[PATCH tty v3 6/6] serial: 8250: Add support for console flow control

John Ogness
2026-04-17T10:24:36Z

The kernel documentation specifies that the console option 'r' can
be used to enable hardware flow control for console writes. The 8250
driver does include code for hardware flow control on the console if
cons_flow is set, but there is no code path that actually sets this.
However, that is not the only issue. The problems are:

1. Specifying the console option 'r' does not lead to cons_flow being
set.

2. Even if cons_flow would be set, serial8250_register_8250_port()
clears it.

3. When the console option 'r' is specified, uart_set_options()
attempts to initialize the port for CRTSCTS. However, afterwards
it does not set the UPSTAT_CTS_ENABLE status bit and therefore on
boot, uart_cts_enabled() is always false. This policy bit is
important for console drivers as a criteria if they may poll CTS.

4. Even though uart_set_options() attempts to initialize the port
for CRTSCTS, the 8250 set_termios() callback does not enable the
RTS signal (TIOCM_RTS) and thus the hardware is not properly
initialized for CTS polling.

5. Even if modem control was properly setup for CTS polling
(TIOCM_RTS), uart_configure_port() clears TIOCM_RTS, thus
breaking CTS polling.

6. wait_for_xmitr() and serial8250_console_write() use cons_flow
to decide if CTS polling should occur. However, the condition
should also include a check that it is not in RS485 mode and
CRTSCTS is actually enabled in the hardware.

Address all these issues as conservatively as possible by gating them
behind checks focussed on the user specifying console hardware flow
control support and the hardware being configured for CTS polling
at the time of the write to the UART.

Since checking the UPSTAT_CTS_ENABLE status bit is a part of the new
condition gate, these changes also support runtime termios updates to
disable/enable CRTSCTS.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
drivers/tty/serial/8250/8250_core.c | 6 +++++-
drivers/tty/serial/8250/8250_port.c | 13 +++++++++++--
drivers/tty/serial/serial_core.c | 21 ++++++++++++++++++++-
include/linux/serial_core.h | 8 ++++++++
4 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 32b894ab26768..a19646e24d883 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -693,6 +693,7 @@ static void serial_8250_overrun_backoff_work(struct work_struct *work)
int serial8250_register_8250_port(const struct uart_8250_port *up)
{
struct uart_8250_port *uart;
+ bool cons_flow;
int ret;

if (up->port.uartclk == 0)
@@ -716,6 +717,9 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
if (uart->port.type == PORT_8250_CIR)
return -ENODEV;

+ /* Preserve specified console flow control. */
+ cons_flow = uart_get_cons_flow(&uart->port);
+
if (uart->port.dev)
uart_remove_one_port(&serial8250_reg, &uart->port);

@@ -746,7 +750,7 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
uart->lsr_save_mask = up->lsr_save_mask;
uart->dma = up->dma;

- uart_set_cons_flow(&uart->port, uart_get_cons_flow(&up->port));
+ uart_set_cons_flow(&uart->port, uart_get_cons_flow(&up->port) | cons_flow);

/* Take tx_loadsz from fifosize if it wasn't set separately */
if (uart->port.fifosize && !uart->tx_loadsz)
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index b3247c55eebd5..f585145a6211a 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1989,7 +1989,7 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits)
tx_ready = wait_for_lsr(up, bits);

/* Wait up to 1s for flow control if necessary */
- if (uart_get_cons_flow(&up->port)) {
+ if (uart_console_hwflow_active(&up->port)) {
for (tmout = 1000000; tmout; tmout--) {
unsigned int msr = serial_in(up, UART_MSR);
up->msr_saved_flags |= msr & MSR_SAVE_FLAGS;
@@ -2786,6 +2786,12 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
serial8250_set_efr(port, termios);
serial8250_set_divisor(port, baud, quot, frac);
serial8250_set_fcr(port, termios);
+ /* Consoles manually poll CTS for hardware flow control. */
+ if (uart_console(port) &&
+ !(port->rs485.flags & SER_RS485_ENABLED)
+ && termios->c_cflag & CRTSCTS) {
+ port->mctrl |= TIOCM_RTS;
+ }
serial8250_set_mctrl(port, port->mctrl);
}

@@ -3355,7 +3361,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
* it regardless of the CTS state. Therefore, only use fifo
* if we don't use control flow.
*/
- !uart_get_cons_flow(&up->port);
+ !uart_console_hwflow_active(&up->port);

if (likely(use_fifo))
serial8250_console_fifo_write(up, s, count);
@@ -3425,6 +3431,9 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
if (ret)
return ret;

+ /* Track user-specified console flow control. */
+ uart_set_cons_flow(port, flow == 'r');
+
if (port->dev)
pm_runtime_get_sync(port->dev);

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 89cebdd278410..840336f95c5f6 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2235,6 +2235,18 @@ uart_set_options(struct uart_port *port, struct console *co,
port->mctrl |= TIOCM_DTR;

port->ops->set_termios(port, &termios, &dummy);
+
+ /*
+ * If console hardware flow control was specified and is supported,
+ * the related policy UPSTAT_CTS_ENABLE must be set to allow console
+ * drivers to identify if CTS should be used for polling.
+ */
+ if (flow == 'r' && (termios.c_cflag & CRTSCTS)) {
+ /* Synchronize @status RMW update against the console. */
+ guard(uart_port_lock_irqsave)(port);
+ port->status |= UPSTAT_CTS_ENABLE;
+ }
+
/*
* Allow the setting of the UART parameters with a NULL console
* too:
@@ -2541,7 +2553,14 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state,
* We probably don't need a spinlock around this, but
*/
scoped_guard(uart_port_lock_irqsave, port) {
- port->mctrl &= TIOCM_DTR;
+ unsigned int mask = TIOCM_DTR;
+
+ /* Console hardware flow control polls CTS. */
+ if (uart_console_hwflow_active(port))
+ mask |= TIOCM_RTS;
+
+ port->mctrl &= mask;
+
if (!(port->rs485.flags & SER_RS485_ENABLED))
port->ops->set_mctrl(port, port->mctrl);
}
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 2327a364ded16..ff1145e86088b 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -1175,6 +1175,14 @@ static inline bool uart_get_cons_flow(const struct uart_port *uport)
return uport->cons_flow;
}

+static inline bool uart_console_hwflow_active(struct uart_port *uport)
+{
+ return uart_console(uport) &&
+ !(uport->rs485.flags & SER_RS485_ENABLED) &&
+ uart_get_cons_flow(uport) &&
+ uart_cts_enabled(uport);
+}
+
/*
* The following are helper functions for the low level drivers.
*/
--
2.47.3