net: phy: microchip: force IRQ polling mode for lan88xx

net: phy: microchip: force IRQ polling mode for lan88xx

[ Upstream commit 30a41ed32d3088cd0d682a13d7f30b23baed7e93 ]

With lan88xx based devices the lan78xx driver can get stuck in an
interrupt loop while bringing the device up, flooding the kernel log
with messages like the following:

lan78xx 2-3:1.0 enp1s0u3: kevent 4 may have been dropped

Removing interrupt support from the lan88xx PHY driver forces the
driver to use polling instead, which avoids the problem.

The issue has been observed with Raspberry Pi devices at least since
4.14 (see [1], bug report for their downstream kernel), as well as
with Nvidia devices [2] in 2020, where disabling interrupts was the
vendor-suggested workaround (together with the claim that phylib
changes in 4.9 made the interrupt handling in lan78xx incompatible).

Iperf reports well over 900Mbits/sec per direction with client in
--dualtest mode, so there does not seem to be a significant impact on
throughput (lan88xx device connected via switch to the peer).

[1] https://github.com/raspberrypi/linux/issues/2447
[2] https://forums.developer.nvidia.com/t/jetson-xavier-and-lan7800-problem/142134/11

Link: https://lore.kernel.org/0901d90d-3f20-4a10-b680-9c978e04ddda@lunn.ch
Fixes: 792aec47d59d ("add microchip LAN88xx phy driver")
Signed-off-by: Fiona Klute <fiona.klute@gmx.de>
Cc: kernel-list@raspberrypi.com
Cc: stable@vger.kernel.org
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250416102413.30654-1-fiona.klute@gmx.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>