[PATCH net v3 12/12] net/sonic: Prevent tx watchdog timeout

[Finn Thain]

Section 5.5.3.2 of the datasheet says,

    If FIFO Underrun, Byte Count Mismatch, Excessive Collision, or
    Excessive Deferral (if enabled) errors occur, transmission ceases.

In this situation, the chip asserts a TXER interrupt rather than TXDN.
But the handler for the TXDN is the only way that the transmit queue
gets restarted. Hence, an aborted transmission can result in a watchdog
timeout.

This problem can be reproduced on congested link, as that can result in
excessive transmitter collisions. Another way to reproduce this is with
a FIFO Underrun, which may be caused by DMA latency.

In event of a TXER interrupt, prevent a watchdog timeout by restarting
transmission.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Tested-by: Stan Johnson <userm57@xxxxxxxxx>
Signed-off-by: Finn Thain <fthain@xxxxxxxxxxxxxxxxxxx>
---
 drivers/net/ethernet/natsemi/sonic.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/natsemi/sonic.c b/drivers/net/ethernet/natsemi/sonic.c
index 27b6f6585527..05e760444a92 100644
--- a/drivers/net/ethernet/natsemi/sonic.c
+++ b/drivers/net/ethernet/natsemi/sonic.c
@@ -415,10 +415,19 @@ static irqreturn_t sonic_interrupt(int irq, void *dev_id)
 			lp->stats.rx_missed_errors += 65536;
 
 		/* transmit error */
-		if (status & SONIC_INT_TXER)
-			if (SONIC_READ(SONIC_TCR) & SONIC_TCR_FU)
-				netif_dbg(lp, tx_err, dev, "%s: tx fifo underrun\n",
-					  __func__);
+		if (status & SONIC_INT_TXER) {
+			u16 tcr = SONIC_READ(SONIC_TCR);
+
+			netif_dbg(lp, tx_err, dev, "%s: TXER intr, TCR %04x\n",
+				  __func__, tcr);
+
+			if (tcr & (SONIC_TCR_EXD | SONIC_TCR_EXC |
+				   SONIC_TCR_FU | SONIC_TCR_BCM)) {
+				/* Aborted transmission. Try again. */
+				netif_stop_queue(dev);
+				SONIC_WRITE(SONIC_CMD, SONIC_CR_TXP);
+			}
+		}
 
 		/* bus retry */
 		if (status & SONIC_INT_BR) {
-- 
2.24.1