[PATCH tty v3 1/6] serial: core: Add dedicated uart_port field for console flow

John Ogness
2026-04-17T10:24:32Z

Currently the UPF_CONS_FLOW bit in the uart_port.flags field is used
by serial console drivers to identify if a user has configured flow
control on the console. This policy is most commonly setup during
early boot, but can be changed at runtime.

The bits in uart_port.flags are either hardware and driver properties
that are initialized before usage or are properties that can be
changed via the tty layer.

The UPF_CONS_FLOW is an exception because it is a console-only policy
that can change at runtime and its setting and usage have nothing to
do with the tty layer. This actually causes a problem for its usage
because uart_port.flags is synchronized by a related tty_port.mutex,
but a console has no relation to a tty (other than sharing the port).

This is probably why console flow control is not properly available
for most serial drivers. And it is hindering being able to provide a
proper implementation. Commit d01f4d181c92 ("serial: core: Privatize
tty->hw_stopped") addressed a similar issue to deal with software
assisted CTS flow state tracking.

Add a new uart_port boolean field "cons_flow" to store the user
configuration for console flow control. Add get/set wrappers to allow
for adding more policies later and/or locking constraint validation.

Mark UPF_CONS_FLOW as deprecated.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
include/linux/serial_core.h | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 666430b478997..2327a364ded16 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -533,6 +533,7 @@ struct uart_port {
#define UPF_HARD_FLOW ((__force upf_t) (UPF_AUTO_CTS | UPF_AUTO_RTS))
/* Port has hardware-assisted s/w flow control */
#define UPF_SOFT_FLOW ((__force upf_t) BIT_ULL(22))
+/* Deprecated: use uart_get_cons_flow()/uart_set_cons_flow() instead. */
#define UPF_CONS_FLOW ((__force upf_t) BIT_ULL(23))
#define UPF_SHARE_IRQ ((__force upf_t) BIT_ULL(24))
#define UPF_EXAR_EFR ((__force upf_t) BIT_ULL(25))
@@ -567,6 +568,7 @@ struct uart_port {
#define UPSTAT_SYNC_FIFO ((__force upstat_t) (1 << 5))

bool hw_stopped; /* sw-assisted CTS flow state */
+ bool cons_flow; /* user specified console flow control */
unsigned int mctrl; /* current modem ctrl settings */
unsigned int frame_time; /* frame timing in ns */
unsigned int type; /* port type */
@@ -1163,6 +1165,16 @@ static inline bool uart_softcts_mode(struct uart_port *uport)
return ((uport->status & mask) == UPSTAT_CTS_ENABLE);
}

+static inline void uart_set_cons_flow(struct uart_port *uport, bool on)
+{
+ uport->cons_flow = on;
+}
+
+static inline bool uart_get_cons_flow(const struct uart_port *uport)
+{
+ return uport->cons_flow;
+}
+
/*
* The following are helper functions for the low level drivers.
*/
--
2.47.3