[PATCH 0/2] thunderbolt: fix wedge under sustained tbnet load on AM4 and AM5

[PATCH 0/2] thunderbolt: fix wedge under sustained tbnet load on AM4 and AM5

[Benjamin Berman]

Greetings Thunderbolt maintainers,

These patches for drivers were tested by me, Benjamin Berman, a
software developer, but they were authored by a coding agent that
had access to and ran the patches against real hardware.

The purpose of these patches was to fix Thunderbolt networking between
Thunderbolt 3 and Thunderbolt 4 (USB4) hosts on AM4 and AM5. I observed
these issues when using nccl across a Thunderbolt daisy chain: the
connection would drop abruptly, and performance was poorer than
expected. In any instance, I had to also update the NVM by exotic
methods on the TB3 controllers for AM4; AM5 generally ships the
Thunderbolt controller NVM in its UEFI patches.

Please advise on next steps for how to improve the patches. I can also
make my testing environment available, since it has a bunch of random
but useful Thunderbolt hardware.

Below is the generatively-authored explanation of the patch, and the
patch itself:

---

Two changes.

1. drivers/thunderbolt/nhi.c — tb_ring_poll_complete() gates the
   unmask on @start_poll rather than @running. Under load on NHIs
   with several rings in NAPI poll, a race with __ring_interrupt()'s
   unconditional mask leaves the ring masked: MSI-X stops, NAPI is
   not rescheduled, carrier stays up, no driver event fires. On NHIs
   without QUIRK_AUTO_CLEAR_INT, stale REG_RING_NOTIFY_BASE state
   blocks MSI-X re-arm. The patch gates on @running, adds a posted-
   write barrier, and clears the ring's pending bit before re-enable.

2. drivers/net/thunderbolt/main.c — TBNET_RING_SIZE=256 and the
   netif_napi_add() weight of 64 produce ~1 % rx_missed_errors on a
   TB4 transit under sustained tbnet bulk traffic. The patch raises
   ring size to 2048 and the NAPI weight to 256.

Hardware tested:
  ASRock X570 Phantom Gaming-ITX/TB3 (AM4), Intel JHL7540 2C TB3
    controller, NVM 50.0
  ASUS ROG STRIX X670E-I GAMING WIFI (AM5), Maple Ridge 4C TB4
    controller, NVM 43.83
  Monoprice USB4 Gen 3 40 Gb/s passive cables
  Linux 6.17.0-22-generic (Ubuntu HWE)

Workload: NCCL 2.28.9 all-reduce over tb-lo, NCCL_ALGO=Tree,
NCCL_PROTO=Simple, three ranks. Pre-patch the connection wedges
under 1 GB transferred. Post-patch a 192 GB run (3000 iterations
of a 64 MiB all-reduce) completes with mask/unmask counters
balanced and rx_missed_errors under 0.005 %.

Built clean against linux.git commit 3b3bea6d4b9c.

Benjamin Berman (2):
  thunderbolt: drop start_poll guard in tb_ring_poll_complete()
  net: thunderbolt: enlarge RX/TX ring and set NAPI weight for sustained
    load

 drivers/net/thunderbolt/main.c |  4 ++--
 drivers/thunderbolt/nhi.c      | 22 +++++++++++++++++++---
 2 files changed, 21 insertions(+), 5 deletions(-)


base-commit: 3b3bea6d4b9c162f9e555905d96b8c1da67ecd5b
--
2.43.0

[PATCH 1/2] thunderbolt: drop start_poll guard in tb_ring_poll_complete()

[Benjamin Berman]

Under concurrent load on a single NHI with several rings simultaneously
in NAPI poll (e.g. a Maple Ridge TB4 transit forwarding tbnet traffic
between two peers), one ring's interrupt enable bit in
REG_RING_INTERRUPT_BASE can stay cleared.  MSI-X stops for that ring,
NAPI is never rescheduled, but carrier is reported up and no driver
event fires.  The ring stays masked until thunderbolt_net is reloaded.

tb_ring_poll_complete() gated the unmask on @start_poll:

	if (ring->start_poll)
		__ring_interrupt_mask(ring, false);

while the ISR path masks unconditionally via __ring_interrupt().  In a
window where @start_poll is observed as NULL by the unmask path while
the paired mask persists, the ring is left permanently masked.

Gate on @running instead and add an ioread32() barrier so the posted
enable reaches the device before the spinlock is dropped.

On NHIs without QUIRK_AUTO_CLEAR_INT a second issue compounds the
first: stale pending status in REG_RING_NOTIFY_BASE can prevent the
hardware from re-arming its MSI-X generator when the ring is
re-enabled.  Clear the ring's bit in REG_RING_INT_CLEAR before setting
the enable bit, mirroring what ring_msix() already does at ISR entry.

