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

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

[John Ogness]

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

[PATCH tty v3 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 get/set 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. A follow-up commit will address this.

Note2: Aside from sh-sci, the samsung_tty driver is also using
       UPF_CONS_FLOW as a platform data configuration flag.

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..c91b0fa7111a7 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_get_cons_flow(&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_get_cons_flow(&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..be6777dfdc532 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_get_cons_flow(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..a9879bc655745 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_get_cons_flow(&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..08cb9ff30506f 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_get_cons_flow(&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..80afa47f09880 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_get_cons_flow(&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..f9b0dbded1f43 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_get_cons_flow(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_get_cons_flow(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_get_cons_flow(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(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..103f03c1fe748 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_get_cons_flow(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_get_cons_flow(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..97019b5ec49e2 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_get_cons_flow(&up->port)) {
 		tmout = 1000000;
 		while (--tmout &&
 		       ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0))
-- 
2.47.3