1000 driver which has since
factored it out. IOMMU tech was new, at the time.

Tested on an I218-V.

Assisted-by: Claude:claude-4-7-opus
Signed-off-by: Matt Vollrath <tactii@gmail.com>
---
 drivers/net/ethernet/intel/e1000e/netdev.c | 34 +++++++++++++++-------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/drivers/net/ethernet/intel/e1000e/netdev.c b/drivers/net/ethernet/intel/e1000e/netdev.c
index 9befdacd6730..b1d6119171df 100644
--- a/drivers/net/ethernet/intel/e1000e/netdev.c
+++ b/drivers/net/ethernet/intel/e1000e/netdev.c
@@ -663,6 +663,8 @@ static void e1000_alloc_rx_buffers(struct e1000_ring *rx_ring,
 		skb = buffer_info->skb;
 		if (skb) {
 			skb_trim(skb, 0);
+			if (likely(buffer_info->dma))
+				goto write_desc;
 			goto map_skb;
 		}
 
@@ -680,10 +682,12 @@ static void e1000_alloc_rx_buffers(struct e1000_ring *rx_ring,
 						  DMA_FROM_DEVICE);
 		if (dma_mapping_error(&pdev->dev, buffer_info->dma)) {
 			dev_err(&pdev->dev, "Rx DMA map failed\n");
+			buffer_info->dma = 0;
 			adapter->rx_dma_failed++;
 			break;
 		}
 
+write_desc:
 		rx_desc = E1000_RX_DESC_EXT(*rx_ring, i);
 		rx_desc->read.buffer_addr = cpu_to_le64(buffer_info->dma);
 
@@ -941,7 +945,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,
 		dma_rmb();	/* read descriptor and rx_buffer_info after status DD */
 
 		skb = buffer_info->skb;
-		buffer_info->skb = NULL;
 
 		prefetch(skb->data - NET_IP_ALIGN);
 
@@ -955,9 +958,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,
 
 		cleaned = true;
 		cleaned_count++;
-		dma_unmap_single(&pdev->dev, buffer_info->dma,
-				 adapter->rx_buffer_len, DMA_FROM_DEVICE);
-		buffer_info->dma = 0;
 
 		length = le16_to_cpu(rx_desc->wb.upper.length);
 
@@ -973,8 +973,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,
 		if (adapter->flags2 & FLAG2_IS_DISCARDING) {
 			/* All receives must fit into a single buffer */
 			e_dbg("Receive packet consumed multiple buffers\n");
-			/* recycle */
-			buffer_info->skb = skb;
 			if (staterr & E1000_RXD_STAT_EOP)
 				adapter->flags2 &= ~FLAG2_IS_DISCARDING;
 			goto next_desc;
@@ -982,8 +980,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,
 
 		if (unlikely((staterr & E1000_RXDEXT_ERR_FRAME_ERR_MASK) &&
 			     !(netdev->features & NETIF_F_RXALL))) {
-			/* recycle */
-			buffer_info->skb = skb;
 			goto next_desc;
 		}
 
@@ -1010,19 +1006,35 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,
 			struct sk_buff *new_skb =
 				napi_alloc_skb(&adapter->napi, length);
 			if (new_skb) {
+				dma_sync_single_for_cpu(&pdev->dev,
+							buffer_info->dma,
+							adapter->rx_buffer_len,
+							DMA_FROM_DEVICE);
 				skb_copy_to_linear_data_offset(new_skb,
 							       -NET_IP_ALIGN,
 							       (skb->data -
 								NET_IP_ALIGN),
 							       (length +
 								NET_IP_ALIGN));
-				/* save the skb in buffer_info as good */
-				buffer_info->skb = skb;
+				dma_sync_single_for_device(&pdev->dev,
+							   buffer_info->dma,
+							   adapter->rx_buffer_len,
+							   DMA_FROM_DEVICE);
 				skb = new_skb;
 			}
 			/* else just continue with the old one */
 		}
-		/* end copybreak code */
+
+		/* If skb was not replaced by copybreak, we are consuming
+		 * the original buffer and must release the DMA mapping.
+		 */
+		if (skb == buffer_info->skb) {
+			buffer_info->skb = NULL;
+			dma_unmap_single(&pdev->dev, buffer_info->dma,
+					 adapter->rx_buffer_len,
+					 DMA_FROM_DEVICE);
+			buffer_info->dma = 0;
+		}
 		skb_put(skb, length);
 
 		/* Receive Checksum Offload */
-- 
2.43.0