Verified on a Maple Ridge 4C transit and two TB3 Titan Ridge endpoints
running NCCL all-reduce over tb-lo: pre-patch the chain wedges in
under 1 GB; post-patch a 192 GB run (3000 iterations of a 64 MiB
all-reduce) completes with mask/unmask counters balanced.

Generated-by: Claude Opus 4.7 <claude-opus-4-7@anthropic.com>
Tested-by: Benjamin Berman <benjamin.s.berman@gmail.com>
Signed-off-by: Benjamin Berman <benjamin.s.berman@gmail.com>
---
 drivers/thunderbolt/nhi.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/thunderbolt/nhi.c b/drivers/thunderbolt/nhi.c
index 2bb2e79ca..bba45ec36 100644
--- a/drivers/thunderbolt/nhi.c
+++ b/drivers/thunderbolt/nhi.c
@@ -389,10 +389,24 @@ static void __ring_interrupt_mask(struct tb_ring *ring, bool mask)
 	u32 val;
 
 	val = ioread32(ring->nhi->iobase + reg);
-	if (mask)
+	if (mask) {
 		val &= ~BIT(bit);
-	else
+	} else {
+		if (!(ring->nhi->quirks & QUIRK_AUTO_CLEAR_INT)) {
+			int cbit = ring_interrupt_index(ring) & 31;
+
+			if (ring->is_tx)
+				iowrite32(BIT(cbit),
+					  ring->nhi->iobase +
+					  REG_RING_INT_CLEAR);
+			else
+				iowrite32(BIT(cbit),
+					  ring->nhi->iobase +
+					  REG_RING_INT_CLEAR +
+					  4 * (ring->nhi->hop_count / 32));
+		}
 		val |= BIT(bit);
+	}
 	iowrite32(val, ring->nhi->iobase + reg);
 }
 
@@ -423,8 +437,10 @@ void tb_ring_poll_complete(struct tb_ring *ring)
 
 	spin_lock_irqsave(&ring->nhi->lock, flags);
 	spin_lock(&ring->lock);
-	if (ring->start_poll)
+	if (ring->running) {
 		__ring_interrupt_mask(ring, false);
+		(void)ioread32(ring->nhi->iobase + REG_RING_INTERRUPT_BASE);
+	}
 	spin_unlock(&ring->lock);
 	spin_unlock_irqrestore(&ring->nhi->lock, flags);
 }
-- 
2.43.0

[PATCH 2/2] net: thunderbolt: enlarge RX/TX ring and set NAPI weight for sustained load

[Benjamin Berman]

The default TBNET_RING_SIZE of 256 and the NAPI_POLL_WEIGHT of 64
implicit in netif_napi_add() are too small for host-to-host Thunderbolt
networking under sustained bulk traffic.  Running NCCL all-reduce over
tb-lo on a three-node chain (two TB3 endpoints plus a TB4 Maple Ridge
transit) produces rx_missed_errors at ~1 % of rx_packets on the transit
and ~0.6 % on the endpoints, with rx_packets stalling against a peer's
continuing tx_packets.

Raise TBNET_RING_SIZE to 2048 (8x) and use netif_napi_add_weight() with
a per-NAPI weight of 256 so tbnet_poll() drains more frames per softirq
invocation.  With matching sysctls (net.core.netdev_budget=1024,
net.core.netdev_budget_usecs=8000) rx_missed_errors stays below 0.005 %
over a 192 GB all-reduce workload on the same hardware.

Generated-by: Claude Opus 4.7 <claude-opus-4-7@anthropic.com>
Tested-by: Benjamin Berman <benjamin.s.berman@gmail.com>
Signed-off-by: Benjamin Berman <benjamin.s.berman@gmail.com>
---
 drivers/net/thunderbolt/main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/thunderbolt/main.c b/drivers/net/thunderbolt/main.c
index 7aae5d915..3a096f7c5 100644
--- a/drivers/net/thunderbolt/main.c
+++ b/drivers/net/thunderbolt/main.c
@@ -31,7 +31,7 @@
 #define TBNET_LOGIN_TIMEOUT	500
 #define TBNET_LOGOUT_TIMEOUT	1000
 
-#define TBNET_RING_SIZE		256
+#define TBNET_RING_SIZE		2048
 #define TBNET_LOGIN_RETRIES	60
 #define TBNET_LOGOUT_RETRIES	10
 #define TBNET_E2E		BIT(0)
@@ -1383,7 +1383,7 @@ static int tbnet_probe(struct tb_service *svc, const struct tb_service_id *id)
 	dev->features = dev->hw_features | NETIF_F_HIGHDMA;
 	dev->hard_header_len += sizeof(struct thunderbolt_ip_frame_header);
 
-	netif_napi_add(dev, &net->napi, tbnet_poll);
+	netif_napi_add_weight(dev, &net->napi, tbnet_poll, 256);
 
 	/* MTU range: 68 - 65522 */
 	dev->min_mtu = ETH_MIN_MTU;
-- 
2.43.0