[PATCH v3] wifi: rtw89: phy: increase RF calibration timeouts for USB transport

Louis Kotze
2026-04-16T04:55:51Z

USB transport adds significant latency to H2C/C2H round-trips used
by RF calibration. The existing timeout values were designed for PCIe
and are too tight for USB, causing "failed to wait RF DACK",
"failed to wait RF TSSI" and similar errors on USB adapters.

Apply a 4x timeout multiplier when the device uses USB transport.
The multiplier is applied in rtw89_phy_rfk_report_wait() so all
calibrations benefit without changing any call sites or PCIe
timeout values.

The 4x multiplier was chosen based on measured data from two
independent testers (RTL8922AU, 6GHz MLO and 2.4/5GHz):

Calibration PCIe timeout Max measured (USB) 4x timeout
PRE_NTFY 5ms 1ms 20ms
DACK 58ms 72ms 232ms
RX_DCK 128ms 374ms 512ms
TSSI normal 20ms 24ms 80ms
TSSI scan 6ms 14ms 24ms
TXGAPK 54ms 18ms 216ms
IQK 84ms 53ms 336ms
DPK 34ms 30ms 136ms

Tested with RTL8922AU on 6GHz MLO (5GHz + 6GHz simultaneous):
25 connect/disconnect cycles with zero failures.

The 4x multiplier was also verified under adverse host conditions
on 5GHz. 5 cycles per scenario, stress-ng as the load generator,
max observed time per calibration:

Calibration PCIe 4x Baseline CPU stress Mem stress Combined
PRE_NTFY 5 20 0 0 0 1
DACK 58 232 71 (!) 71 (!) 71 (!) 71 (!)
RX_DCK 128 512 23 22 22 23
IQK 84 336 53 53 53 53
DPK 34 136 23 23 26 23
TSSI 20 80 6 9 14 9
TXGAPK 54 216 16 16 16 16

Legend: (!) = exceeds PCIe budget but within 4x budget.

Two observations from that matrix:

1. DACK exceeds the stock PCIe budget (58ms) in baseline on 5GHz
on this hardware. Without the 4x multiplier, DACK fails
-ETIMEDOUT deterministically on every connect, no stress
needed. This is the condition the patch addresses.

2. Calibration times appear dominated by USB transport round-trip
latency rather than host load, though hardware and external
component factors may also contribute. DACK stays at 71ms
across all four scenarios. Host-side stress has essentially
zero effect on observed calibration duration. Bumping the
multiplier above 4x would not address a failure mode that
this stress matrix produces.

Reported-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Link: https://github.com/Lucid-Duck/rtw89-usb3-gap/tree/main/evidence/crash-2026-04-11
Signed-off-by: Louis Kotze <loukot@gmail.com>
Tested-by: Devin Wittmayer <lucid_duck@justthetip.ca> # RTL8922AU (BrosTrend BE6500)
Tested-by: Devin Wittmayer <lucid_duck@justthetip.ca> # RTL8852AU (D-Link DWA-X1850 A1)
Tested-by: Devin Wittmayer <lucid_duck@justthetip.ca> # RTL8852AU (D-Link DWA-X1850 B1)
Tested-by: Devin Wittmayer <lucid_duck@justthetip.ca> # RTL8852BU (BrosTrend AX4L)
Tested-by: Devin Wittmayer <lucid_duck@justthetip.ca> # RTL8852CU (EDUP AX5400)
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
---
Changes since v2:
- Reword commit message per Ping-Ke review: remove v1 reference
from permanent changelog, use "condition" instead of "bug",
acknowledge hardware factors in calibration timing rather than
asserting I/O bound.
- Add Tested-by tags from Devin Wittmayer across 4 chipsets
(RTL8922AU, RTL8852AU, RTL8852BU, RTL8852CU) on Framework 13 +
Fedora 43 (6.19.11) and Raspberry Pi 5 + Pi OS (6.12.47).
- Add Reported-by for independent confirmation including xHCI hard
lockup evidence (CPU5 deadlock in usb_unanchor_urb after DACK
timeout triggered driver recovery).

v2: https://lore.kernel.org/linux-wireless/20260415111339.453602-1-loukot@gmail.com/
v1: https://lore.kernel.org/linux-wireless/20260410080017.82946-1-loukot@gmail.com/

drivers/net/wireless/realtek/rtw89/phy.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/drivers/net/wireless/realtek/rtw89/phy.c b/drivers/net/wireless/realtek/rtw89/phy.c
index e70d0e283..1f249c297 100644
--- a/drivers/net/wireless/realtek/rtw89/phy.c
+++ b/drivers/net/wireless/realtek/rtw89/phy.c
@@ -3956,6 +3956,14 @@ int rtw89_phy_rfk_report_wait(struct rtw89_dev *rtwdev, const char *rfk_name,
struct rtw89_rfk_wait_info *wait = &rtwdev->rfk_wait;
unsigned long time_left;

+ /*
+ * USB transport adds latency to H2C/C2H round-trips, so RF
+ * calibrations take longer than on PCIe. Apply a 4x multiplier
+ * to avoid spurious timeouts.
+ */
+ if (rtwdev->hci.type == RTW89_HCI_TYPE_USB)
+ ms *= 4;
+
/* Since we can't receive C2H event during SER, use a fixed delay. */
if (test_bit(RTW89_FLAG_SER_HANDLING, rtwdev->flags)) {
fsleep(1000 * ms / 2);

base-commit: 1e33ef7657531b2361d53cca25f375b5626e76a9
--
2.53.0