[net-next PATCH RFC 5/8] net: mvneta: remove copybreak, prefetch and use build_skb

[Jesper Dangaard Brouer <brouer@redhat.com>]

From: Ilias Apalodimas <ilias.apalodimas@linaro.org>

The driver memcpy for packets < 256b and it's recycle tricks are not
needed anymore.  As previous patch introduces buffer recycling using
the page_pool API (although recycling doesn't get fully activated in
this patch).  After this switch to using build_skb().

This patch implicit fixes a driver bug where the memory is copied
prior to it's syncing for the CPU, in the < 256b case (as this case is
removed).

We also remove the data prefetch completely. The original driver had
the prefetch misplaced before any dma_sync operations took place.
Based on Jesper's analysis even if the prefetch is placed after
any DMA sync ops it ends up hurting performance.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 drivers/net/ethernet/marvell/mvneta.c |   81 +++++++++------------------------
 1 file changed, 22 insertions(+), 59 deletions(-)

diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
index 2354421fe96f..78f1fcdc1f00 100644
--- a/drivers/net/ethernet/marvell/mvneta.c
+++ b/drivers/net/ethernet/marvell/mvneta.c
@@ -643,7 +643,6 @@ static int txq_number = 8;
 static int rxq_def;
 
 static int rx_copybreak __read_mostly = 256;
-static int rx_header_size __read_mostly = 128;
 
 /* HW BM need that each port be identify by a unique ID */
 static int global_port_id;
@@ -1823,7 +1822,7 @@ static int mvneta_rx_refill(struct mvneta_port *pp,
 
 	phys_addr = page_pool_get_dma_addr(page);
 
-	phys_addr += pp->rx_offset_correction;
+	phys_addr += pp->rx_offset_correction + NET_SKB_PAD;
 	mvneta_rx_desc_fill(rx_desc, phys_addr, page, rxq);
 	return 0;
 }
@@ -1944,14 +1943,12 @@ static int mvneta_rx_swbm(struct napi_struct *napi,
 		struct page *page;
 		dma_addr_t phys_addr;
 		u32 rx_status, index;
-		int rx_bytes, skb_size, copy_size;
-		int frag_num, frag_size, frag_offset;
+		int frag_num, frag_size;
+		int rx_bytes;
 
 		index = rx_desc - rxq->descs;
 		page = (struct page *)rxq->buf_virt_addr[index];
 		data = page_address(page);
-		/* Prefetch header */
-		prefetch(data);
 
 		phys_addr = rx_desc->buf_phys_addr;
 		rx_status = rx_desc->status;
@@ -1969,49 +1966,25 @@ static int mvneta_rx_swbm(struct napi_struct *napi,
 			rx_bytes = rx_desc->data_size -
 				   (ETH_FCS_LEN + MVNETA_MH_SIZE);
 
-			/* Allocate small skb for each new packet */
-			skb_size = max(rx_copybreak, rx_header_size);
-			rxq->skb = netdev_alloc_skb_ip_align(dev, skb_size);
-			if (unlikely(!rxq->skb)) {
-				netdev_err(dev,
-					   "Can't allocate skb on queue %d\n",
-					   rxq->id);
-				dev->stats.rx_dropped++;
-				rxq->skb_alloc_err++;
-				continue;
-			}
-			copy_size = min(skb_size, rx_bytes);
-
-			/* Copy data from buffer to SKB, skip Marvell header */
-			memcpy(rxq->skb->data, data + MVNETA_MH_SIZE,
-			       copy_size);
-			skb_put(rxq->skb, copy_size);
-			rxq->left_size = rx_bytes - copy_size;
 
-			mvneta_rx_csum(pp, rx_status, rxq->skb);
-			if (rxq->left_size == 0) {
-				int size = copy_size + MVNETA_MH_SIZE;
-
-				dma_sync_single_range_for_cpu(dev->dev.parent,
-							      phys_addr, 0,
-							      size,
-							      DMA_FROM_DEVICE);
+			dma_sync_single_range_for_cpu(dev->dev.parent,
+						      phys_addr, 0,
+						      rx_bytes,
+						      DMA_FROM_DEVICE);
 
-				/* leave the descriptor and buffer untouched */
-			} else {
-				/* refill descriptor with new buffer later */
-				rx_desc->buf_phys_addr = 0;
+			rxq->skb = build_skb(data, PAGE_SIZE);
+			if (!rxq->skb)
+				break;
 
-				frag_num = 0;
-				frag_offset = copy_size + MVNETA_MH_SIZE;
-				frag_size = min(rxq->left_size,
-						(int)(PAGE_SIZE - frag_offset));
-				skb_add_rx_frag(rxq->skb, frag_num, page,
-						frag_offset, frag_size,
-						PAGE_SIZE);
-				page_pool_unmap_page(rxq->page_pool, page);
-				rxq->left_size -= frag_size;
-			}
+			rx_desc->buf_phys_addr = 0;
+			frag_num = 0;
+			skb_reserve(rxq->skb, MVNETA_MH_SIZE + NET_SKB_PAD);
+			skb_put(rxq->skb, rx_bytes < PAGE_SIZE ? rx_bytes :
+				PAGE_SIZE);
+			mvneta_rx_csum(pp, rx_status, rxq->skb);
+			page_pool_unmap_page(rxq->page_pool, page);
+			rxq->left_size = rx_bytes < PAGE_SIZE ? 0 : rx_bytes -
+				PAGE_SIZE;
 		} else {
 			/* Middle or Last descriptor */
 			if (unlikely(!rxq->skb)) {
@@ -2019,24 +1992,14 @@ static int mvneta_rx_swbm(struct napi_struct *napi,
 					 rx_status);
 				continue;
 			}
-			if (!rxq->left_size) {
-				/* last descriptor has only FCS */
-				/* and can be discarded */
-				dma_sync_single_range_for_cpu(dev->dev.parent,
-							      phys_addr, 0,
-							      ETH_FCS_LEN,
-							      DMA_FROM_DEVICE);
-				/* leave the descriptor and buffer untouched */
-			} else {
+			if (rxq->left_size) {
 				/* refill descriptor with new buffer later */
 				rx_desc->buf_phys_addr = 0;
 
 				frag_num = skb_shinfo(rxq->skb)->nr_frags;
-				frag_offset = 0;
-				frag_size = min(rxq->left_size,
-						(int)(PAGE_SIZE - frag_offset));
+				frag_size = min(rxq->left_size, (int)PAGE_SIZE);
 				skb_add_rx_frag(rxq->skb, frag_num, page,
-						frag_offset, frag_size,
+						0, frag_size,
 						PAGE_SIZE);
 
 				page_pool_unmap_page(rxq->page_pool, page);