[PATCH 2/3] can: xilinx_can: enable NAPI before starting the controller

[Patrick Zhang <patrzhan@amazon.com>]

xcan_open() calls xcan_chip_start(), which writes XCAN_IER_OFFSET to
unmask the RX interrupts and then sets XCAN_SRR_CEN_MASK to enable the
controller, and only then calls napi_enable(). A frame that arrives in
that window raises the RX interrupt. xcan_interrupt() clears the RX
bits in XCAN_IER_OFFSET and calls napi_schedule(). NAPI is still
disabled, so napi_schedule_prep() finds NAPI_STATE_SCHED set and the
schedule is dropped. Only xcan_rx_poll() unmasks RX again, and it never
runs. The interface receives nothing until the next close. Transmit
still works and no rx_errors subcounter moves, so the stall is silent.
Every re-open (ip link down/up, a driver rebind) runs the same race
again.

The window is only the tail of xcan_chip_start(), so the failure is
rare. Adding an mdelay(50) at the end of xcan_chip_start() makes it
reproducible with continuous traffic from another node and a single
"ip link set down/up" on the test device.

Enable NAPI before the controller starts, so that an early RX interrupt
can schedule the poll. NAPI can be enabled while the controller is in
reset, because nothing schedules it until an interrupt arrives. If
xcan_chip_start() fails, disable NAPI again. Otherwise the next open
would call napi_enable() on an already enabled instance and trip the
BUG_ON() in napi_enable_locked().

This is the same race as in ctucanfd, fixed by the previous patch.

Tested on a Versal board.

Fixes: b1201e44f50b ("can: xilinx CAN controller support")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Patrick Zhang <patrzhan@amazon.com>
---
 drivers/net/can/xilinx_can.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/xilinx_can.c b/drivers/net/can/xilinx_can.c
index 43d7f2282..5c2412bf9 100644
--- a/drivers/net/can/xilinx_can.c
+++ b/drivers/net/can/xilinx_can.c
@@ -1570,18 +1570,22 @@ static int xcan_open(struct net_device *ndev)
 	if (ret)
 		goto err_irq;
 
+	/* Keep before xcan_chip_start(): the ISR masks RX interrupts and only
+	 * the poll unmasks them.
+	 */
+	napi_enable(&priv->napi);
 	ret = xcan_chip_start(ndev);
 	if (ret < 0) {
 		netdev_err(ndev, "xcan_chip_start failed!\n");
 		goto err_candev;
 	}
 
-	napi_enable(&priv->napi);
 	netif_start_queue(ndev);
 
 	return 0;
 
 err_candev:
+	napi_disable(&priv->napi);
 	close_candev(ndev);
 err_irq:
 	free_irq(ndev->irq, ndev);
-- 
2.50.1