net: stmmac: Set page_pool_params.max_len to a precise size

DMA engine will always write no more than dma_buf_sz bytes of a received
frame into a page buffer, the remaining spaces are unused or used by CPU
exclusively.
Setting page_pool_params.max_len to almost the full size of page(s) helps
nothing more, but wastes more CPU cycles on cache maintenance.

For a standard MTU of 1500, then dma_buf_sz is assigned to 1536, and this
patch brings ~16.9% driver performance improvement in a TCP RX
throughput test with iPerf tool on a single isolated Cortex-A65 CPU
core, from 2.43 Gbits/sec increased to 2.84 Gbits/sec.

Signed-off-by: Furong Xu <0x1207@gmail.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Yanteng Si <si.yanteng@linux.dev>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Furong Xu

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -2059,7 +2059,7 @@ static int __alloc_dma_rx_desc_resources(struct stmmac_priv *priv,
 	pp_params.dev = priv->device;
 	pp_params.dma_dir = xdp_prog ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;
 	pp_params.offset = stmmac_rx_offset(priv);
-	pp_params.max_len = STMMAC_MAX_RX_BUF_SIZE(num_pages);
+	pp_params.max_len = dma_conf->dma_buf_sz;
 
 	rx_q->page_pool = page_pool_create(&pp_params);
 	if (IS_ERR(rx_q->page_pool)) {

drivers/net/ethernet/stmicro/stmmac/stmmac_xdp.h

@@ -4,7 +4,6 @@
 #ifndef _STMMAC_XDP_H_
 #define _STMMAC_XDP_H_
 
-#define STMMAC_MAX_RX_BUF_SIZE(num)	(((num) * PAGE_SIZE) - XDP_PACKET_HEADROOM)
 #define STMMAC_RX_DMA_ATTR	(DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING)
 
 int stmmac_xdp_setup_pool(struct stmmac_priv *priv, struct xsk_buff_pool *pool,