[PATCH tty v4 0/6] 8250: Add console flow control

[PATCH tty v4 0/6] 8250: Add console flow control

[John Ogness]

Hi,

This is v4 of a series to implement console flow control for the
8250 serial driver. v3 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.

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_cons_flow_enabled(&E->port)

@r3@
struct uart_port *U;
@@

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

@r4@
struct uart_port *U;
@@

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

Changes for v4:

- Rename uart_get_cons_flow() to uart_cons_flow_enabled().

- Rename uart_set_cons_flow() to uart_set_cons_flow_enabled().

Changes for v3:

- 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 for v2:

- 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/20260417102423.40984-1-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: a2083fd1fa7aa0ef5cd8fd92396da0de2d0654b0
-- 
2.47.3

[PATCH tty v4 2/6] serial: Replace driver usage of UPF_CONS_FLOW

[John Ogness]

Rather than using the UPF_CONS_FLOW bit of uart_port.flags to track
the user configuration of console flow control, use the newly added
uart_port.cons_flow (via its getter/setter functions).

A coccinelle script was used to perform the search/replace.

Note1: The sh-sci driver is blindly copying platform data configuration
       flags to uart_port.flags. Thus UPF_CONS_FLOW could get set for
       uart_port.flags. A follow-up commit will address this.

Note2: The samsung_tty driver is using UPF_CONS_FLOW as a platform data
       configuration flag. However, the driver explicitly checks for
       this configuration flag and thus setting UPF_CONS_FLOW in
       uart_port.flags is avoided.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 drivers/tty/serial/8250/8250_port.c | 4 ++--
 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_txx9.c    | 4 ++--
 drivers/tty/serial/sunsu.c          | 2 +-
 8 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index af78cc02f38e7..dbed5e5767541 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1988,7 +1988,7 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits)
 	wait_for_lsr(up, bits);
 
 	/* Wait up to 1s for flow control if necessary */
-	if (up->port.flags & UPF_CONS_FLOW) {
+	if (uart_cons_flow_enabled(&up->port)) {
 		for (tmout = 1000000; tmout; tmout--) {
 			unsigned int msr = serial_in(up, UART_MSR);
 			up->msr_saved_flags |= msr & MSR_SAVE_FLAGS;
@@ -3351,7 +3351,7 @@ 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.
 		 */
-		!(up->port.flags & UPF_CONS_FLOW);
+		!uart_cons_flow_enabled(&up->port);
 
 	if (likely(use_fifo))
 		serial8250_console_fifo_write(up, s, count);
diff --git a/drivers/tty/serial/bcm63xx_uart.c b/drivers/tty/serial/bcm63xx_uart.c
index 51df9d2d8bfc5..544695cb184c3 100644
--- a/drivers/tty/serial/bcm63xx_uart.c
+++ b/drivers/tty/serial/bcm63xx_uart.c
@@ -675,7 +675,7 @@ static void wait_for_xmitr(struct uart_port *port)
 	}
 
 	/* Wait up to 1s for flow control if necessary */
-	if (port->flags & UPF_CONS_FLOW) {
+	if (uart_cons_flow_enabled(port)) {
 		tmout = 1000000;
 		while (--tmout) {
 			unsigned int val;
diff --git a/drivers/tty/serial/omap-serial.c b/drivers/tty/serial/omap-serial.c
index 0b85f47ff19e0..a689d190940cf 100644
--- a/drivers/tty/serial/omap-serial.c
+++ b/drivers/tty/serial/omap-serial.c
@@ -1092,7 +1092,7 @@ static void __maybe_unused wait_for_xmitr(struct uart_omap_port *up)
 	} while (!uart_lsr_tx_empty(status));
 
 	/* Wait up to 1s for flow control if necessary */
-	if (up->port.flags & UPF_CONS_FLOW) {
+	if (uart_cons_flow_enabled(&up->port)) {
 		for (tmout = 1000000; tmout; tmout--) {
 			unsigned int msr = serial_in(up, UART_MSR);
 
diff --git a/drivers/tty/serial/pch_uart.c b/drivers/tty/serial/pch_uart.c
index 6729d8e83c3c5..2f72cd62e2488 100644
--- a/drivers/tty/serial/pch_uart.c
+++ b/drivers/tty/serial/pch_uart.c
@@ -1444,7 +1444,7 @@ static void wait_for_xmitr(struct eg20t_port *up, int bits)
 	}
 
