[PATCH tty v10 0/2] Convert 8250 to NBCON, take 2

[PATCH tty v10 0/2] Convert 8250 to NBCON, take 2

[John Ogness]

Hi,

This is v10 of a series to convert the 8250 driver to an NBCON
console, providing both threaded and atomic printing
implementations. v9 of this series is here [0].

Note that v5 was pulled into the 6.14 merge window, but was
then reverted before the 6.14-rc1 release due to kernel test
robot problems. It turned out that the problems were related
to general 8250 driver issues. Those issues have now been
addressed mainline, paving the way for a second take at
converting the 8250 driver to NBCON.

The changes since v9:

- In serial8250_fifo_wait_for_lsr_thre(), always exit the
  unsafe section, regardless of wait_for_lsr() results.

John Ogness

[0] https://lore.kernel.org/lkml/20260724103654.133654-1-john.ogness@linutronix.de

John Ogness (2):
  serial: 8250: Switch to nbcon console, take 2
  Revert "serial: 8250: drop lockdep annotation from
    serial8250_clear_IER()"

 drivers/tty/serial/8250/8250.h      |   4 +-
 drivers/tty/serial/8250/8250_core.c |  65 +++++++--
 drivers/tty/serial/8250/8250_dw.c   |   2 +-
 drivers/tty/serial/8250/8250_port.c | 210 +++++++++++++++++++++++-----
 include/linux/serial_8250.h         |  16 ++-
 5 files changed, 245 insertions(+), 52 deletions(-)


base-commit: a5339effdd523b4b3502fba5358d44e83c76a565
-- 
2.47.3

[PATCH tty v10 1/2] serial: 8250: Switch to nbcon console, take 2

[John Ogness]

Implement the necessary callbacks to switch the 8250 console driver
to perform as an nbcon console.

Add implementations for the nbcon console callbacks:

  ->write_atomic()
  ->write_thread()
  ->device_lock()
  ->device_unlock()

and add CON_NBCON to the initial @flags.

All hardware access in the callbacks is within unsafe sections.
The ->write_atomic() and ->write_thread() callbacks allow safe
handover/takeover per byte and add a preceding newline if they
take over from another context mid-line.

For the ->write_atomic() callback, a new irq_work is used to defer
modem control since it may be called from a context that does not
allow waking up tasks. During suspend/resume the irq_work is not
used as this has been shown to cause suspend problems for some
hardware. Upon resume, any pending modem control is performed.

Note: A new __serial8250_clear_IER() is introduced for direct
clearing of UART_IER during console writing (which will not be
holding the port lock for atomic printing or KDB/KGDB). This
allows restoring a lockdep check to serial8250_clear_IER() in
a follow-up commit.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 drivers/tty/serial/8250/8250.h      |   4 +-
 drivers/tty/serial/8250/8250_core.c |  65 +++++++--
 drivers/tty/serial/8250/8250_dw.c   |   2 +-
 drivers/tty/serial/8250/8250_port.c | 207 +++++++++++++++++++++++-----
 include/linux/serial_8250.h         |  16 ++-
 5 files changed, 242 insertions(+), 52 deletions(-)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index b62f88eec881f..d672b014ade02 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -178,7 +178,9 @@ static unsigned int __maybe_unused serial_icr_read(struct uart_8250_port *up,
 
 void serial8250_clear_fifos(struct uart_8250_port *p);
 void serial8250_clear_and_reinit_fifos(struct uart_8250_port *p);
-void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up, unsigned int count);
+void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up,
+				       struct nbcon_write_context *wctxt,
+				       unsigned int count);
 
 void serial8250_rpm_get(struct uart_8250_port *p);
 void serial8250_rpm_put(struct uart_8250_port *p);
diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index c0e8a4efbdcc8..5042a8e8c6fb1 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -390,12 +390,34 @@ void __init serial8250_register_ports(struct uart_driver *drv, struct device *de
 
 #ifdef CONFIG_SERIAL_8250_CONSOLE
 
-static void univ8250_console_write(struct console *co, const char *s,
-				   unsigned int count)
+static void univ8250_console_write_atomic(struct console *co,
+					  struct nbcon_write_context *wctxt)
 {
 	struct uart_8250_port *up = &serial8250_ports[co->index];
 
