[PATCH tty v2 0/2] 8250: Add console flow control

[PATCH tty v2 0/2] 8250: Add console flow control

[John Ogness]

Hi,

This is v2 of a series to implement console flow control for the
8250 serial driver. v1 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.

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/20260331141502.6233-1-john.ogness@linutronix.de

John Ogness (2):
  serial: 8250: Check LSR timeout on console flow control
  serial: 8250: Add support for console hardware flow control

 drivers/tty/serial/8250/8250_core.c |  6 +++++-
 drivers/tty/serial/8250/8250_port.c | 22 ++++++++++++++++++----
 drivers/tty/serial/serial_core.c    | 21 ++++++++++++++++++++-
 include/linux/serial_core.h         |  8 ++++++++
 4 files changed, 51 insertions(+), 6 deletions(-)


base-commit: a1a81aef99e853dec84241d701fbf587d713eb5b
-- 
2.47.3

[PATCH tty v2 1/2] serial: 8250: Check LSR timeout on console flow control

[John Ogness]

wait_for_xmitr() calls wait_for_lsr() to wait for the transmission
registers to be empty. wait_for_lsr() can timeout after a reasonable
amount of time.

When console flow control is active (UPF_CONS_FLOW), wait_for_xmitr()
additionally polls CTS, waiting for the peer to signal that it is
ready to receive more data.

If hardware flow control is enabled (auto CTS) and the peer deasserts
CTS, wait_for_lsr() will timeout. If additionally console flow
control is active and while polling CTS the peer asserts CTS, the
console will assume it can immediately transmit, even though the
transmission registers may not be empty. This can lead to data loss.

Avoid this problem by performing an extra wait_for_lsr() upon CTS
assertion if wait_for_lsr() previously timed out.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 drivers/tty/serial/8250/8250_port.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index af78cc02f38e7..a739350e634f9 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1984,16 +1984,20 @@ static bool wait_for_lsr(struct uart_8250_port *up, int bits)
 static void wait_for_xmitr(struct uart_8250_port *up, int bits)
 {
 	unsigned int tmout;
+	bool tx_ready;
 
-	wait_for_lsr(up, bits);
+	tx_ready = wait_for_lsr(up, bits);
 
 	/* Wait up to 1s for flow control if necessary */
 	if (up->port.flags & UPF_CONS_FLOW) {
 		for (tmout = 1000000; tmout; tmout--) {
 			unsigned int msr = serial_in(up, UART_MSR);
 			up->msr_saved_flags |= msr & MSR_SAVE_FLAGS;
-			if (msr & UART_MSR_CTS)
+			if (msr & UART_MSR_CTS) {
+				if (!tx_ready)
+					wait_for_lsr(up, bits);
 				break;
+			}
 			udelay(1);
 			touch_nmi_watchdog();
 		}
-- 
2.47.3

[PATCH tty v2 2/2] serial: 8250: Add support for console hardware flow control

[John Ogness]

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
the UPF_CONS_FLOW flag is set, but there is no code path that sets
this flag. However, that is not the only issue. The problems are:

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

2. Even if UPF_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 the
   UPF_CONS_FLOW bit 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 | 14 ++++++++++++--
 drivers/tty/serial/serial_core.c    | 21 ++++++++++++++++++++-
 include/linux/serial_core.h         |  8 ++++++++
 4 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index a428e88938eb7..ff4c9972d4576 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;
+	upf_t console_hwflow;
 	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 hardware flow control. */
+	console_hwflow = uart->port.flags & UPF_CONS_FLOW;
+
 	if (uart->port.dev)
 		uart_remove_one_port(&serial8250_reg, &uart->port);
 
@@ -729,7 +733,7 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
 	uart->port.fifosize     = up->port.fifosize;
 	uart->port.regshift     = up->port.regshift;
 	uart->port.iotype       = up->port.iotype;
-	uart->port.flags        = up->port.flags | UPF_BOOT_AUTOCONF;
+	uart->port.flags        = up->port.flags | UPF_BOOT_AUTOCONF | console_hwflow;
 	uart->bugs		= up->bugs;
 	uart->port.mapbase      = up->port.mapbase;
 	uart->port.mapsize      = up->port.mapsize;
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index a739350e634f9..6c8830943b0c0 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 (up->port.flags & UPF_CONS_FLOW) {
+	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.
 		 */
-		!(up->port.flags & UPF_CONS_FLOW);
+		!uart_console_hwflow_active(&up->port);
 
 	if (likely(use_fifo))
 		serial8250_console_fifo_write(up, s, count);
@@ -3425,6 +3431,10 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
 	if (ret)
 		return ret;
 
+	/* Allow user-specified hardware flow control. */
+	if (flow == 'r')
+		port->flags |= UPF_CONS_FLOW;
+
 	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..a9ea10df4fb8b 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_irq)(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 666430b478997..07bd3bd6c8355 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -1163,6 +1163,14 @@ static inline bool uart_softcts_mode(struct uart_port *uport)
 	return ((uport->status & mask) == UPSTAT_CTS_ENABLE);
 }
 
+static inline bool uart_console_hwflow_active(struct uart_port *uport)
+{
+	return uart_console(uport) &&
+	       !(uport->rs485.flags & SER_RS485_ENABLED) &&
+	       (uport->flags & UPF_CONS_FLOW) &&
+	       uart_cts_enabled(uport);
+}
+
 /*
  * The following are helper functions for the low level drivers.
  */
-- 
2.47.3