[PATCH v3 net-next 3/3] amd-xgbe: add TX descriptor cleanup for link-down

[Raju Rangoju <Raju.Rangoju@amd.com>]

Add intelligent TX descriptor cleanup mechanism to reclaim abandoned
descriptors when the physical link goes down.

When the link goes down while TX packets are in-flight, the hardware
stops processing descriptors with the OWN bit still set. The current
driver waits indefinitely for these descriptors to complete, which
never happens. This causes:

  - TX ring exhaustion (no descriptors available for new packets)
  - Memory leaks (skbs never freed)
  - DMA mapping leaks (mappings never unmapped)
  - Network stack backpressure buildup

Add force-cleanup mechanism in xgbe_tx_poll() that detects link-down
state and reclaims abandoned descriptors. The helper functions and DMA
optimizations support efficient TX shutdown:
  - xgbe_wait_for_dma_tx_complete(): Wait for DMA completion with
    link-down optimization
  - Restructure xgbe_disable_tx() for proper shutdown sequence

Implementation:
  1. Check link state at the start of tx_poll
  2. If link is down, set force_cleanup flag
  3. For descriptors that hardware hasn't completed (!tx_complete):
     - If force_cleanup: treat as completed and reclaim resources
     - If link up: break and wait for hardware (normal behavior)

The cleanup process:
  - Frees skbs that will never be transmitted
  - Unmaps DMA mappings
  - Resets descriptors for reuse
  - Does NOT count as successful transmission (correct statistics)

Benefits:
  - Prevents TX ring starvation
  - Eliminates memory and DMA mapping leaks
  - Enables fast link recovery when link comes back up
  - Critical for link aggregation failover scenarios

Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
---
Changes since v2:
 - remove the stale function xgbe_reset_tx_queues(), otherwise it would
   result in undefined symbol during linking.

 drivers/net/ethernet/amd/xgbe/xgbe-common.h |  4 ++
 drivers/net/ethernet/amd/xgbe/xgbe-dev.c    | 77 ++++++++++++++++++---
 drivers/net/ethernet/amd/xgbe/xgbe-drv.c    | 43 ++++++++++--
 3 files changed, 108 insertions(+), 16 deletions(-)

diff --git a/drivers/net/ethernet/amd/xgbe/xgbe-common.h b/drivers/net/ethernet/amd/xgbe/xgbe-common.h
index c17900a49595..66807d67e984 100644
--- a/drivers/net/ethernet/amd/xgbe/xgbe-common.h
+++ b/drivers/net/ethernet/amd/xgbe/xgbe-common.h
@@ -330,6 +330,10 @@
 #define MAC_ISR_SMI_WIDTH		1
 #define MAC_ISR_TSIS_INDEX		12
 #define MAC_ISR_TSIS_WIDTH		1
+#define MAC_ISR_LS_INDEX		24
+#define MAC_ISR_LS_WIDTH		2
+#define MAC_ISR_LSI_INDEX		0
+#define MAC_ISR_LSI_WIDTH		1
 #define MAC_MACA1HR_AE_INDEX		31
 #define MAC_MACA1HR_AE_WIDTH		1
 #define MAC_MDIOIER_SNGLCOMPIE_INDEX	12
diff --git a/drivers/net/ethernet/amd/xgbe/xgbe-dev.c b/drivers/net/ethernet/amd/xgbe/xgbe-dev.c
index b7bf74c6bb47..2de974213090 100644
--- a/drivers/net/ethernet/amd/xgbe/xgbe-dev.c
+++ b/drivers/net/ethernet/amd/xgbe/xgbe-dev.c
@@ -3276,28 +3276,83 @@ static void xgbe_enable_tx(struct xgbe_prv_data *pdata)
 	XGMAC_IOWRITE_BITS(pdata, MAC_TCR, TE, 1);
 }
 
-static void xgbe_disable_tx(struct xgbe_prv_data *pdata)
+/**
+ * xgbe_wait_for_dma_tx_complete - Wait for DMA to complete pending TX
+ * @pdata: driver private data
+ *
+ * Wait for the DMA TX channels to complete all pending descriptors.
+ * This ensures no frames are in-flight before we disable the transmitter.
+ * If link is down, return immediately as TX will never complete.
+ *
+ * Return: 0 on success, -ETIMEDOUT on timeout
+ */
+static int xgbe_wait_for_dma_tx_complete(struct xgbe_prv_data *pdata)
 {
+	struct xgbe_channel *channel;
+	struct xgbe_ring *ring;
+	unsigned long timeout;
 	unsigned int i;
+	bool complete;
 
-	/* Prepare for Tx DMA channel stop */
-	for (i = 0; i < pdata->tx_q_count; i++)
-		xgbe_prepare_tx_stop(pdata, i);
+	/* If link is down, TX will never complete - skip waiting */
+	if (!pdata->phy.link)
+		return 0;
 
-	/* Disable MAC Tx */
-	XGMAC_IOWRITE_BITS(pdata, MAC_TCR, TE, 0);
+	timeout = jiffies + (XGBE_DMA_STOP_TIMEOUT * HZ);
 
-	/* Disable each Tx queue */
-	for (i = 0; i < pdata->tx_q_count; i++)
-		XGMAC_MTL_IOWRITE_BITS(pdata, i, MTL_Q_TQOMR, TXQEN, 0);
+	do {
+		complete = true;
 