-	serial8250_console_write(up, s, count);
+	serial8250_console_write(up, wctxt, true);
+}
+
+static void univ8250_console_write_thread(struct console *co,
+					  struct nbcon_write_context *wctxt)
+{
+	struct uart_8250_port *up = &serial8250_ports[co->index];
+
+	serial8250_console_write(up, wctxt, false);
+}
+
+static void univ8250_console_device_lock(struct console *co, unsigned long *flags)
+{
+	struct uart_port *up = &serial8250_ports[co->index].port;
+
+	__uart_port_lock_irqsave(up, flags);
+}
+
+static void univ8250_console_device_unlock(struct console *co, unsigned long flags)
+{
+	struct uart_port *up = &serial8250_ports[co->index].port;
+
+	__uart_port_unlock_irqrestore(up, flags);
 }
 
 static int univ8250_console_setup(struct console *co, char *options)
@@ -496,12 +518,15 @@ static int univ8250_console_match(struct console *co, char *name, int idx,
 
 static struct console univ8250_console = {
 	.name		= "ttyS",
-	.write		= univ8250_console_write,
+	.write_atomic	= univ8250_console_write_atomic,
+	.write_thread	= univ8250_console_write_thread,
+	.device_lock	= univ8250_console_device_lock,
+	.device_unlock	= univ8250_console_device_unlock,
 	.device		= uart_console_device,
 	.setup		= univ8250_console_setup,
 	.exit		= univ8250_console_exit,
 	.match		= univ8250_console_match,
-	.flags		= CON_PRINTBUFFER | CON_ANYTIME,
+	.flags		= CON_PRINTBUFFER | CON_ANYTIME | CON_NBCON,
 	.index		= -1,
 	.data		= &serial8250_reg,
 };
@@ -584,13 +609,19 @@ void serial8250_suspend_port(int line)
 	struct uart_8250_port *up = &serial8250_ports[line];
 	struct uart_port *port = &up->port;
 
-	if (!console_suspend_enabled && uart_console(port) &&
-	    port->type != PORT_8250) {
-		unsigned char canary = 0xa5;
+	if (uart_console(port)) {
+		/* No irq_work may be queued when suspending */
+		scoped_guard(uart_port_lock_irq, port)
+			up->console_msr_work_allow = false;
+		irq_work_sync(&up->console_msr_work);
 
-		serial_out(up, UART_SCR, canary);
-		if (serial_in(up, UART_SCR) == canary)
-			up->canary = canary;
+		if (!console_suspend_enabled && port->type != PORT_8250) {
+			unsigned char canary = 0xa5;
+
+			serial_out(up, UART_SCR, canary);
+			if (serial_in(up, UART_SCR) == canary)
+				up->canary = canary;
+		}
 	}
 
 	uart_suspend_port(&serial8250_reg, port);
@@ -620,6 +651,18 @@ void serial8250_resume_port(int line)
 		port->uartclk = 921600*16;
 	}
 	uart_resume_port(&serial8250_reg, port);
+
+	if (uart_console(port)) {
+
+		guard(uart_port_lock_irq)(port);
+
+		/* irq_work allowed again */
+		up->console_msr_work_allow = true;
+
+		/* Handle any pending MSR changes */
+		if (up->msr_saved_flags)
+			serial8250_modem_status(up);
+	}
 }
 EXPORT_SYMBOL(serial8250_resume_port);
 
diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 5fba913f33010..51d026f20825a 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -156,7 +156,7 @@ static int dw8250_idle_enter(struct uart_port *p)
 	 *
 	 * FIXME: frame_time delay is too long with very low baudrates.
 	 */
-	serial8250_fifo_wait_for_lsr_thre(up, p->fifosize);
+	serial8250_fifo_wait_for_lsr_thre(up, NULL, p->fifosize);
 	ndelay(p->frame_time);
 
 	serial_port_out(p, UART_MCR, up->mcr | UART_MCR_LOOP);
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 8c241ec7f4f29..5bcbb6deb12f7 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -704,7 +704,12 @@ static void serial8250_clear_interrupts(struct uart_port *port)
 	serial_port_in(port, UART_MSR);
 }
 
