[PATCH] phy: marvell: a3700-comphy: Guard missing polarity argument

["Štěpán Dalecký" <daleckystepan@gmail.com>]

The Armada 3700 COMPHY binding specifies #phy-cells = <1>, so PHY
references supply only the port argument. However,
mvebu_a3700_comphy_xlate() unconditionally reads args[1] to select TX
and RX polarity inversion. The PHY core does not initialize its
of_phandle_args structure, and the phandle parser fills only the
supplied cells, leaving args[1] uninitialized.

Using the unused cell can program unintended SerDes polarity. On a
Turris MOX with a Peridot switch running OpenWrt with Linux 6.18.54,
the CPU link reports RX CRC errors and fails to pass traffic. The
switch PCS does not record a partner advertisement despite link-up.

Check args_count before reading args[1], leaving polarity inversion
disabled when the second argument was not supplied. This does not
change the binding or introduce a supported two-cell interface.

The same change restores 2500BASE-X communication on the affected
MOX with no CRC errors. Cold boots, reboots and interface down/up
cycles were also tested successfully on Linux 6.18.54.

Fixes: 934337080c6c ("phy: marvell: phy-mvebu-a3700-comphy: Add native kernel implementation")
Cc: stable@vger.kernel.org
Closes: https://github.com/openwrt/openwrt/issues/24069
Assisted-by: LLM
Signed-off-by: Štěpán Dalecký <daleckystepan@gmail.com>
---
GitHub Copilot assisted with comparing the working and failing logs,
identifying the unused argument read, preparing the fix and changelog,
and checking the patch. The submitter performed the MOX hardware tests.

The reported hardware tests used OpenWrt with Linux 6.18.54. Mainline
runtime testing has not been performed. An isolated ARM64 mainline build
was attempted on macOS but failed while building host fixdep because
the host Clang could not use -fuse-ld=lld; the driver was not compiled.
A host C test of the changed expressions with poisoned unused argument
slots passed, as did strict checkpatch.

 drivers/phy/marvell/phy-mvebu-a3700-comphy.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/phy/marvell/phy-mvebu-a3700-comphy.c b/drivers/phy/marvell/phy-mvebu-a3700-comphy.c
index 1d1db1737422..d19f0fa9ba7c 100644
--- a/drivers/phy/marvell/phy-mvebu-a3700-comphy.c
+++ b/drivers/phy/marvell/phy-mvebu-a3700-comphy.c
@@ -1232,8 +1232,8 @@ static struct phy *mvebu_a3700_comphy_xlate(struct device *dev,
 		return ERR_PTR(-EINVAL);
 	}
 
-	lane->invert_tx = args->args[1] & BIT(0);
-	lane->invert_rx = args->args[1] & BIT(1);
+	lane->invert_tx = args->args_count > 1 && (args->args[1] & BIT(0));
+	lane->invert_rx = args->args_count > 1 && (args->args[1] & BIT(1));
 
 	return phy;
 }

base-commit: 551c722f40809618230001baccf219193e22fc5a
-- 
2.54.0 (Apple Git-157)