[PATCH 1/3] net: atl1c: fix soft lockup on out-of-range tpd_cons read

[Gajdos Tamás]

The hardware can report an out-of-range tpd_cons (seen as 0xffff)
while the PCIe link/MAC is resetting. An out-of-range value can
never be reached and the loop below would spin forever. To avoid
a soft lockup treat it as "nothing new to clean" instead.

Reproduced on two machines, same NIC (Qualcomm Atheros AR8151 v2.0,
4-port), triggered by rebooting a Mikrotik CCR2004 PCIe card that
the ports are directly linked to:

- Ubuntu 26.04.1 LTS, kernel 7.0.0-31-generic. The link-flap
  precursor, before the lockup was captured with a full trace
  elsewhere:

    atl1c 0000:05:00.0 enp5s0f0: NETDEV WATCHDOG: CPU: 4: transmit queue 2 timed out 489984 ms
    atl1c 0000:05:00.0: MAC state machine can't be idle since disabled for 10ms second
    atl1c 0000:05:00.0: atl1c: enp5s0f0 NIC Link is Up<65535 Mbps Full Duplex>

  65535 (0xffff) here is the same value tpd_cons reads back once the
  loop below gets stuck.

- Proxmox VE, kernel 7.0.14-11-pve. Same NIC/trigger, this time
  caught by the soft lockup watchdog with a full stack trace:

    watchdog: BUG: soft lockup - CPU#12 stuck for 354s! [napi/eth%d-0:329]
    CPU: 12 UID: 0 PID: 329 Comm: napi/eth%d-0 Tainted: P O L 7.0.14-11-pve #1 PREEMPT(lazy)
    RIP: 0010:atl1c_clean_tx+0x142/0x2d0 [atl1c]
    Call Trace:
     <TASK>
     __napi_poll+0x32/0x1e0
     napi_threaded_poll_loop+0x286/0x2e0
     napi_threaded_poll+0xfd/0x140
     kthread+0xf7/0x130
     ret_from_fork+0x2da/0x3a0
     ret_from_fork_asm+0x1a/0x30
     </TASK>

Fixes: 43250ddd75a35d ("atl1c: Atheros L1C Gigabit Ethernet driver")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Gajdos Tamás <tamas@xxxxxxxxxxxxx>
---
 drivers/net/ethernet/atheros/atl1c/atl1c_main.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/ethernet/atheros/atl1c/atl1c_main.c b/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
index 7efa3fc257..e58f1d2c26 100644
--- a/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
+++ b/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
@@ -1602,6 +1602,9 @@ static int atl1c_clean_tx(struct napi_struct *napi, int budget)
 	AT_READ_REGW(&adapter->hw, atl1c_qregs[tpd_ring->num].tpd_cons,
 		     &hw_next_to_clean);
 
+	if (unlikely(hw_next_to_clean >= tpd_ring->count))
+		hw_next_to_clean = next_to_clean;
+
 	while (next_to_clean != hw_next_to_clean) {
 		buffer_info = &tpd_ring->buffer_info[next_to_clean];
 		if (buffer_info->skb) {
-- 
2.53.0