-static void serial8250_clear_IER(struct uart_8250_port *up)
+/*
+ * Only to be directly used by serial8250_console_write() and
+ * serial8250_put_poll_char(), which do not require the port lock.
+ * Use serial8250_clear_IER() instead for all other cases.
+ */
+static void __serial8250_clear_IER(struct uart_8250_port *up)
 {
 	if (up->capabilities & UART_CAP_UUE)
 		serial_out(up, UART_IER, UART_IER_UUE);
@@ -712,6 +717,11 @@ static void serial8250_clear_IER(struct uart_8250_port *up)
 		serial_out(up, UART_IER, 0);
 }
 
+static inline void serial8250_clear_IER(struct uart_8250_port *up)
+{
+	__serial8250_clear_IER(up);
+}
+
 /*
  * This is a quickie test to see how big the FIFO is.
  * It doesn't work at all the time, more's the pity.
@@ -1284,9 +1294,6 @@ void serial8250_em485_stop_tx(struct uart_8250_port *p, bool toggle_ier)
 {
 	unsigned char mcr = serial8250_in_MCR(p);
 
-	/* Port locked to synchronize UART_IER access against the console. */
-	lockdep_assert_held_once(&p->port.lock);
-
 	if (p->port.rs485.flags & SER_RS485_RTS_AFTER_SEND)
 		mcr |= UART_MCR_RTS;
 	else
@@ -1302,6 +1309,16 @@ void serial8250_em485_stop_tx(struct uart_8250_port *p, bool toggle_ier)
 		serial8250_clear_and_reinit_fifos(p);
 
 		if (toggle_ier) {
+			/*
+			 * Port locked to synchronize UART_IER access against
+			 * the console. The lockdep_assert must be restricted
+			 * to this condition because only here is it
+			 * guaranteed that the port lock is held. The other
+			 * hardware access in this function is synchronized
+			 * by console ownership.
+			 */
+			lockdep_assert_held_once(&p->port.lock);
+
 			p->ier |= UART_IER_RLSI | UART_IER_RDI;
 			serial_port_out(&p->port, UART_IER, p->ier);
 		}
@@ -2053,10 +2070,13 @@ static void serial8250_put_poll_char(struct uart_port *port,
 
 	guard(serial8250_rpm)(up);
 	/*
-	 *	First save the IER then disable the interrupts
+	 * First, save the IER, then disable the interrupts. The special
+	 * variant to clear the IER is used because KDB/KGDB printing
+	 * is synchronized via CPU quiescence without holding the port
+	 * lock.
 	 */
 	ier = serial_port_in(port, UART_IER);
-	serial8250_clear_IER(up);
+	__serial8250_clear_IER(up);
 
 	wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
 	/*
@@ -3198,13 +3218,32 @@ void serial8250_set_defaults(struct uart_8250_port *up)
 }
 EXPORT_SYMBOL_GPL(serial8250_set_defaults);
 
-void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up, unsigned int count)
+void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up,
+				       struct nbcon_write_context *wctxt,
+				       unsigned int count)
 {
 	unsigned int i;
 
+	/*
+	 * For console writing, enter/exit an unsafe section for each byte
+	 * in order to pass the ownership as quickly as possible if a higher
+	 * priority context wants ownership. Otherwise, an attempt to take
+	 * over the ownership might timeout. The new owner will wait for
+	 * UART_LSR_THRE before reusing the fifo.
+	 */
 	for (i = 0; i < count; i++) {
-		if (wait_for_lsr(up, UART_LSR_THRE))
+		bool tx_ready;
+
+		if (wctxt && !nbcon_enter_unsafe(wctxt))
 			return;
+
+		tx_ready = wait_for_lsr(up, UART_LSR_THRE);
+
+		if (wctxt)
+			nbcon_exit_unsafe(wctxt);
+
+		if (tx_ready)
+			break;
 	}
 }
 EXPORT_SYMBOL_NS_GPL(serial8250_fifo_wait_for_lsr_thre, "SERIAL_8250");
@@ -3213,7 +3252,11 @@ EXPORT_SYMBOL_NS_GPL(serial8250_fifo_wait_for_lsr_thre, "SERIAL_8250");
 
