[PATCH v7 05/12] serial: 8250_dw: derive RX trigger thresholds from the FIFO depth
From: James Hilliard
Date: Tue Nov 03 2026 - 16:44:44 EST
DW_apb_uart uses receive thresholds of one byte, one quarter full,
one half full and two bytes below full. The generic 16550A table describes
these encodings only for a 16-byte FIFO. For example, the half-full setting
on a 64-byte DW FIFO is reported as eight bytes instead of 32, and a sysfs
request for 32 bytes selects the 62-byte encoding.
Build a per-port threshold table from the discovered FIFO depth in dwlib
and install receive-trigger callbacks for CPR-described FIFOs, including
LPSS ports. Keep the setup helper available to integrations which obtain
the depth without probing the optional DW registers. Validate that each
threshold is positive and representable before constructing the table.
Serialize FCR updates with termios and the port lock, acquire a runtime PM
reference before accessing the device, and propagate resume errors without
changing the shadow or hardware. Update only the trigger field so changing
the threshold preserves the FIFO contents.
The FCR mapping is documented in the DW_apb_uart databook, section 6,
FIFO Control Register. The existing default FCR encoding is unchanged.
Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
---
drivers/tty/serial/8250/8250_dw.c | 7 ++-
drivers/tty/serial/8250/8250_dwlib.c | 94 +++++++++++++++++++++++++++++++++++-
drivers/tty/serial/8250/8250_dwlib.h | 4 +-
drivers/tty/serial/8250/8250_lpss.c | 4 +-
4 files changed, 104 insertions(+), 5 deletions(-)
diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index a546cf2de7b6..613907b8a40f 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -648,7 +648,12 @@ static int dw8250_setup_dma(struct uart_port *p)
static int dw8250_setup(struct uart_port *p)
{
- dw8250_setup_port(p);
+ int ret;
+
+ ret = dw8250_setup_port(p);
+ if (ret)
+ return ret;
+
return dw8250_setup_dma(p);
}
diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
index 91b3607b64e6..004817cc8f28 100644
--- a/drivers/tty/serial/8250/8250_dwlib.c
+++ b/drivers/tty/serial/8250/8250_dwlib.c
@@ -1,18 +1,27 @@
// SPDX-License-Identifier: GPL-2.0+
/* Synopsys DesignWare 8250 library. */
+#include <linux/array_size.h>
#include <linux/bitops.h>
#include <linux/bitfield.h>
#include <linux/delay.h>
#include <linux/device.h>
#include <linux/kernel.h>
+#include <linux/limits.h>
#include <linux/math.h>
+#include <linux/mutex.h>
+#include <linux/pm_runtime.h>
#include <linux/property.h>
#include <linux/serial_8250.h>
#include <linux/serial_core.h>
#include "8250_dwlib.h"
+/* DesignWare FIFO-relative receive thresholds */
+#define DW_UART_RXTRIG_QUARTER_DIV 4
+#define DW_UART_RXTRIG_HALF_DIV 2
+#define DW_UART_RXTRIG_FULL_MARGIN 2
+
/*
* divisor = div(I) + div(F)
* "I" means integer, "F" means fractional
@@ -208,11 +217,86 @@ static const struct serial_rs485 dw8250_rs485_supported = {
SER_RS485_ADDR_DEST,
};
-void dw8250_setup_port(struct uart_port *p)
+/* The trigger is the masked FCR receive trigger field, in register position */
+static unsigned int dw8250_rxtrig_bytes(struct uart_port *p, unsigned int trigger)
+{
+ switch (trigger) {
+ case UART_FCR_TRIGGER_1:
+ return 1;
+ case UART_FCR_TRIGGER_4:
+ return p->fifosize / DW_UART_RXTRIG_QUARTER_DIV;
+ case UART_FCR_TRIGGER_8:
+ return p->fifosize / DW_UART_RXTRIG_HALF_DIV;
+ default:
+ return p->fifosize - DW_UART_RXTRIG_FULL_MARGIN;
+ }
+}
+
+static int dw8250_get_rxtrig(struct uart_port *p)
+{
+ struct dw8250_port_data *pd = p->private_data;
+ struct uart_8250_port *up = up_to_u8250p(p);
+
+ guard(uart_port_lock_irqsave)(p);
+
+ return pd->rxtrig_bytes[UART_FCR_R_TRIG_BITS(up->fcr)];
+}
