kernel generic API rx_trigger to set low level RX FIFO threshold or idle timeout

sllin/linux-patches/0001-serial-add-rx_trigger-API-to-uart_ops.patch

@@ -0,0 +1,71 @@
+From 3d941169431ab9c362412798b8e1702ba4c8a800 Mon Sep 17 00:00:00 2001
+From: Daniel Trnka <[email protected]>
+Date: Sun, 7 Sep 2025 09:34:29 +0200
+Subject: [PATCH 1/2] serial: add rx_trigger API to uart_ops
+
+Introduce rx_trigger() callback in struct uart_ops to configure
+RX FIFO trigger conditions based on the number of received bytes
+or an idle timeout in nanoseconds. This allows communication
+protocols to react immediately to incoming data without waiting
+for the RX buffer to fill or for a timeout.
+---
+ include/linux/serial_core.h      | 21 +++++++++++++++++++++
+ include/uapi/linux/serial_core.h |  5 +++++
+ 2 files changed, 26 insertions(+)
+
+diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
+index 84b4648ead7..ef2bfe2478d 100644
+--- a/include/linux/serial_core.h
++++ b/include/linux/serial_core.h
+@@ -344,6 +344,25 @@ struct gpio_desc;
+  *	Locking: none.
+  *	Interrupts: caller dependent.
+  *
++ * @rx_trigger: ``int (*)(struct uart_port *port, int mode,
++ *		int *rx_trigger_bytes, int *rx_trigger_idle_time)``
++ *
++ * Configure the RX trigger condition based on either the number of received
++ * bytes or a period of RX line idleness, expressed in nanoseconds.
++ *
++ * Supported @mode:
++ *  UART_RX_TRIGGER_MODE_SET
++ *		Apply the trigger using the values in @rx_trigger_bytes and
++ *		@rx_trigger_idle_time.
++ *  UART_RX_TRIGGER_MODE_CHECK_ROUND_DOWN
++ *  UART_RX_TRIGGER_MODE_CHECK_ROUND_UP
++ *		Round the requested values down or up to the nearest hardware-
++ *		supported value. The adjusted values are written back to
++ *		@rx_trigger_bytes and @rx_trigger_idle_time.
++ *  UART_RX_TRIGGER_MODE_GET
++ *		Return the currently configured trigger values via
++ *		@rx_trigger_bytes and @rx_trigger_idle_time.
++ *
+  * @poll_init: ``int ()(struct uart_port *port)``
+  *
+  *	Called by kgdb to perform the minimal hardware initialization needed to
+@@ -399,6 +418,8 @@ struct uart_ops {
+ 	void		(*config_port)(struct uart_port *, int);
+ 	int		(*verify_port)(struct uart_port *, struct serial_struct *);
+ 	int		(*ioctl)(struct uart_port *, unsigned int, unsigned long);
++	int		(*rx_trigger)(struct uart_port *, int mode, int *rx_trigger_bytes,
++		int *rx_trigger_idle_time);
+ #ifdef CONFIG_CONSOLE_POLL
+ 	int		(*poll_init)(struct uart_port *);
+ 	void		(*poll_put_char)(struct uart_port *, unsigned char);
+diff --git a/include/uapi/linux/serial_core.h b/include/uapi/linux/serial_core.h
+index 9c007a10633..e3445d4edc9 100644
+--- a/include/uapi/linux/serial_core.h
++++ b/include/uapi/linux/serial_core.h
+@@ -234,4 +234,9 @@
+ /* Generic type identifier for ports which type is not important to userspace. */
+ #define PORT_GENERIC	(-1)
+
++#define UART_RX_TRIGGER_MODE_SET 0
++#define UART_RX_TRIGGER_MODE_CHECK_ROUND_DOWN 1
++#define UART_RX_TRIGGER_MODE_CHECK_ROUND_UP 2
++#define UART_RX_TRIGGER_MODE_GET 3
++
+ #endif /* _UAPILINUX_SERIAL_CORE_H */
+-- 
+2.34.1
+