 static void serial8250_console_putchar(struct uart_port *port, unsigned char ch)
 {
+	struct uart_8250_port *up = up_to_u8250p(port);
+
 	serial_port_out(port, UART_TX, ch);
+
+	up->console_line_ended = (ch == '\n');
 }
 
 static void serial8250_console_wait_putchar(struct uart_port *port, unsigned char ch)
@@ -3257,20 +3300,29 @@ static void serial8250_console_restore(struct uart_8250_port *up)
  * to get empty.
  */
 static void serial8250_console_fifo_write(struct uart_8250_port *up,
-					  const char *s, unsigned int count)
+					  struct nbcon_write_context *wctxt)
 {
-	const char *end = s + count;
 	unsigned int fifosize = up->tx_loadsz;
 	struct uart_port *port = &up->port;
+	const char *s = wctxt->outbuf;
+	const char *end = s + wctxt->len;
 	unsigned int tx_count = 0;
 	bool cr_sent = false;
 	unsigned int i;
 
 	while (s != end) {
 		/* Allow timeout for each byte of a possibly full FIFO */
-		serial8250_fifo_wait_for_lsr_thre(up, fifosize);
+		serial8250_fifo_wait_for_lsr_thre(up, wctxt, fifosize);
 
+		/*
+		 * Fill the FIFO. If a handover or takeover occurs, writing
+		 * must be aborted since wctxt->outbuf and wctxt->len are no
+		 * longer valid.
+		 */
 		for (i = 0; i < fifosize && s != end; ++i) {
+			if (!nbcon_enter_unsafe(wctxt))
+				return;
+
 			if (*s == '\n' && !cr_sent) {
 				serial8250_console_putchar(port, '\r');
 				cr_sent = true;
@@ -3278,6 +3330,8 @@ static void serial8250_console_fifo_write(struct uart_8250_port *up,
 				serial8250_console_putchar(port, *s++);
 				cr_sent = false;
 			}
+
+			nbcon_exit_unsafe(wctxt);
 		}
 		tx_count = i;
 	}
@@ -3286,39 +3340,58 @@ static void serial8250_console_fifo_write(struct uart_8250_port *up,
 	 * Allow timeout for each byte written since the caller will only wait
 	 * for UART_LSR_BOTH_EMPTY using the timeout of a single character
 	 */
-	serial8250_fifo_wait_for_lsr_thre(up, tx_count);
+	serial8250_fifo_wait_for_lsr_thre(up, wctxt, tx_count);
+}
+
+static void serial8250_console_byte_write(struct uart_8250_port *up,
+					  struct nbcon_write_context *wctxt)
+{
+	struct uart_port *port = &up->port;
+	const char *s = wctxt->outbuf;
+	const char *end = s + wctxt->len;
+
+	/*
+	 * Write out the message. If a handover or takeover occurs, writing
+	 * must be aborted since wctxt->outbuf and wctxt->len are no longer
+	 * valid.
+	 */
+	while (s != end) {
+		if (!nbcon_enter_unsafe(wctxt))
+			return;
+
+		uart_console_write(port, s++, 1, serial8250_console_wait_putchar);
+
+		nbcon_exit_unsafe(wctxt);
+	}
 }
 
 /*
- *	Print a string to the serial port trying not to disturb
- *	any possible real use of the port...
- *
- *	The console_lock must be held when we get here.
+ * Print a string to the serial port trying not to disturb
+ * any possible real use of the port...
  *
- *	Doing runtime PM is really a bad idea for the kernel console.
- *	Thus, we assume the function is called when device is powered up.
+ * Doing runtime PM is really a bad idea for the kernel console.
+ * Thus, assume it is called when device is powered up.
  */
-void serial8250_console_write(struct uart_8250_port *up, const char *s,
-			      unsigned int count)
+void serial8250_console_write(struct uart_8250_port *up,
+			      struct nbcon_write_context *wctxt,
+			      bool is_atomic)
 {
 	struct uart_8250_em485 *em485 = up->em485;
 	struct uart_port *port = &up->port;
-	unsigned long flags;
-	unsigned int ier, use_fifo;
-	int locked = 1;
-
-	touch_nmi_watchdog();
+	unsigned int ier;
+	bool use_fifo;
 
