linux-kernel - [PATCH v4 14/15] can: grcan: Update echo skb handling to match variable length CANFD frame

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Message-ID: <20260128144921.5458-15-arun.muthusamy@gaisler.com>
Date: Wed, 28 Jan 2026 15:49:20 +0100
From: Arun Muthusamy <arun.muthusamy@...sler.com>
To: robh@...nel.org,
	krzk+dt@...nel.org,
	conor+dt@...nel.org,
	mkl@...gutronix.de,
	mailhol@...nel.org
Cc: devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-can@...r.kernel.org,
	Arun Muthusamy <arun.muthusamy@...sler.com>
Subject: [PATCH v4 14/15] can: grcan: Update echo skb handling to match variable length CANFD frame

Refactor echo socket buffer management by introducing dedicated indices
for current and next echo slots.

- Introduce "echo_skb_idx" to keep track of the current packet index in
  the echo buffer, and "next_echo_idx" for the next available slot.
- Adjust memory allocation for echo skb to calculate the number of slots
  based on slot size.
- Enhance logic in catch_up_echo_skb() to correctly process and free echo
  skbs.
- Initialize "next_echo_idx" in grcan_set_mode() to ensure proper starting
  conditions when the device enters proper modes.
- Improve memory and index handling in grcan_start_xmit() and
  added a check to stop the network queue when necessary.

Signed-off-by: Arun Muthusamy <arun.muthusamy@...sler.com>
---
 drivers/net/can/grcan.c | 53 ++++++++++++++++++++++++++++++-----------
 1 file changed, 39 insertions(+), 14 deletions(-)

diff --git a/drivers/net/can/grcan.c b/drivers/net/can/grcan.c
index d838fb0698f4..e6f3cd36ea66 100644
--- a/drivers/net/can/grcan.c
+++ b/drivers/net/can/grcan.c
@@ -298,6 +298,15 @@ struct grcan_priv {

 	struct sk_buff **echo_skb;	/* We allocate this on our own */

+	/*
+	 * Since the CAN FD frame has a variable length, this variable is used
+	 * to keep track of the index of the CAN echo skb (socket buffer) frame.
+	 */
+	u32 echo_skb_idx;
+
+	/* Next echo skb free slot index */
+	u32 next_echo_idx;
+
 	/* The echo skb pointer, pointing into echo_skb and indicating which
 	 * frames can be echoed back. See the "Notes on the tx cyclic buffer
 	 * handling"-comment for grcan_start_xmit for more details.
@@ -567,7 +576,7 @@ static int catch_up_echo_skb(struct net_device *dev, int budget, bool echo)
 	struct grcan_registers __iomem *regs = priv->regs;
 	struct grcan_dma *dma = &priv->dma;
 	struct net_device_stats *stats = &dev->stats;
-	int i, work_done;
+	int work_done;

 	/* Updates to priv->eskbp and wake-ups of the queue needs to
 	 * be atomic towards the reads of priv->eskbp and shut-downs
@@ -578,19 +587,22 @@ static int catch_up_echo_skb(struct net_device *dev, int budget, bool echo)
 	for (work_done = 0; work_done < budget || budget < 0; work_done++) {
 		if (priv->eskbp == txrd)
 			break;
-		i = priv->eskbp / GRCAN_MSG_SIZE;
-		if (echo) {
-			/* Normal echo of messages */
-			stats->tx_packets++;
-			stats->tx_bytes += can_get_echo_skb(dev, i, NULL);
-		} else {
-			/* For cleanup of untransmitted messages */
-			can_free_echo_skb(dev, i, NULL);
-		}

 		priv->eskbp = grcan_ring_add(priv->eskbp, GRCAN_MSG_SIZE,
 					     dma->tx.size);
 		txrd = grcan_read_reg(&regs->txrd);
+
+		/* Grab the packet once the packet is send or free untransmitted packet */
+		if (priv->eskbp == txrd) {
+			if (echo) {
+				/* Normal echo of messages */
+				stats->tx_packets++;
+				stats->tx_bytes += can_get_echo_skb(dev, priv->echo_skb_idx, NULL);
+			} else {
+				/* For cleanup of untransmitted messages */
+				can_free_echo_skb(dev, priv->echo_skb_idx, NULL);
+			}
+		}
 	}
 	return work_done;
 }
@@ -1098,6 +1110,7 @@ static int grcan_set_mode(struct net_device *dev, enum can_mode mode)
 			if (!(priv->can.ctrlmode & CAN_CTRLMODE_LISTENONLY))
 				netif_wake_queue(dev);
 		}
+		priv->next_echo_idx = 0;
 		spin_unlock_irqrestore(&priv->lock, flags);
 		return err;
 	}
@@ -1109,6 +1122,7 @@ static int grcan_open(struct net_device *dev)
 	struct grcan_priv *priv = netdev_priv(dev);
 	struct grcan_dma *dma = &priv->dma;
 	unsigned long flags;
+	u32 nr_echo_slots;
 	int err;

 	/* Allocate memory */
@@ -1119,13 +1133,15 @@ static int grcan_open(struct net_device *dev)
 		return err;
 	}

-	priv->echo_skb = kcalloc(dma->tx.size, sizeof(*priv->echo_skb),
+	nr_echo_slots = dma->tx.size / GRCAN_MSG_SIZE;
+
+	priv->echo_skb = kcalloc(nr_echo_slots, sizeof(*priv->echo_skb),
 				 GFP_KERNEL);
 	if (!priv->echo_skb) {
 		err = -ENOMEM;
 		goto exit_free_dma_buffers;
 	}
-	priv->can.echo_skb_max = dma->tx.size;
+	priv->can.echo_skb_max = nr_echo_slots;
 	priv->can.echo_skb = priv->echo_skb;

 	/* Get can device up */
@@ -1485,8 +1501,11 @@ static netdev_tx_t grcan_start_xmit(struct sk_buff *skb,
 	space = grcan_txspace(dma->tx.size, txwr, priv->eskbp);
 	bds = grcan_numbds(len);

-	if (unlikely(space == 1))
+	if (unlikely(space < bds || priv->can.echo_skb[priv->echo_skb_idx])) {
 		netif_stop_queue(dev);
+		spin_unlock_irqrestore(&priv->lock, flags);
+		return NETDEV_TX_BUSY;
+	}

 	spin_unlock_irqrestore(&priv->lock, flags);
 	/* End of critical section*/
@@ -1565,7 +1584,13 @@ static netdev_tx_t grcan_start_xmit(struct sk_buff *skb,
 	 * can_put_echo_skb would be an error unless other measures are
 	 * taken.
 	 */
-	can_put_echo_skb(skb, dev, slotindex, 0);
+
+	priv->echo_skb_idx = priv->next_echo_idx;
+
+	can_put_echo_skb(skb, dev, priv->next_echo_idx, 0);
+
+	/* Move to the next index in the echo skb buffer */
+	priv->next_echo_idx = (priv->next_echo_idx + 1) % priv->can.echo_skb_max;

 	/* Make sure everything is written before allowing hardware to
 	 * read from the memory
--
2.51.0