[PATCH] wifi: mt76: dma: run threaded NAPI kthreads with SCHED_FIFO low priority

[Fil Dunsky via B4 Relay <devnull+filipp.dunsky.gmail.com@kernel.org>]

From: Fil Dunsky <filipp.dunsky@gmail.com>

mt76 enables threaded NAPI for the dma rx queues, but the napi kthreads
stay SCHED_OTHER, unlike the usb and sdio rx/status workers and the tx
worker, which already use sched_set_fifo_low().

On MT7621 (MIPS 1004Kc, 2 cores / 4 threads, PREEMPT_NONE, HZ=100) with
MT7612E (mt76x2e) as a 5 GHz client receiving ~200 Mbit/s while
userspace keeps the CPUs busy, the napi kthread is regularly delayed by
10-30 ms or more and the 256-entry rx ring fills up completely
(instrumented: ~500 full-ring events in 5 minutes). Occasionally the
chip's rx DMA does not resume after such an event: WPDMA_GLO_CFG reports
rx and tx DMA busy, RXQ_STA shows the PBF rx queue at RX_MAX_PCNT, the
host ring has free buffers, beacons are lost, then tx DMA stops (dma_idx
stuck with frames queued), the tx hang watchdog fires, followed by
"MCU message ... timed out" and a second restart. A series of these
sometimes hangs the whole system.

With the napi kthreads at SCHED_FIFO 1 the ring overflowed once in 5
minutes, no watchdog resets occurred in repeated 5 minute runs and rx
throughput in the same setup went from ~210 to ~280 Mbit/s. A larger rx
ring (1024 entries) did not help: it still overflowed and resets were
more frequent.

Set the napi kthread priority right after netif_napi_add(), as done for
the other workers.

Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
---
Tested on Securifi Almond 3 (MT7621, MT7612E + MT7602E) with OpenWrt
25.12.6 (kernel 6.12.112, mt76 2026-03-19 39c960c3 plus this change):
iperf3 bidirectional, 4 streams, 5 GHz client link at -66 dBm, with a
userspace proxy and a display daemon running. Without the change 2-3
watchdog resets in most 5 minute runs and several full-system hangs;
with it 0 resets over 6 runs.

A similar reset chain under full CPU load on MT7620A + MT7612E is
reported in https://github.com/openwrt/openwrt/issues/8984
---
 drivers/net/wireless/mediatek/mt76/dma.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/wireless/mediatek/mt76/dma.c b/drivers/net/wireless/mediatek/mt76/dma.c
index a67880a92..8e66fad7b 100644
--- a/drivers/net/wireless/mediatek/mt76/dma.c
+++ b/drivers/net/wireless/mediatek/mt76/dma.c
@@ -1128,6 +1128,11 @@ mt76_dma_rx_queue_init(struct mt76_dev *dev, enum mt76_rxq_id qid,
 		       int (*poll)(struct napi_struct *napi, int budget))
 {
 	netif_napi_add(dev->napi_dev, &dev->napi[qid], poll);
+	/* Same as the usb/sdio rx workers: under CPU load a SCHED_OTHER napi
+	 * thread can be starved long enough for the rx ring to overflow.
+	 */
+	if (dev->napi[qid].thread)
+		sched_set_fifo_low(dev->napi[qid].thread);
 	mt76_dma_rx_fill_buf(dev, &dev->q_rx[qid], false);
 	napi_enable(&dev->napi[qid]);
 }

---
base-commit: 6474fa070f2b8013b4b87350b775b8c3be6e8aac
change-id: 20261020-b4-mt76-napi-fifo-dc5ef4fc24b4

Best regards,
--  
Fil Dunsky <filipp.dunsky@gmail.com>