Subject: Re: Re: [PATCH net v2] net: stmmac: fix transmit queue timed out after resume

From: "Russell King (Oracle)" <[email-masked]>
Date: Thu, 15 Jan 2026 12:09:18 +0000

On Thu, Jan 15, 2026 at 03:08:53PM +0800, Tao Wang wrote:
> > While I agree with the change for stmmac_tso_xmit(), please explain why
> > the change in stmmac_free_tx_buffer() is necessary.
> >
> > It seems to me that if this is missing in stmmac_free_tx_buffer(), the
> > driver should have more problems than just TSO.
> 
> The change in stmmac_free_tx_buffer() is intended to be generic for all
> users of last_segment, not only for the TSO path.

However, transmit is a hotpath, so work needs to be minimised for good
performance. We don't want anything that is unnecessary in these paths.

If we always explicitly set .last_segment when adding any packet to the
ring, then there is absolutely no need to also do so when freeing them.

Also, I think there's a similar issue with .is_jumbo.

So, I think it would make more sense to have some helpers for setting
up the tx_skbuff_dma entry. Maybe something like the below? I'll see
if I can measure the performance impact of this later today, but I
can't guarantee I'll get to that.

The idea here is to ensure that all members with the exception of
xsk_meta are fully initialised when an entry is populated.

I haven't removed anything in the tx_q->tx_skbuff_dma entry release
path yet, but with this in place, we should be able to eliminate the
clearance of these in stmmac_tx_clean() and stmmac_free_tx_buffer().

Note that the driver assumes setting .buf to zero means the entry is
cleared. dma_addr_t is a cookie which is device specific, and zero
may be a valid DMA cookie. Only DMA_MAPPING_ERROR is invalid, and
can be assumed to hold any meaning in driver code. So that needs
fixing as well.

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index a8a78fe7d01f..0e605d0f6a94 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -1874,6 +1874,34 @@ static int init_dma_rx_desc_rings(struct net_device *dev,
 	return ret;
 }
 
+static void stmmac_set_tx_dma_entry(struct stmmac_tx_queue *tx_q,
+				    unsigned int entry,
+				    enum stmmac_txbuf_type type,
+				    dma_addr_t addr, size_t len,
+				    bool map_as_page)
+{
+	tx_q->tx_skbuff_dma[entry].buf = addr;
+	tx_q->tx_skbuff_dma[entry].len = len;
+	tx_q->tx_skbuff_dma[entry].buf_type = type;
+	tx_q->tx_skbuff_dma[entry].map_as_page = map_as_page;
+	tx_q->tx_skbuff_dma[entry].last_segment = false;
+	tx_q->tx_skbuff_dma[entry].is_jumbo = false;
+}
+
+static void stmmac_set_tx_skb_dma_entry(struct stmmac_tx_queue *tx_q,
+					unsigned int entry, dma_addr_t addr,
+					size_t len, bool map_as_page)
+{
+	stmmac_set_tx_dma_entry(tx_q, entry, STMMAC_TXBUF_T_SKB, addr, len,
+				map_as_page);
+}
+
+static void stmmac_set_tx_dma_last_segment(struct stmmac_tx_queue *tx_q,
+					   unsigned int entry)
+{
+	tx_q->tx_skbuff_dma[entry].last_segment = true;
+}
+
 /**
  * __init_dma_tx_desc_rings - init the TX descriptor ring (per queue)
  * @priv: driver private structure
@@ -1919,11 +1947,8 @@ static int __init_dma_tx_desc_rings(struct stmmac_priv *priv,
 			p = tx_q->dma_tx + i;
 
 		stmmac_clear_desc(priv, p);
+		stmmac_set_tx_skb_dma_entry(tx_q, i, 0, 0, false);
 
-		tx_q->tx_skbuff_dma[i].buf = 0;
-		tx_q->tx_skbuff_dma[i].map_as_page = false;
-		tx_q->tx_skbuff_dma[i].len = 0;
-		tx_q->tx_skbuff_dma[i].last_segment = false;
 		tx_q->tx_skbuff[i] = NULL;
 	}
 
@@ -2649,19 +2674,15 @@ static bool stmmac_xdp_xmit_zc(struct stmmac_priv *priv, u32 queue, u32 budget)
 		meta = xsk_buff_get_metadata(pool, xdp_desc.addr);
 		xsk_buff_raw_dma_sync_for_device(pool, dma_addr, xdp_desc.len);
 