-	/* Disable each Tx DMA channel */
+		for (i = 0; i < pdata->channel_count; i++) {
+			channel = pdata->channel[i];
+			ring = channel->tx_ring;
+			if (!ring)
+				continue;
+
+			/* Check if DMA has processed all descriptors */
+			if (ring->dirty != ring->cur) {
+				complete = false;
+				break;
+			}
+		}
+
+		if (complete)
+			return 0;
+
+		usleep_range(100, 200);
+	} while (time_before(jiffies, timeout));
+
+	netif_warn(pdata, drv, pdata->netdev,
+		   "timeout waiting for DMA TX to complete\n");
+	return -ETIMEDOUT;
+}
+
+static void xgbe_disable_tx(struct xgbe_prv_data *pdata)
+{
+	unsigned int i;
+
+	/* Step 1: Wait for DMA to complete pending descriptors */
+	xgbe_wait_for_dma_tx_complete(pdata);
+
+	/* Step 2: Disable each Tx DMA channel to stop
+	 * processing new descriptors
+	 */
 	for (i = 0; i < pdata->channel_count; i++) {
 		if (!pdata->channel[i]->tx_ring)
 			break;
-
 		XGMAC_DMA_IOWRITE_BITS(pdata->channel[i], DMA_CH_TCR, ST, 0);
 	}
+
+	/* Step 3: Wait for MTL TX queues to drain */
+	for (i = 0; i < pdata->tx_q_count; i++)
+		xgbe_prepare_tx_stop(pdata, i);
+
+	/* Step 4: Disable MTL TX queues */
+	for (i = 0; i < pdata->tx_q_count; i++)
+		XGMAC_MTL_IOWRITE_BITS(pdata, i, MTL_Q_TQOMR, TXQEN, 0);
+
+	/* Step 5: Disable MAC TX last */
+	XGMAC_IOWRITE_BITS(pdata, MAC_TCR, TE, 0);
 }
 
 static void xgbe_prepare_rx_stop(struct xgbe_prv_data *pdata,
diff --git a/drivers/net/ethernet/amd/xgbe/xgbe-drv.c b/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
index 6886d3b33ffe..2d6d00e3689b 100644
--- a/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
+++ b/drivers/net/ethernet/amd/xgbe/xgbe-drv.c
@@ -2169,6 +2169,7 @@ static int xgbe_tx_poll(struct xgbe_channel *channel)
 	struct net_device *netdev = pdata->netdev;
 	struct netdev_queue *txq;
 	int processed = 0;
+	int force_cleanup;
 	unsigned int tx_packets = 0, tx_bytes = 0;
 	unsigned int cur;
 
@@ -2185,13 +2186,41 @@ static int xgbe_tx_poll(struct xgbe_channel *channel)
 
 	txq = netdev_get_tx_queue(netdev, channel->queue_index);
 
+	/* Smart descriptor cleanup during link-down conditions.
+	 *
+	 * When link is down, hardware stops processing TX descriptors (OWN bit
+	 * remains set). Enable intelligent cleanup to reclaim these abandoned
+	 * descriptors and maintain TX queue health.
+	 *
+	 * This cleanup mechanism enables:
+	 * - Continuous TX queue availability for new packets when link recovers
+	 * - Clean resource management (skbs, DMA mappings, descriptors)
+	 * - Fast failover in link aggregation scenarios
+	 */
+	force_cleanup = !pdata->phy.link;
+
 	while ((processed < XGBE_TX_DESC_MAX_PROC) &&
 	       (ring->dirty != cur)) {
 		rdata = XGBE_GET_DESC_DATA(ring, ring->dirty);
 		rdesc = rdata->rdesc;
 
-		if (!hw_if->tx_complete(rdesc))
-			break;
+		if (!hw_if->tx_complete(rdesc)) {
+			if (!force_cleanup)
+				break;
+			/* Link-down descriptor cleanup: reclaim abandoned
+			 * resources.
+			 *
+			 * Hardware has stopped processing this descriptor, so
+			 * perform intelligent cleanup to free skbs and reclaim
+			 * descriptors for future use when link recovers.
+			 *
+			 * These are not counted as successful transmissions
+			 * since packets never reached the wire.
+			 */
+			netif_dbg(pdata, tx_err, netdev,
+				  "force-freeing stuck TX desc %u (link down)\n",
+				  ring->dirty);
+		}
 
 		/* Make sure descriptor fields are read after reading the OWN
 		 * bit */
@@ -2200,9 +2229,13 @@ static int xgbe_tx_poll(struct xgbe_channel *channel)
 		if (netif_msg_tx_done(pdata))
 			xgbe_dump_tx_desc(pdata, ring, ring->dirty, 1, 0);
 
-		if (hw_if->is_last_desc(rdesc)) {
-			tx_packets += rdata->tx.packets;
-			tx_bytes += rdata->tx.bytes;
+		/* Only count packets actually transmitted (not force-cleaned)
+		 */
+		if (!force_cleanup || hw_if->is_last_desc(rdesc)) {
+			if (hw_if->is_last_desc(rdesc)) {
+				tx_packets += rdata->tx.packets;
+				tx_bytes += rdata->tx.bytes;
+			}
 		}
 
 		/* Free the SKB and reset the descriptor for re-use */
-- 
2.34.1