serial: max310x: honour rs485 properties from per-channel DT subnode

The MAX310x DT binding pulls in /schemas/serial/rs485.yaml via its allOf
list, advertising the rs485-* properties defined there - none of which
were honoured at runtime, because the driver never called
uart_get_rs485_mode().

All channels share the parent SPI/I2C device, so uart_get_rs485_mode()
called directly on each port would read the same chip-level fwnode for
every call. Walk dev->of_node's children for the "serial@N" subnode
with matching reg, and temporarily retarget the parent device's fwnode
while uart_get_rs485_mode() runs, so each channel picks up its own
subnode's properties. Probe is serialised, so the swap is safe.

For single-channel variants (max3107, max3108), fall back to the chip's
own fwnode when no subnode is present, so existing DTs that declare
rs485 properties at the top level keep working.

Signed-off-by: Tapio Reijonen <tapio.reijonen@vaisala.com>
Link: https://patch.msgid.link/20260615-b4-max310x-rs485-dt-v3-3-7e79f064bdd7@vaisala.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 37 insertions, 0 deletions

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index 3e26bdf8806b..022502986c5f 100644
--- a/drivers/tty/serial/max310x.c
+++ b/drivers/tty/serial/max310x.c
@@ -1452,6 +1452,9 @@ static int max310x_probe(struct device *dev, const struct max310x_devtype *devty
 #endif
 
 	for (i = 0; i < devtype->nr; i++) {
+		struct fwnode_handle *saved_fwnode = dev_fwnode(dev);
+		struct device_node *port_np = NULL;
+		struct device_node *child;
 		unsigned int line;
 
 		line = find_first_zero_bit(max310x_lines, MAX310X_UART_NRMAX);
@@ -1461,6 +1464,40 @@ static int max310x_probe(struct device *dev, const struct max310x_devtype *devty
 		}
 		s->p[i].port.line = line;
 
+		/* Locate the matching "serial@i" DT subnode, if any. */
+		for_each_available_child_of_node(dev->of_node, child) {
+			u32 reg;
+
+			if (!of_node_name_eq(child, "serial"))
+				continue;
+			if (of_property_read_u32(child, "reg", &reg))
+				continue;
+			if (reg == i) {
+				port_np = child;
+				break;
+			}
+		}
+
+		/*
+		 * Temporarily retarget dev's fwnode to the per-port subnode
+		 * so uart_get_rs485_mode() picks up the per-port properties.
+		 * For single-port variants, fall back to the chip's own
+		 * fwnode so legacy DTs that declare rs485 properties at the
+		 * top level keep working.
+		 */
+		if (port_np) {
+			device_set_node(dev, of_fwnode_handle(port_np));
+			ret = uart_get_rs485_mode(&s->p[i].port);
+			device_set_node(dev, saved_fwnode);
+			of_node_put(port_np);
+			if (ret)
+				goto out_uart;
+		} else if (devtype->nr == 1) {
+			ret = uart_get_rs485_mode(&s->p[i].port);
+			if (ret)
+				goto out_uart;
+		}
+
 		/* Register port */
 		ret = uart_add_one_port(&max310x_uart, &s->p[i].port);
 		if (ret)