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

Arun Muthusamy
2026-01-28T14:49:34Z

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@gaisler.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