[PATCH net] r8169: enable the RxOverflow interrupt again

[Jure Rebernik <jure.rebernik@luxonis.com>]

Commit 6994520a3328 ("r8169: disable interrupt source RxOverflow")
removed RxOverflow (RxDescUnavail in the vendor driver) from the
interrupt mask on the grounds that NAPI is scheduled anyway when the
chip runs out of RX descriptors. On RTL8125B that does not hold.

When the RTL8125B runs out of RX descriptors it sets RxOverflow and
stops receiving. It does not start again when the driver returns
descriptors to it, only when RxOverflow is cleared in IntrStatus.
rtl8169_interrupt() clears only the bits it read on entry. If the ring
fills while NAPI runs with the interrupt mask cleared, RxOverflow is
set after that read and stays set: NAPI empties the ring, unmasks the
interrupts and completes, the stopped receiver raises no RxOK, and
RxOverflow is no longer an interrupt source. Reception stays stopped,
and every frame is counted in rx_missed, until another event such as
TxOK makes the handler clear all status bits.

This was observed on a machine in real use: an RTL8125B (XID 641) on
the PCIe bus of an arm64 Qualcomm QCS8550, running v7.2.6. With UDP
at 800 Mbit/s inbound and nothing sent by the host, reception stopped
for seconds in 13 of 20 30 s runs, losing 5% to 31% of the datagrams,
and a single transmitted packet restarted it. To confirm the cause,
a debugfs dump added to a test build of the driver showed, during each
stop, IntrMask 0x2f, IntrStatus 0x50, NAPI idle and all 256 RX
descriptors owned by the NIC. Writing only RxOverflow to IntrStatus
from user space, through the sysfs PCI resource mapping and with
nothing transmitted, restarted reception in 17 of 17 stops.
Reproducing the stall itself needs no kernel modification.

With this change backported to v7.2.6, 0 of 20 runs stopped and the
worst run lost 0.03%, on two RTL8125B boards. It was build-tested,
not run, on this tree, where the single-vector path RTL8125B uses
is the same as in v7.2. The multi-vector mapping used by RTL8127 is
not changed and was not tested.

Restore the mask of the chips that had RxOverflow before that commit.
Only RTL8125B was tested.

The investigation was done with an AI coding assistant (Claude Code):
it wrote the HIL test scripts, the debugfs instrumentation and the
register tools used above, proposed this change and drafted this
changelog. I reviewed the change and the evidence, and the test
results above come from runs on our hardware.

Fixes: 6994520a3328 ("r8169: disable interrupt source RxOverflow")
Cc: stable@vger.kernel.org
Signed-off-by: Jure Rebernik <jure.rebernik@luxonis.com>
---

Notes:
    This restores RxOverflow for every chip that had it before 6994520a3328,
    but I could only test RTL8125B. If you prefer, I can send a v2 that sets
    it for the RTL8125 family only (rtl_is_8125()).

 drivers/net/ethernet/realtek/r8169_main.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
index cd4ebc654dd0..62709160d354 100644
--- a/drivers/net/ethernet/realtek/r8169_main.c
+++ b/drivers/net/ethernet/realtek/r8169_main.c
@@ -6086,7 +6086,9 @@ static void rtl_set_irq_mask(struct rtl8169_private *tp)
 		tp->irq_mask = RxOK | RxErr | TxOK | TxErr | LinkChg;
 
 		if (tp->mac_version <= RTL_GIGA_MAC_VER_06)
-			tp->irq_mask |= SYSErr | RxFIFOOver;
+			tp->irq_mask |= SYSErr | RxOverflow | RxFIFOOver;
+		else
+			tp->irq_mask |= RxOverflow;
 	}
 }
 
-- 
2.39.5