[PATCH tty v7 0/2] Convert 8250 to NBCON, take 2

[PATCH tty v7 0/2] Convert 8250 to NBCON, take 2

[John Ogness]

Hi,

This is v7 of a series to convert the 8250 driver to an NBCON
console, providing both threaded and atomic printing
implementations. v6 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.

Based on Sashiko feedback, the changes since v6:

- Add irq_work_sync() calls to make sure there is no pending
  modem status work when suspending or unregistering the
  console.

- In the put_poll_char() callback, use __serial8250_clear_IER()
  because KDB/KGDB do not acquire the port lock to print.

- In serial8250_console_write(), re-enter unsafe section after
  printing text, even if ownership was _not_ lost. The final
  actions need to be performed within an unsafe section.

John Ogness

[0] https://lore.kernel.org/lkml/20260720103242.7265-1-john.ogness@linutronix.de

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

 drivers/tty/serial/8250/8250.h      |   4 +-
 drivers/tty/serial/8250/8250_core.c |  45 ++++++-
 drivers/tty/serial/8250/8250_dw.c   |   2 +-
 drivers/tty/serial/8250/8250_port.c | 200 +++++++++++++++++++++++-----
 include/linux/serial_8250.h         |  16 ++-
 5 files changed, 222 insertions(+), 45 deletions(-)


base-commit: 782f4dbd1794b4f30dc116a7ca42c5962c409be8
-- 
2.47.3

[PATCH tty v7 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 may not be
holding the port lock for atomic printing). 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 |  45 ++++++-
 drivers/tty/serial/8250/8250_dw.c   |   2 +-
 drivers/tty/serial/8250/8250_port.c | 197 +++++++++++++++++++++++-----
 include/linux/serial_8250.h         |  16 ++-
 5 files changed, 219 insertions(+), 45 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..e08da7e22f9f2 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,
 };
@@ -584,6 +609,10 @@ void serial8250_suspend_port(int line)
 	struct uart_8250_port *up = &serial8250_ports[line];
 	struct uart_port *port = &up->port;
 
+	/* No irq_work may be queued when suspending. */
+	up->avoid_modem_status_work = true;
+	irq_work_sync(&up->modem_status_work);
+
 	if (!console_suspend_enabled && uart_console(port) &&
 	    port->type != PORT_8250) {
 		unsigned char canary = 0xa5;
@@ -620,6 +649,12 @@ void serial8250_resume_port(int line)
 		port->uartclk = 921600*16;
 	}
 	uart_resume_port(&serial8250_reg, port);
+
+	/* irq_work allowed again. Handle MSR now if pending. */
+	up->avoid_modem_status_work = false;
+	guard(uart_port_lock_irqsave)(port);
+	if (uart_console(port) && 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..8753f86d633ef 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 may 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 +2070,12 @@ 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
+	 * occurs without acquiring 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);
 	/*
@@ -3198,11 +3217,22 @@ 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 (i = 0; i < count; i++) {
+		/*
+		 * Pass the ownership as quickly as possible to a higher
+		 * priority context. Otherwise, its attempt to take over
+		 * the ownership might timeout. The new owner will wait
+		 * for UART_LSR_THRE before reusing the fifo.
+		 */
+		if (wctxt && !nbcon_can_proceed(wctxt))
+			return;
+
 		if (wait_for_lsr(up, UART_LSR_THRE))
 			return;
 	}
@@ -3213,7 +3243,11 @@ 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 +3291,29 @@ 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 +3321,8 @@ 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 +3331,57 @@ 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...
+ * 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.
- *
- *	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 console printing may
+	 * occur 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 +3395,18 @@ 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 +3427,20 @@ 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 the 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 +3463,29 @@ 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) {
+		/*
+		 * For atomic, it must be deferred to irq_work because this
+		 * may be a context that does not permit waking up tasks.
+		 */
+		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->avoid_modem_status_work)
+				irq_work_queue(&up->modem_status_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 +3503,23 @@ static unsigned int probe_baud(struct uart_port *port)
 	return (port->uartclk / 16) / quot;
 }
 
