[PATCH v5 12/14] serial: 8250: allow UART drivers to override rx_trig_bytes handling

The rx_trig_bytes sysfs attribute currently relies on 8250-internal
helper functions and assumes a fixed mapping between trigger levels and
FIFO behavior.

Some UARTs provide hardware-specific RX trigger mechanisms that do not
fit this model. Add optional uart_port callbacks for setting and getting
the RX trigger level, and use them when provided, while preserving the
existing 8250 helpers as the default fallback.

Signed-off-by: Crescent Hsieh <[email protected]>
---
 drivers/tty/serial/8250/8250_core.c |  4 ++++
 drivers/tty/serial/8250/8250_port.c | 14 ++++++++++++--
 include/linux/serial_core.h         |  2 ++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 0a3355eb4bc3..052c0020a860 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -792,6 +792,10 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
 		uart->port.get_divisor = up->port.get_divisor;
 	if (up->port.set_divisor)
 		uart->port.set_divisor = up->port.set_divisor;
+	if (up->port.get_rxtrig)
+		uart->port.get_rxtrig = up->port.get_rxtrig;
+	if (up->port.set_rxtrig)
+		uart->port.set_rxtrig = up->port.set_rxtrig;
 	if (up->port.startup)
 		uart->port.startup = up->port.startup;
 	if (up->port.shutdown)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 6325784e5e73..c2b46ef01d2f 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2999,9 +2999,14 @@ static ssize_t rx_trig_bytes_show(struct device *dev,
 	struct device_attribute *attr, char *buf)
 {
 	struct tty_port *port = dev_get_drvdata(dev);
+	struct uart_state *state = container_of(port, struct uart_state, port);
+	struct uart_port *uport = state->uart_port;
 	int rxtrig_bytes;
 
-	rxtrig_bytes = do_serial8250_get_rxtrig(port);
+	if (uport->get_rxtrig)
+		rxtrig_bytes = uport->get_rxtrig(uport);
+	else
+		rxtrig_bytes = do_serial8250_get_rxtrig(port);
 	if (rxtrig_bytes < 0)
 		return rxtrig_bytes;
 
@@ -3044,6 +3049,8 @@ static ssize_t rx_trig_bytes_store(struct device *dev,
 	struct device_attribute *attr, const char *buf, size_t count)
 {
 	struct tty_port *port = dev_get_drvdata(dev);
+	struct uart_state *state = container_of(port, struct uart_state, port);
+	struct uart_port *uport = state->uart_port;
 	unsigned char bytes;
 	int ret;
 
@@ -3054,7 +3061,10 @@ static ssize_t rx_trig_bytes_store(struct device *dev,
 	if (ret < 0)
 		return ret;
 
-	ret = do_serial8250_set_rxtrig(port, bytes);
+	if (uport->set_rxtrig)
+		ret = uport->set_rxtrig(uport, bytes);
+	else
+		ret = do_serial8250_set_rxtrig(port, bytes);
 	if (ret < 0)
 		return ret;
 
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index f6ba3e7b09f1..13b0d4117036 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -459,6 +459,8 @@ struct uart_port {
 					       unsigned int baud,
 					       unsigned int quot,
 					       unsigned int quot_frac);
+	int			(*get_rxtrig)(struct uart_port *port);
+	int			(*set_rxtrig)(struct uart_port *port, unsigned char bytes);
 	int			(*startup)(struct uart_port *port);
 	void			(*shutdown)(struct uart_port *port);
 	void			(*throttle)(struct uart_port *port);
-- 
2.45.2