-	if (oops_in_progress)
-		locked = uart_port_trylock_irqsave(port, &flags);
-	else
-		uart_port_lock_irqsave(port, &flags);
+	if (!nbcon_enter_unsafe(wctxt))
+		return;
 
 	/*
-	 *	First save the IER then disable the interrupts
+	 * First, save the IER, then disable the interrupts. The special
+	 * variant to clear the IER is used because emergency and panic
+	 * printing is synchronized only by nbcon ownership without
+	 * holding the port lock.
 	 */
 	ier = serial_port_in(port, UART_IER);
-	serial8250_clear_IER(up);
+	__serial8250_clear_IER(up);
 
 	/* check scratch reg to see if port powered off during system sleep */
 	if (up->canary && (up->canary != serial_port_in(port, UART_SCR))) {
@@ -3332,6 +3405,18 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
 		mdelay(port->rs485.delay_rts_before_send);
 	}
 
+	/* If ownership was lost, no writing is allowed */
+	if (!nbcon_can_proceed(wctxt))
+		goto skip_write;
+
+	/*
+	 * If console printer did not fully output the previous line, it must
+	 * have been handed or taken over. Insert a newline in order to
+	 * maintain clean output.
+	 */
+	if (!up->console_line_ended)
+		uart_console_write(port, "\n", 1, serial8250_console_wait_putchar);
+
 	use_fifo = (up->capabilities & UART_CAP_FIFO) &&
 		/*
 		 * BCM283x requires to check the fifo
@@ -3352,10 +3437,20 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
 		 */
 		!uart_console_hwflow_active(&up->port);
 
+	nbcon_exit_unsafe(wctxt);
+
 	if (likely(use_fifo))
-		serial8250_console_fifo_write(up, s, count);
+		serial8250_console_fifo_write(up, wctxt);
 	else
-		uart_console_write(port, s, count, serial8250_console_wait_putchar);
+		serial8250_console_byte_write(up, wctxt);
+skip_write:
+	/*
+	 * Re-enter an unsafe section in order to perform final actions
+	 * (such as re-enabling interrupts). If ownership was lost, this
+	 * context must reacquire ownership.
+	 */
+	while (!nbcon_enter_unsafe(wctxt))
+		nbcon_reacquire_nobuf(wctxt);
 
 	/*
 	 *	Finally, wait for transmitter to become empty
@@ -3378,11 +3473,25 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
 	 *	call it if we have saved something in the saved flags
 	 *	while processing with interrupts off.
 	 */
-	if (up->msr_saved_flags)
-		serial8250_modem_status(up);
+	if (up->msr_saved_flags) {
+		if (is_atomic) {
+			/*
+			 * For atomic, MSR handling must be deferred to
+			 * irq_work because this may be a context that does
+			 * not permit waking up tasks.
+			 *
+			 * But no irq_work may be queued when suspending.
+			 * In that case, the MSR handling will occur during
+			 * resume in serial8250_resume_port().
+			 */
+			if (up->console_msr_work_allow)
+				irq_work_queue(&up->console_msr_work);
+		} else {
+			serial8250_modem_status(up);
+		}
+	}
 
-	if (locked)
-		uart_port_unlock_irqrestore(port, flags);
+	nbcon_exit_unsafe(wctxt);
 }
 
 static unsigned int probe_baud(struct uart_port *port)
@@ -3400,8 +3509,24 @@ static unsigned int probe_baud(struct uart_port *port)
 	return (port->uartclk / 16) / quot;
 }
 
+/*
+ * irq_work handler to perform modem control during console output.
+ * Only triggered via ->write_atomic() callback because it may be
+ * in a scheduler or NMI context, unable to wake tasks.
+ */
+static void console_msr_handler(struct irq_work *iwp)
+{
+	struct uart_8250_port *up = container_of(iwp, struct uart_8250_port, console_msr_work);
+	struct uart_port *port = &up->port;
+
+	guard(uart_port_lock)(port);
+
+	serial8250_modem_status(up);
+}
+
 int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
 {
+	struct uart_8250_port *up = up_to_u8250p(port);
 	int baud = 9600;
 	int bits = 8;
 	int parity = 'n';
@@ -3411,6 +3536,10 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
 	if (!port->iobase && !port->membase)
 		return -ENODEV;
 