-		tx_q->tx_skbuff_dma[entry].buf_type = STMMAC_TXBUF_T_XSK_TX;
-
 		/* To return XDP buffer to XSK pool, we simple call
 		 * xsk_tx_completed(), so we don't need to fill up
 		 * 'buf' and 'xdpf'.
 		 */
-		tx_q->tx_skbuff_dma[entry].buf = 0;
-		tx_q->xdpf[entry] = NULL;
+		stmmac_set_tx_dma_entry(tx_q, entry, STMMAC_TXBUF_T_XSK_TX,
+					0, xdp_desc.len, false);
+		stmmac_set_tx_dma_last_segment(tx_q, entry);
 
-		tx_q->tx_skbuff_dma[entry].map_as_page = false;
-		tx_q->tx_skbuff_dma[entry].len = xdp_desc.len;
-		tx_q->tx_skbuff_dma[entry].last_segment = true;
-		tx_q->tx_skbuff_dma[entry].is_jumbo = false;
+		tx_q->xdpf[entry] = NULL;
 
 		stmmac_set_desc_addr(priv, tx_desc, dma_addr);
 
@@ -2836,6 +2857,9 @@ static int stmmac_tx_clean(struct stmmac_priv *priv, int budget, u32 queue,
 			tx_q->tx_skbuff_dma[entry].map_as_page = false;
 		}
 
+		/* This looks at tx_q->tx_skbuff_dma[tx_q->dirty_tx].is_jumbo
+		 * and tx_q->tx_skbuff_dma[tx_q->dirty_tx].last_segment
+		 */
 		stmmac_clean_desc3(priv, tx_q, p);
 
 		tx_q->tx_skbuff_dma[entry].last_segment = false;
@@ -4494,10 +4518,8 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
 	 * this DMA buffer right after the DMA engine completely finishes the
 	 * full buffer transmission.
 	 */
-	tx_q->tx_skbuff_dma[tx_q->cur_tx].buf = des;
-	tx_q->tx_skbuff_dma[tx_q->cur_tx].len = skb_headlen(skb);
-	tx_q->tx_skbuff_dma[tx_q->cur_tx].map_as_page = false;
-	tx_q->tx_skbuff_dma[tx_q->cur_tx].buf_type = STMMAC_TXBUF_T_SKB;
+	stmmac_set_tx_skb_dma_entry(tx_q, tx_q->cur_tx, des, skb_headlen(skb),
+				    false);
 
 	/* Prepare fragments */
 	for (i = 0; i < nfrags; i++) {
@@ -4512,17 +4534,14 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
 		stmmac_tso_allocator(priv, des, skb_frag_size(frag),
 				     (i == nfrags - 1), queue);
 
-		tx_q->tx_skbuff_dma[tx_q->cur_tx].buf = des;
-		tx_q->tx_skbuff_dma[tx_q->cur_tx].len = skb_frag_size(frag);
-		tx_q->tx_skbuff_dma[tx_q->cur_tx].map_as_page = true;
-		tx_q->tx_skbuff_dma[tx_q->cur_tx].buf_type = STMMAC_TXBUF_T_SKB;
+		stmmac_set_tx_skb_dma_entry(tx_q, tx_q->cur_tx, des,
+					    skb_frag_size(frag), true);
 	}
 
-	tx_q->tx_skbuff_dma[tx_q->cur_tx].last_segment = true;
+	stmmac_set_tx_dma_last_segment(tx_q, tx_q->cur_tx);
 
 	/* Only the last descriptor gets to point to the skb. */
 	tx_q->tx_skbuff[tx_q->cur_tx] = skb;
-	tx_q->tx_skbuff_dma[tx_q->cur_tx].buf_type = STMMAC_TXBUF_T_SKB;
 
 	/* Manage tx mitigation */
 	tx_packets = (tx_q->cur_tx + 1) - first_tx;
@@ -4774,23 +4793,18 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		if (dma_mapping_error(priv->device, des))
 			goto dma_map_err; /* should reuse desc w/o issues */
 