sllin/linux-patches/0002-serial-imx-implementation-of-rx_trigger-uart_ops.patch

@@ -0,0 +1,94 @@
+From 639d5c8ccaf36220422f0da422e73577f4276c4b Mon Sep 17 00:00:00 2001
+From: Daniel Trnka <[email protected]>
+Date: Sun, 7 Sep 2025 09:46:01 +0200
+Subject: [PATCH 2/2] serial: imx: implementation of rx_trigger uart_ops
+
+- rx_trigger_bytes can be between 1 and 32
+- rx_trigger_idle_time not supported (AGTIM fixed at 8 characters)
+---
+ drivers/tty/serial/imx.c | 55 ++++++++++++++++++++++++++++++++++++++++
+ 1 file changed, 55 insertions(+)
+
+diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
+index 500dfc009d0..6ac5988ba34 100644
+--- a/drivers/tty/serial/imx.c
++++ b/drivers/tty/serial/imx.c
+@@ -175,6 +175,7 @@
+ #define DRIVER_NAME "IMX-uart"
+
+ #define UART_NR 8
++#define FIFO_MAX_DEPTH 32
+
+ /* i.MX21 type uart runs on all i.mx except i.MX1 and i.MX6q */
+ enum imx_uart_type {
+@@ -2054,6 +2055,59 @@ static int imx_uart_rs485_config(struct uart_port *port, struct ktermios *termio
+ 	return 0;
+ }
+
++/*
++ * rx_trigger_bytes can be between 1 and 32
++ * rx_trigger_idle_time is unsupported - AGTIM is fixed at 8 characters
++ */
++static int imx_uart_rx_trigger(struct uart_port *port, int mode,
++		int *rx_trigger_bytes, int *rx_trigger_idle_time)
++{
++	struct imx_port *sport = to_imx_port(port);
++	u32 ufcr;
++	unsigned int txwl, rxwl;
++
++	if (rx_trigger_idle_time) {
++		return -ENOTSUPP;
++	}
++
++	if (!rx_trigger_bytes) {
++		return -EINVAL;
++	}
++
++	/* read currently set FIFO thresholds */
++	ufcr = imx_uart_readl(sport, UFCR);
++	rxwl = (ufcr >> UFCR_RXTL_SHF) & UFCR_RXTL_MASK;
++	txwl = ufcr >> UFCR_TXTL_SHF;
++
++	switch (mode) {
++		case UART_RX_TRIGGER_MODE_SET:
++			if (*rx_trigger_bytes < 1 || *rx_trigger_bytes > FIFO_MAX_DEPTH) {
++				return -EINVAL;
++			}
++
++			imx_uart_setup_ufcr(sport, txwl, *rx_trigger_bytes);
++			break;
++
++		case UART_RX_TRIGGER_MODE_CHECK_ROUND_DOWN:
++		case UART_RX_TRIGGER_MODE_CHECK_ROUND_UP:
++			if (*rx_trigger_bytes < 1) {
++				*rx_trigger_bytes = 1;
++			}
++
++			if (*rx_trigger_bytes > FIFO_MAX_DEPTH) {
++				*rx_trigger_bytes = FIFO_MAX_DEPTH;
++			}
++
++			break;
++		case UART_RX_TRIGGER_MODE_GET:
++			*rx_trigger_bytes = rxwl;
++			break;
++		default:
++			return -ENOTSUPP;
++	}
++	return 0;
++}
++
+ static const struct uart_ops imx_uart_pops = {
+ 	.tx_empty	= imx_uart_tx_empty,
+ 	.set_mctrl	= imx_uart_set_mctrl,
+@@ -2070,6 +2124,7 @@ static const struct uart_ops imx_uart_pops = {
+ 	.type		= imx_uart_type,
+ 	.config_port	= imx_uart_config_port,
+ 	.verify_port	= imx_uart_verify_port,
++	.rx_trigger	= imx_uart_rx_trigger,
+ #if defined(CONFIG_CONSOLE_POLL)
+ 	.poll_init      = imx_uart_poll_init,
+ 	.poll_get_char  = imx_uart_poll_get_char,
+-- 
+2.34.1
+

sllin/sllin.c

@@ -65,6 +65,7 @@
 #include <linux/hrtimer.h>
 #include <linux/version.h>
 #include "linux/lin_bus.h"
+#include <linux/serial_core.h>
 
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(4,11,0)
 #include <uapi/linux/sched/types.h>
@@ -1678,6 +1679,20 @@ static int sllin_open_common(struct tty_struct *tty, bool setup_master)
 	 */
 	rtnl_lock();
 
+#ifdef UART_RX_TRIGGER_MODE_SET
+	struct uart_state *state = tty->driver_data;
+	if (state) {
+		struct uart_port *uport = state->uart_port;
+		if (uport && uport->ops->rx_trigger) {
+			int rx_trigger_bytes = 1;
+			err = uport->ops->rx_trigger(uport, UART_RX_TRIGGER_MODE_SET, &rx_trigger_bytes, NULL);
+			if (err) {
+				pr_err("sllin: Failed to configure rx_trigger: %d", err);
+			}
+		}
+	}
+#endif
+
 	/* Collect hanged up channels. */
 	sll_sync();