+	up->console_line_ended = true;
+	up->console_msr_work_allow = true;
+	init_irq_work(&up->console_msr_work, console_msr_handler);
+
 	if (options)
 		uart_parse_options(options, &baud, &parity, &bits, &flow);
 	else if (probe)
@@ -3431,6 +3560,10 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
 
 int serial8250_console_exit(struct uart_port *port)
 {
+	struct uart_8250_port *up = up_to_u8250p(port);
+
+	irq_work_sync(&up->console_msr_work);
+
 	if (port->dev)
 		pm_runtime_put_sync(port->dev);
 
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index a95b2d143d248..eba36710bc476 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -150,8 +150,20 @@ struct uart_8250_port {
 #define LSR_SAVE_FLAGS UART_LSR_BRK_ERROR_BITS
 	u16			lsr_saved_flags;
 	u16			lsr_save_mask;
+
+	/*
+	 * Track when a console line has been fully written to the
+	 * hardware, i.e. true when the most recent byte written to
+	 * UART_TX by the console was '\n'.
+	 */
+	bool			console_line_ended;
+
+	/* Allow queuing irq_work for MSR handling */
+	bool			console_msr_work_allow;
+
 #define MSR_SAVE_FLAGS UART_MSR_ANY_DELTA
 	unsigned char		msr_saved_flags;
+	struct irq_work		console_msr_work;
 
 	struct uart_8250_dma	*dma;
 	const struct uart_8250_ops *ops;
@@ -203,8 +215,8 @@ void serial8250_tx_chars(struct uart_8250_port *up);
 unsigned int serial8250_modem_status(struct uart_8250_port *up);
 void serial8250_init_port(struct uart_8250_port *up);
 void serial8250_set_defaults(struct uart_8250_port *up);
-void serial8250_console_write(struct uart_8250_port *up, const char *s,
-			      unsigned int count);
+void serial8250_console_write(struct uart_8250_port *up,
+			      struct nbcon_write_context *wctxt, bool in_atomic);
 int serial8250_console_setup(struct uart_port *port, char *options, bool probe);
 int serial8250_console_exit(struct uart_port *port);
 
-- 
2.47.3

[PATCH tty v10 2/2] Revert "serial: 8250: drop lockdep annotation from serial8250_clear_IER()"

[John Ogness]

This reverts commit 3d9e6f556e235ddcdc9f73600fdd46fe1736b090.

The 8250 driver no longer depends on @oops_in_progress and
will no longer violate the port->lock locking constraints.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 drivers/tty/serial/8250/8250_port.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 5bcbb6deb12f7..12162a8928922 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -719,6 +719,9 @@ static void __serial8250_clear_IER(struct uart_8250_port *up)
 
 static inline void serial8250_clear_IER(struct uart_8250_port *up)
 {
+	/* Port locked to synchronize UART_IER access against the console */
+	lockdep_assert_held_once(&up->port.lock);
+
 	__serial8250_clear_IER(up);
 }
 
-- 
2.47.3

Re: [PATCH tty v10 1/2] serial: 8250: Switch to nbcon console, take 2

[John Ogness]

On 2026-07-27, Sashiko wrote:
> > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > index 8c241ec7f4f29..5bcbb6deb12f7 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> [ ... ]
> > @@ -3332,6 +3405,18 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
> >  		mdelay(port->rs485.delay_rts_before_send);
> >  	}
> >  
> > +	/* If ownership was lost, no writing is allowed */
> > +	if (!nbcon_can_proceed(wctxt))
> > +		goto skip_write;
> > +
> > +	/*
> > +	 * If console printer did not fully output the previous line, it must
> > +	 * have been handed or taken over. Insert a newline in order to
> > +	 * maintain clean output.
> > +	 */
> > +	if (!up->console_line_ended)
> > +		uart_console_write(port, "\n", 1, serial8250_console_wait_putchar);
> > +
>
> Will this cause the newline characters to be silently dropped if the
> hardware FIFO is already full from a preempted context?
>
> When uart_console_write() calls serial8250_console_wait_putchar(), it
> relies on wait_for_lsr() to wait for transmitter room. The timeout in
> wait_for_lsr() is calculated based on frame_time, which is only enough
> for a single character. If the preempted context left a full FIFO (e.g.,
> 16-64 bytes), the wait_for_lsr() timeout will expire before the FIFO
> drains, causing the hardware to drop the injected newline characters.