+
+static int dw8250_set_rxtrig(struct uart_port *p, u16 bytes)
+{
+ struct dw8250_port_data *pd = p->private_data;
+ struct uart_8250_port *up = up_to_u8250p(p);
+ int fcr, ret;
+
+ fcr = serial8250_bytes_to_fcr(pd->rxtrig_bytes, bytes);
+ if (fcr < 0)
+ return fcr;
+
+ guard(mutex)(&p->state->port.mutex);
+
+ PM_RUNTIME_ACQUIRE_IF_ENABLED(p->dev, pm);
+ ret = PM_RUNTIME_ACQUIRE_ERR(&pm);
+ if (ret)
+ return ret;
+
+ guard(uart_port_lock_irqsave)(p);
+
+ up->fcr &= ~UART_FCR_TRIGGER_MASK;
+ up->fcr |= fcr;
+ /* Changing the trigger alone preserves the FIFO contents */
+ serial_port_out(p, UART_FCR, up->fcr);
+
+ return 0;
+}
+
+int dw8250_setup_rxtrig(struct uart_port *p)
+{
+ struct dw8250_port_data *pd = p->private_data;
+
+ /* Each FIFO-relative threshold must be positive and fit in the table */
+ if (p->fifosize < DW_UART_RXTRIG_QUARTER_DIV ||
+ p->fifosize - DW_UART_RXTRIG_FULL_MARGIN > U16_MAX)
+ return dev_err_probe(p->dev, -EINVAL,
+ "invalid FIFO size %u\n", p->fifosize);
+
+ for (unsigned int i = 0; i < ARRAY_SIZE(pd->rxtrig_bytes); i++) {
+ unsigned int trigger = FIELD_PREP(UART_FCR_TRIGGER_MASK, i);
+
+ pd->rxtrig_bytes[i] = dw8250_rxtrig_bytes(p, trigger);
+ }
+ p->get_rxtrig = dw8250_get_rxtrig;
+ p->set_rxtrig = dw8250_set_rxtrig;
+
+ return 0;
+}
+EXPORT_SYMBOL_NS_GPL(dw8250_setup_rxtrig, "SERIAL_8250");
+
+int dw8250_setup_port(struct uart_port *p)
{
struct dw8250_port_data *pd = p->private_data;
struct uart_8250_port *up = up_to_u8250p(p);
u32 reg, old_dlf;
+ int ret;
pd->hw_rs485_support = dw8250_detect_rs485_hw(p);
if (pd->hw_rs485_support) {
@@ -250,7 +334,7 @@ void dw8250_setup_port(struct uart_port *p)
dev_dbg(p->dev, "CPR is not available, using 0x%08x instead\n", reg);
}
if (!reg)
- return;
+ return 0;
/* Select the type based on FIFO */
if (reg & DW_UART_CPR_FIFO_MODE) {
@@ -258,6 +342,10 @@ void dw8250_setup_port(struct uart_port *p)
p->flags |= UPF_FIXED_TYPE;
p->fifosize = DW_UART_CPR_FIFO_SIZE(reg);
up->capabilities = UART_CAP_FIFO | UART_CAP_NOTEMT;
+
+ ret = dw8250_setup_rxtrig(p);
+ if (ret)
+ return ret;
}
if (reg & DW_UART_CPR_AFCE_MODE)
@@ -270,5 +358,7 @@ void dw8250_setup_port(struct uart_port *p)
pd->srbr = DW_UART_SRBR_0;
else
pd->srbr = UART_RX;
+
+ return 0;
}
EXPORT_SYMBOL_GPL(dw8250_setup_port);
diff --git a/drivers/tty/serial/8250/8250_dwlib.h b/drivers/tty/serial/8250/8250_dwlib.h
index 7191cabfb086..44a5d0c2feba 100644
--- a/drivers/tty/serial/8250/8250_dwlib.h
+++ b/drivers/tty/serial/8250/8250_dwlib.h
@@ -88,6 +88,7 @@ struct dw8250_port_data {
/* Hardware configuration */
u32 cpr_value;
u8 dlf_size;
+ u16 rxtrig_bytes[UART_FCR_R_TRIG_MAX_STATE];
/* RS485 variables */
bool hw_rs485_support;
@@ -100,7 +101,8 @@ struct dw8250_port_data {
unsigned int dw8250_get_divisor_dlf(struct uart_port *p, unsigned int baud, unsigned int *frac);
void dw8250_set_divisor_dlf(struct uart_port *p, unsigned int quot_frac);
void dw8250_do_set_termios(struct uart_port *p, struct ktermios *termios, const struct ktermios *old);
-void dw8250_setup_port(struct uart_port *p);
+int dw8250_setup_rxtrig(struct uart_port *p);
+int dw8250_setup_port(struct uart_port *p);
static inline u32 dw8250_readl_ext(struct uart_port *p, int offset)
{
diff --git a/drivers/tty/serial/8250/8250_lpss.c b/drivers/tty/serial/8250/8250_lpss.c
index f1b4b975571a..7ace0515038e 100644
--- a/drivers/tty/serial/8250/8250_lpss.c
+++ b/drivers/tty/serial/8250/8250_lpss.c
@@ -350,7 +350,9 @@ static int lpss8250_probe(struct pci_dev *pdev, const struct pci_device_id *id)
if (ret)
return ret;
- dw8250_setup_port(&uart.port);
+ ret = dw8250_setup_port(&uart.port);
+ if (ret)
+ goto err_exit;
ret = lpss8250_dma_setup(lpss, &uart);
if (ret)
--
2.53.0