[PATCH RFC net 3/6] net: macb: mask TXUBR during TX NAPI poll to prevent IRQ storms

[Steffen Dirkwinkel <lists@steffen.cc>]

From: Christian Taedcke <christian.taedcke@weidmueller.com>

macb_interrupt() defers TX completion handling to NAPI, but when it
schedules the poll it only masks TCOMP, even though TXUBR is enabled
alongside it (both are part of MACB_TX_INT_FLAGS). macb_tx_poll() is
asymmetric in the same way and only re-enables TCOMP. TXUBR is thus
left unmasked while responsibility for handling it has been deferred
to NAPI.

Unlike an edge event, TXUBR is a persistent condition: the controller
keeps it asserted for as long as the transmitter reads a buffer
descriptor whose used bit is set. Leaving a level-triggered source
enabled while NAPI owns its processing means the interrupt refires
immediately after the handler returns, before the poll has had a
chance to clear the underlying condition. This turns into a hard
interrupt storm that pegs a CPU in the (threaded) MAC IRQ handler and,
on PREEMPT_RT, triggers RT throttling ("sched: RT throttling
activated"), taking the network interface down.

Several situations can keep the used-bit read asserted across a poll -
for example unreaped completed descriptors still sitting at tx_tail,
or a transmit restart racing with macb_start_xmit(). The specific
trigger does not matter: as long as the source stays unmasked, any
persistent assertion is enough to storm, so the interrupt handling
itself must be made self-limiting.

Mask TXUBR together with TCOMP in the IDR write when scheduling the TX
NAPI, and re-enable both from the napi_complete path in
macb_tx_poll(), making the TX interrupt mask/unmask symmetric and
consistent with how the driver already treats every other
NAPI-serviced source. The pending TXUBR is still recorded in
queue->txubr_pending before masking and acted on by macb_tx_restart(),
so no event is lost. A persistent TXUBR now degrades to NAPI-paced
polling instead of a CPU-pegging hard interrupt storm.

Fixes: 138badbc21a0 ("net: macb: use NAPI for TX completion path")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Christian Taedcke <christian.taedcke@weidmueller.com>
---
 drivers/net/ethernet/cadence/macb_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 97494933eb1e..14c8299cb7f3 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -1979,7 +1979,7 @@ static int macb_tx_poll(struct napi_struct *napi, int budget)
 		    (unsigned int)(queue - bp->queues), work_done, budget);
 
 	if (work_done < budget && napi_complete_done(napi, work_done)) {
-		queue_writel(queue, IER, MACB_BIT(TCOMP));
+		queue_writel(queue, IER, MACB_BIT(TCOMP) | MACB_BIT(TXUBR));
 
 		/* Packet completions only seem to propagate to raise
 		 * interrupts when interrupts are enabled at the time, so if
@@ -2174,7 +2174,8 @@ static irqreturn_t macb_interrupt(int irq, void *dev_id)
 
 		if (status & (MACB_BIT(TCOMP) |
 			      MACB_BIT(TXUBR))) {
-			queue_writel(queue, IDR, MACB_BIT(TCOMP));
+			queue_writel(queue, IDR, MACB_BIT(TCOMP) |
+						 MACB_BIT(TXUBR));
 			macb_queue_isr_clear(bp, queue, MACB_BIT(TCOMP) |
 							MACB_BIT(TXUBR));
 			if (status & MACB_BIT(TXUBR)) {

-- 
2.56.0