Sashiko is correct. For v11 I am writing helper functions to handle
@console_line_ended and the console line printing for fifo and
non-fifo. This causes a bit more churn than I would like to see at this
point (v11) but it will handle all situations, has increased code
re-use, and should be easier to read/maintain.

> [ ... ]
> >	while (!nbcon_enter_unsafe(wctxt))
> >		nbcon_reacquire_nobuf(wctxt);
> >
> >  	/*
> >  	 *	Finally, wait for transmitter to become empty
> >  	 *	and restore the IER
> >  	 */
> >  	wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
> >  
> > +	if (em485) {
> > +		mdelay(port->rs485.delay_rts_after_send);
> > +		if (em485->tx_stopped)
> > +			up->rs485_stop_tx(up, false);
> > +	}
> > +
> >  	serial_port_out(port, UART_IER, ier);
>
> Could the hardware state be corrupted here if a hostile takeover occurs
> while waiting for the transmitter to empty?
>
> The wait_for_xmitr() call can take a significant amount of time to let the
> FIFO drain. If a system panic triggers a hostile console takeover during
> this window, the panic context might reconfigure the hardware. Once the
> original context finishes waiting, it will unconditionally execute
> serial_port_out(), potentially undoing the panic context's configuration
> or causing interrupt storms.
>
> Should we check nbcon_can_proceed(wctxt) again before modifying the
> hardware state?

nbcon_can_proceed() is not enough because these final actions are
necessary. It would need to be:

	nbcon_exit_unsafe(wctxt);
	while (!nbcon_enter_unsafe(wctxt))
		nbcon_reacquire_nobuf(wctxt);

Although this does not close the window, it does make it quite
small. And if we are really in the unlikely situation of:

- a panic occurs
- the panic is unable to stop all the CPUs
- a console printer is actively printing from a non-stopped CPU
- the panic CPU performs a hostile takeover

then we should probably do everything we can (within reason) to avoid
interfering with the panic printing.

As Sashiko points out, the wait_for_xmitr() and mdelay() calls could be
relatively large. I agree that it makes sense to peform unsafe toggles
in these two locations. I will add that for v11. It will look something
like this:

@@ -3481,10 +3487,21 @@ void serial8250_console_write(struct uart_8250_port *up,
 
 	if (em485) {
 		mdelay(port->rs485.delay_rts_after_send);
+
+		/* Toggle unsafe after possibly long delay */
+		nbcon_exit_unsafe(wctxt);
+		while (!nbcon_enter_unsafe(wctxt))
+			nbcon_reacquire_nobuf(wctxt);
+
 		if (em485->tx_stopped)
 			up->rs485_stop_tx(up, false);
 	}
 
+	/* Toggle unsafe after possibly long delay */
+	nbcon_exit_unsafe(wctxt);
+	while (!nbcon_enter_unsafe(wctxt))
+		nbcon_reacquire_nobuf(wctxt);
+
 	serial_port_out(port, UART_IER, ier);
 
 	/*

> > @@ -3400,8 +3509,24 @@ static unsigned int probe_baud(struct uart_port *port)
> >  	return (port->uartclk / 16) / quot;
> >  }
> >  
> > +/*
> > + * irq_work handler to perform modem control during console output.
> > + * Only triggered via ->write_atomic() callback because it may be
> > + * in a scheduler or NMI context, unable to wake tasks.
> > + */
> > +static void console_msr_handler(struct irq_work *iwp)
> > +{
> > +	struct uart_8250_port *up = container_of(iwp, struct uart_8250_port, console_msr_work);
> > +	struct uart_port *port = &up->port;
> > +
> > +	guard(uart_port_lock)(port);
> > +
> > +	serial8250_modem_status(up);
> > +}
>
> Does this code introduce a race condition that could cause invalid hardware
> accesses on a suspended device?
>
> If CPU1 is executing serial8250_console_write(), it reads
> up->console_msr_work_allow locklessly before queueing the work...

Sashiko is not correct. Both code blocks (reader and writer) are within
unsafe sections and therefore synchronized.

John