 	/* Wait up to 1s for flow control if necessary */
-	if (up->port.flags & UPF_CONS_FLOW) {
+	if (uart_cons_flow_enabled(&up->port)) {
 		unsigned int tmout;
 		for (tmout = 1000000; tmout; tmout--) {
 			unsigned int msr = ioread8(up->membase + UART_MSR);
diff --git a/drivers/tty/serial/pxa.c b/drivers/tty/serial/pxa.c
index fea0255067ccd..10fc8990579b5 100644
--- a/drivers/tty/serial/pxa.c
+++ b/drivers/tty/serial/pxa.c
@@ -573,7 +573,7 @@ static void wait_for_xmitr(struct uart_pxa_port *up)
 	} while (!uart_lsr_tx_empty(status));
 
 	/* Wait up to 1s for flow control if necessary */
-	if (up->port.flags & UPF_CONS_FLOW) {
+	if (uart_cons_flow_enabled(&up->port)) {
 		tmout = 1000000;
 		while (--tmout &&
 		       ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0))
diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
index e27806bf2cf3e..2f94fc798cffb 100644
--- a/drivers/tty/serial/samsung_tty.c
+++ b/drivers/tty/serial/samsung_tty.c
@@ -319,7 +319,7 @@ static void s3c24xx_serial_stop_tx(struct uart_port *port)
 	ourport->tx_enabled = 0;
 	ourport->tx_in_progress = 0;
 
-	if (port->flags & UPF_CONS_FLOW)
+	if (uart_cons_flow_enabled(port))
 		s3c24xx_serial_rx_enable(port);
 
 	ourport->tx_mode = 0;
@@ -493,7 +493,7 @@ static void s3c24xx_serial_start_tx(struct uart_port *port)
 	struct tty_port *tport = &port->state->port;
 
 	if (!ourport->tx_enabled) {
-		if (port->flags & UPF_CONS_FLOW)
+		if (uart_cons_flow_enabled(port))
 			s3c24xx_serial_rx_disable(port);
 
 		ourport->tx_enabled = 1;
@@ -781,7 +781,7 @@ static void s3c24xx_serial_rx_drain_fifo(struct s3c24xx_uart_port *ourport)
 		uerstat = rd_regl(port, S3C2410_UERSTAT);
 		ch = rd_reg(port, S3C2410_URXH);
 
-		if (port->flags & UPF_CONS_FLOW) {
+		if (uart_cons_flow_enabled(port)) {
 			bool txe = s3c24xx_serial_txempty_nofifo(port);
 
 			if (ourport->rx_enabled) {
@@ -1830,7 +1830,7 @@ static int s3c24xx_serial_init_port(struct s3c24xx_uart_port *ourport,
 
 	if (cfg->uart_flags & UPF_CONS_FLOW) {
 		dev_dbg(port->dev, "enabling flow control\n");
-		port->flags |= UPF_CONS_FLOW;
+		uart_set_cons_flow_enabled(port, true);
 	}
 
 	/* sort our the physical and virtual addresses for each UART */
diff --git a/drivers/tty/serial/serial_txx9.c b/drivers/tty/serial/serial_txx9.c
index 436a559234dfe..4ae9a45c8e3a3 100644
--- a/drivers/tty/serial/serial_txx9.c
+++ b/drivers/tty/serial/serial_txx9.c
@@ -422,7 +422,7 @@ static void wait_for_xmitr(struct uart_port *up)
 		udelay(1);
 
