[PATCH net 2/2] net: pse-pd: si3474: validate the channels of a PI

[Piotr Kubik <piotr@kubik.pl>]

si3474_get_of_channels() accepted channel 8, one past the last channel.
The driver then accessed channel 4 instead: quad 1, channel index 0.

It also accepted a 4-pair PI whose two channels are not a channel pair
of one quad. The chip forms a 4-pair port only from channels 0+1 or 2+3
of a quad (2P/4P mode bit in POWER_ALLOCATION, PORT_REMAP default), and
the driver sends every register access of a PI to the quad of its first
channel, so such a PI enabled and measured the wrong channels.

Reject both at probe.

Discovered by an AI code review agent; no shipped device tree triggers
it. Validated and tested on an SDG-9000 board (MediaTek MT7988A, two
Si3474) with Linux 6.18 and a modified device tree: one chip with a
channel at reg 8, the other with a 4-pair PI on channels 1 and 2. The
driver now rejects both at probe, and the unmodified device tree still
probes all eight PIs. No kernel selftest covers PSE hardware.

Fixes: a2317231df4b ("net: pse-pd: Add Si3474 PSE controller driver")
Signed-off-by: Piotr Kubik <piotr@kubik.pl>
---
 drivers/net/pse-pd/si3474.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/net/pse-pd/si3474.c b/drivers/net/pse-pd/si3474.c
index 99e96f6bdefc..2f18714c95f1 100644
--- a/drivers/net/pse-pd/si3474.c
+++ b/drivers/net/pse-pd/si3474.c
@@ -250,9 +250,9 @@ static int si3474_get_of_channels(struct si3474_priv *priv)
 					"Failed to read channel reg property\n");
 				return ret;
 			}
-			if (chan_id > SI3474_MAX_CHANS) {
+			if (chan_id >= SI3474_MAX_CHANS) {
 				dev_err(&priv->client[0]->dev,
-					"Incorrect channel number: %d\n", chan_id);
+					"Incorrect channel number: %u\n", chan_id);
 				return -EINVAL;
 			}
 
@@ -267,6 +267,16 @@ static int si3474_get_of_channels(struct si3474_priv *priv)
 				pi->np);
 			return -EINVAL;
 		}
+
+		/* The 2P/4P mode bit pairs channels 0+1 and 2+3 of a quad */
+		if (pairset_found &&
+		    (priv->pi[pi_no].chan[0] ^ 1) != priv->pi[pi_no].chan[1]) {
+			dev_err(&priv->client[0]->dev,
+				"PI %pOF: channels %u and %u are not a channel pair\n",
+				pi->np, priv->pi[pi_no].chan[0],
+				priv->pi[pi_no].chan[1]);
+			return -EINVAL;
+		}
 	}
 
 	return 0;
-- 
2.55.0