+/*
+ * irq_work handler to perform modem control. Only triggered via
+ * ->write_atomic() callback because it may be in a scheduler or
+ * NMI context, unable to wake tasks.
+ */
+static void modem_status_handler(struct irq_work *iwp)
+{
+	struct uart_8250_port *up = container_of(iwp, struct uart_8250_port, modem_status_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 +3529,9 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
 	if (!port->iobase && !port->membase)
 		return -ENODEV;
 
+	up->console_line_ended = true;
+	init_irq_work(&up->modem_status_work, modem_status_handler);
+
 	if (options)
 		uart_parse_options(options, &baud, &parity, &bits, &flow);
 	else if (probe)
@@ -3431,6 +3552,10 @@ 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->modem_status_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..12c2f6aadd420 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;
+
+	/* Do not queue irq_work for MSR handling when suspending. */
+	bool			avoid_modem_status_work;
+
 #define MSR_SAVE_FLAGS UART_MSR_ANY_DELTA
 	unsigned char		msr_saved_flags;
+	struct irq_work		modem_status_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 v7 2/2] serial: 8250: Revert "drop lockdep annotation from serial8250_clear_IER()"

[John Ogness]

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

This reverts commit 3d9e6f556e235ddcdc9f73600fdd46fe1736b090.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 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 8753f86d633ef..31386dd283ef8 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 v7 2/2] serial: 8250: Revert "drop lockdep annotation from serial8250_clear_IER()"

[Sebastian Andrzej Siewior]

On 2026-07-20 16:00:04 [+0206], John Ogness wrote:
> The 8250 driver no longer depends on @oops_in_progress and
> will no longer violate the port->lock locking constraints.
> 
> This reverts commit 3d9e6f556e235ddcdc9f73600fdd46fe1736b090.

If needed, could this be please replaced with
  3d9e6f556e235 ("serial: 8250: drop lockdep annotation from serial8250_clear_IER()")

instead the bare number, please?

> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Sebastian

Re: [PATCH tty v7 2/2] serial: 8250: Revert "drop lockdep annotation from serial8250_clear_IER()"

[Andy Shevchenko]

On Mon, Jul 20, 2026 at 04:03:12PM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-07-20 16:00:04 [+0206], John Ogness wrote:
> > The 8250 driver no longer depends on @oops_in_progress and
> > will no longer violate the port->lock locking constraints.
> > 
> > This reverts commit 3d9e6f556e235ddcdc9f73600fdd46fe1736b090.
> 
> If needed, could this be please replaced with
>   3d9e6f556e235 ("serial: 8250: drop lockdep annotation from serial8250_clear_IER()")
> 
> instead the bare number, please?

IIRC this is a requirement for stable that they will able to find the reverts
without having a Fixes tags.


On top of that we may add the standard format into the commit message.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH tty v7 1/2] serial: 8250: Switch to nbcon console, take 2

[Andy Shevchenko]

On Mon, Jul 20, 2026 at 04:00:03PM +0206, John Ogness wrote:
> 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 may not be
> holding the port lock for atomic printing). This allows restoring
> a lockdep check to serial8250_clear_IER() in a follow-up commit.

...

> +	/* irq_work allowed again. Handle MSR now if pending. */
> +	up->avoid_modem_status_work = false;
> +	guard(uart_port_lock_irqsave)(port);

To make guard()() visible, we usually add a blank line before and after
guard()() line. I dunno if scoped_guard() makes more sense here as Petr
proposed, I'm fine with either.

> +	if (uart_console(port) && up->msr_saved_flags)
> +		serial8250_modem_status(up);

...

> +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) {

Can len == 0? If not, I would write this as do {} while ().

> +		if (!nbcon_enter_unsafe(wctxt))
> +			return;
> +
> +		uart_console_write(port, s++, 1, serial8250_console_wait_putchar);
> +
> +		nbcon_exit_unsafe(wctxt);
> +	}
>  }

...

> +/*
> + * irq_work handler to perform modem control. Only triggered via
> + * ->write_atomic() callback because it may be in a scheduler or
> + * NMI context, unable to wake tasks.
> + */
> +static void modem_status_handler(struct irq_work *iwp)
> +{
> +	struct uart_8250_port *up = container_of(iwp, struct uart_8250_port, modem_status_work);
> +	struct uart_port *port = &up->port;
> +
> +	guard(uart_port_lock)(port);

+ blank line.

> +	serial8250_modem_status(up);
> +}

-- 
With Best Regards,
Andy Shevchenko