[PATCH tty v3 0/6] 8250: Add console flow control

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

Hi,

This is v3 of a series to implement console flow control for the
8250 serial driver. v2 is here [0].

The 8250 driver already has code in place to support console flow
control. However, there is no way to activate it and it is
incomplete. This series provides the necessary missing pieces while
attempting to be as conservative as possible, so as not to introduce
any side effects into the many 8250 variants or other non-8250 serial
drivers.

Note that as of v3 I am now deprecating UPF_CONS_FLOW in favor of a
separate boolean field. In the commit message (patch 1) I explain my
reasoning for this decision.

For patch 2 I used the following Coccinelle script to perform the
modifications...

===== BEGIN cons_flow.cocci =====
// SPDX-License-Identifier: GPL-2.0-only
// Options: --all-includes

virtual patch

@r1@
type T1;
identifier U;
@@

T1 {
...
struct uart_port U;
...
};

@r2@
r1.T1 *E;
@@

- (E->port.flags & UPF_CONS_FLOW)
+ uart_get_cons_flow(&E->port)

@r3@
struct uart_port *U;
@@

- (U->flags & UPF_CONS_FLOW)
+ uart_get_cons_flow(U)

@r4@
struct uart_port *U;
@@

- U->flags |= UPF_CONS_FLOW
+ uart_set_cons_flow(U, true)
===== END cons_flow.cocci =====

Changes since v2:

- Deprecate UPF_CONS_FLOW. Provide separate boolean with wrappers as
alternative.

- Update all UPF_CONS_FLOW users to new cons_flow wrappers.

- Use irqsave variant of spin lock for status update.

- When 8250 console flow control is not specified, clear the policy.

Changes since v1:

- Prepend a patch to perform an extra LSR wait after CTS assertion if
the initial LSR wait timed out.

- Close a window in serial8250_register_8250_port() where console
flow control was briefly disabled.

- Add port lock synchronization to the port->status RMW update in
uart_set_options().

John Ogness

[0] https://lore.kernel.org/lkml/20260410144949.16581-1-john.ogness@linutronix.de

John Ogness (6):
serial: core: Add dedicated uart_port field for console flow
serial: Replace driver usage of UPF_CONS_FLOW
serial: sh-sci: Avoid deprecated UPF_CONS_FLOW
serial: 8250: Set cons_flow on port registration
serial: 8250: Check LSR timeout on console flow control
serial: 8250: Add support for console flow control

drivers/tty/serial/8250/8250_core.c | 6 ++++++
drivers/tty/serial/8250/8250_port.c | 21 +++++++++++++++++----
drivers/tty/serial/bcm63xx_uart.c | 2 +-
drivers/tty/serial/omap-serial.c | 2 +-
drivers/tty/serial/pch_uart.c | 2 +-
drivers/tty/serial/pxa.c | 2 +-
drivers/tty/serial/samsung_tty.c | 8 ++++----
drivers/tty/serial/serial_core.c | 21 ++++++++++++++++++++-
drivers/tty/serial/serial_txx9.c | 4 ++--
drivers/tty/serial/sh-sci.c | 5 ++++-
drivers/tty/serial/sunsu.c | 2 +-
include/linux/serial_core.h | 20 ++++++++++++++++++++
12 files changed, 78 insertions(+), 17 deletions(-)

base-commit: a1a81aef99e853dec84241d701fbf587d713eb5b
--
2.47.3