 	/* Wait up to 1s for flow control if necessary */
-	if (up->flags & UPF_CONS_FLOW) {
+	if (uart_cons_flow_enabled(up)) {
 		tmout = 1000000;
 		while (--tmout &&
 		       (sio_in(up, TXX9_SICISR) & TXX9_SICISR_CTSS))
@@ -857,7 +857,7 @@ serial_txx9_console_write(struct console *co, const char *s, unsigned int count)
 	 *	Disable flow-control if enabled (and unnecessary)
 	 */
 	flcr = sio_in(up, TXX9_SIFLCR);
-	if (!(up->flags & UPF_CONS_FLOW) && (flcr & TXX9_SIFLCR_TES))
+	if (!uart_cons_flow_enabled(up) && (flcr & TXX9_SIFLCR_TES))
 		sio_out(up, TXX9_SIFLCR, flcr & ~TXX9_SIFLCR_TES);
 
 	uart_console_write(up, s, count, serial_txx9_console_putchar);
diff --git a/drivers/tty/serial/sunsu.c b/drivers/tty/serial/sunsu.c
index 6505a1930da9a..4ff3e5c41dab9 100644
--- a/drivers/tty/serial/sunsu.c
+++ b/drivers/tty/serial/sunsu.c
@@ -1245,7 +1245,7 @@ static void wait_for_xmitr(struct uart_sunsu_port *up)
 	} while (!uart_lsr_tx_empty(status));
 
 	/* Wait up to 1s for flow control if necessary */
-	if (up->port.flags & UPF_CONS_FLOW) {
+	if (uart_cons_flow_enabled(&up->port)) {
 		tmout = 1000000;
 		while (--tmout &&
 		       ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0))
-- 
2.47.3

Re: [PATCH tty v4 2/6] serial: Replace driver usage of UPF_CONS_FLOW

[John Ogness]

Hi,

(This email is particularly directed at the Samsung folks.)

Responding to Sashiko:

https://sashiko.dev/#/patchset/20260506121606.5805-1-john.ogness%40linutronix.de

On 2026-05-06, Sashiko wrote:
>> diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
>> index e27806bf2cf3e..2f94fc798cffb 100644
>> --- a/drivers/tty/serial/samsung_tty.c
>> +++ b/drivers/tty/serial/samsung_tty.c
>> @@ -319,7 +319,7 @@ static void s3c24xx_serial_stop_tx(struct uart_port *port)
>>  	ourport->tx_enabled = 0;
>>  	ourport->tx_in_progress = 0;
>>  
>> -	if (port->flags & UPF_CONS_FLOW)
>> +	if (uart_cons_flow_enabled(port))
>>  		s3c24xx_serial_rx_enable(port);
>
> This isn't a new bug introduced by this patch, but does this code
> result in a recursive spinlock acquisition and deadlock when console
> flow control is enabled?
>
> The serial core framework calls port->ops->stop_tx() with the
> port->lock spinlock already held. When uart_cons_flow_enabled(port)
> evaluates to true, s3c24xx_serial_stop_tx() invokes
> s3c24xx_serial_rx_enable().
>
> Looking at s3c24xx_serial_rx_enable(), it unconditionally attempts to
> acquire the exact same lock again:
>
> static void s3c24xx_serial_rx_enable(struct uart_port *port)
> {
> 	struct s3c24xx_uart_port *ourport = to_ourport(port);
> 	unsigned long flags;
> 	...
> 	uart_port_lock_irqsave(port, &flags);
> 	...
> }
>
> Because spinlocks are not recursive in the kernel, it appears this
> sequence would cause an immediate deadlock on the same CPU.

Although out of scope for my series, the reported issue is real. Perhaps
something for Krzysztof or Alim to look into.

>> @@ -493,7 +493,7 @@ static void s3c24xx_serial_start_tx(struct uart_port *port)
>>  	struct tty_port *tport = &port->state->port;
>>  
>>  	if (!ourport->tx_enabled) {
>> -		if (port->flags & UPF_CONS_FLOW)
>> +		if (uart_cons_flow_enabled(port))
>>  			s3c24xx_serial_rx_disable(port);
>
> Similarly here, s3c24xx_serial_start_tx() is also called with
> port->lock held, and s3c24xx_serial_rx_disable() will attempt to
> acquire port->lock again:
>
> static void s3c24xx_serial_rx_disable(struct uart_port *port)
> {
> 	struct s3c24xx_uart_port *ourport = to_ourport(port);
> 	unsigned long flags;
> 	...
> 	uart_port_lock_irqsave(port, &flags);
> 	...
> }
>
> Could this pre-existing locking issue in the samsung_tty driver be
> addressed so that the rx enable/disable helpers do not try to take the
> port lock when it is already held by the caller?

Also legitimate. But out of scope for my series.

John Ogness