-		tx_q->tx_skbuff_dma[entry].buf = des;
-
+		stmmac_set_tx_skb_dma_entry(tx_q, entry, des, len, true);
 		stmmac_set_desc_addr(priv, desc, des);
 
-		tx_q->tx_skbuff_dma[entry].map_as_page = true;
-		tx_q->tx_skbuff_dma[entry].len = len;
-		tx_q->tx_skbuff_dma[entry].last_segment = last_segment;
-		tx_q->tx_skbuff_dma[entry].buf_type = STMMAC_TXBUF_T_SKB;
-
 		/* Prepare the descriptor and set the own bit too */
 		stmmac_prepare_tx_desc(priv, desc, 0, len, csum_insertion,
 				priv->mode, 1, last_segment, skb->len);
 	}
 
+	stmmac_set_tx_dma_last_segment(tx_q, entry);
+
 	/* Only the last descriptor gets to point to the skb. */
 	tx_q->tx_skbuff[entry] = skb;
-	tx_q->tx_skbuff_dma[entry].buf_type = STMMAC_TXBUF_T_SKB;
 
 	/* According to the coalesce parameter the IC bit for the latest
 	 * segment is reset and the timer re-started to clean the tx status.
@@ -4869,14 +4883,13 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		if (dma_mapping_error(priv->device, des))
 			goto dma_map_err;
 
-		tx_q->tx_skbuff_dma[first_entry].buf = des;
-		tx_q->tx_skbuff_dma[first_entry].buf_type = STMMAC_TXBUF_T_SKB;
-		tx_q->tx_skbuff_dma[first_entry].map_as_page = false;
+		stmmac_set_tx_skb_dma_entry(tx_q, first_entry, des, nopaged_len,
+					    false);
 
 		stmmac_set_desc_addr(priv, first, des);
 
-		tx_q->tx_skbuff_dma[first_entry].len = nopaged_len;
-		tx_q->tx_skbuff_dma[first_entry].last_segment = last_segment;
+		if (last_segment)
+			stmmac_set_tx_dma_last_segment(tx_q, first_entry);
 
 		if (unlikely((skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) &&
 			     priv->hwts_tx_en)) {
@@ -5064,6 +5077,7 @@ static int stmmac_xdp_xmit_xdpf(struct stmmac_priv *priv, int queue,
 	struct stmmac_tx_queue *tx_q = &priv->dma_conf.tx_queue[queue];
 	bool csum = !priv->plat->tx_queues_cfg[queue].coe_unsupported;
 	unsigned int entry = tx_q->cur_tx;
+	enum stmmac_txbuf_type buf_type;
 	struct dma_desc *tx_desc;
 	dma_addr_t dma_addr;
 	bool set_ic;
@@ -5091,7 +5105,7 @@ static int stmmac_xdp_xmit_xdpf(struct stmmac_priv *priv, int queue,
 		if (dma_mapping_error(priv->device, dma_addr))
 			return STMMAC_XDP_CONSUMED;
 
-		tx_q->tx_skbuff_dma[entry].buf_type = STMMAC_TXBUF_T_XDP_NDO;
+		buf_type = STMMAC_TXBUF_T_XDP_NDO;
 	} else {
 		struct page *page = virt_to_page(xdpf->data);
 
@@ -5100,14 +5114,12 @@ static int stmmac_xdp_xmit_xdpf(struct stmmac_priv *priv, int queue,
 		dma_sync_single_for_device(priv->device, dma_addr,
 					   xdpf->len, DMA_BIDIRECTIONAL);
 
-		tx_q->tx_skbuff_dma[entry].buf_type = STMMAC_TXBUF_T_XDP_TX;
+		buf_type = STMMAC_TXBUF_T_XDP_TX;
 	}
 
-	tx_q->tx_skbuff_dma[entry].buf = dma_addr;
-	tx_q->tx_skbuff_dma[entry].map_as_page = false;
-	tx_q->tx_skbuff_dma[entry].len = xdpf->len;
-	tx_q->tx_skbuff_dma[entry].last_segment = true;
-	tx_q->tx_skbuff_dma[entry].is_jumbo = false;
+	stmmac_set_tx_dma_entry(tx_q, entry, buf_type, dma_addr, xdpf->len,
+				false);
+	stmmac_set_tx_dma_last_segment(tx_q, entry);
 
 	tx_q->xdpf[entry] = xdpf;

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