[RFC PATCH v1 20/25] serial: 8250: implement write_atomic

[John Ogness <john.ogness@linutronix.de>]

Implement a non-sleeping NMI-safe write_atomic console function in
order to support emergency printk messages.

Since interrupts need to be disabled during transmit, all usage of
the IER register was wrapped with access functions that use the
console_atomic_lock function to synchronize register access while
tracking the state of the interrupts. This was necessary because
write_atomic is can be calling from an NMI context that has
preempted write_atomic.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 drivers/tty/serial/8250/8250.h      |   4 +
 drivers/tty/serial/8250/8250_core.c |  19 +++--
 drivers/tty/serial/8250/8250_dma.c  |   5 +-
 drivers/tty/serial/8250/8250_port.c | 154 +++++++++++++++++++++++++++---------
 include/linux/serial_8250.h         |   5 ++
 5 files changed, 139 insertions(+), 48 deletions(-)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index ebfb0bd5bef5..1d36337d40a2 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -255,3 +255,7 @@ static inline int serial_index(struct uart_port *port)
 {
 	return port->minor - 64;
 }
+
+void set_ier(struct uart_8250_port *up, unsigned char ier);
+void clear_ier(struct uart_8250_port *up);
+void restore_ier(struct uart_8250_port *up);
diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index e441221e04b9..1daea9709ce2 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -265,7 +265,7 @@ static void serial8250_timeout(struct timer_list *t)
 static void serial8250_backup_timeout(struct timer_list *t)
 {
 	struct uart_8250_port *up = from_timer(up, t, timer);
-	unsigned int iir, ier = 0, lsr;
+	unsigned int iir, lsr;
 	unsigned long flags;
 
 	spin_lock_irqsave(&up->port.lock, flags);
@@ -274,10 +274,8 @@ static void serial8250_backup_timeout(struct timer_list *t)
 	 * Must disable interrupts or else we risk racing with the interrupt
 	 * based handler.
 	 */
-	if (up->port.irq) {
-		ier = serial_in(up, UART_IER);
-		serial_out(up, UART_IER, 0);
-	}
+	if (up->port.irq)
+		clear_ier(up);
 
 	iir = serial_in(up, UART_IIR);
 
@@ -300,7 +298,7 @@ static void serial8250_backup_timeout(struct timer_list *t)
 		serial8250_tx_chars(up);
 
 	if (up->port.irq)
-		serial_out(up, UART_IER, ier);
+		restore_ier(up);
 
 	spin_unlock_irqrestore(&up->port.lock, flags);
 
@@ -578,6 +576,14 @@ serial8250_register_ports(struct uart_driver *drv, struct device *dev)
 
 #ifdef CONFIG_SERIAL_8250_CONSOLE
 
+static void univ8250_console_write_atomic(struct console *co, const char *s,
+					  unsigned int count)
+{
+	struct uart_8250_port *up = &serial8250_ports[co->index];
+
+	serial8250_console_write_atomic(up, s, count);
+}
+
 static void univ8250_console_write(struct console *co, const char *s,
 				   unsigned int count)
 {
@@ -663,6 +669,7 @@ static int univ8250_console_match(struct console *co, char *name, int idx,
 
 static struct console univ8250_console = {
 	.name		= "ttyS",
+	.write_atomic	= univ8250_console_write_atomic,
 	.write		= univ8250_console_write,
 	.device		= uart_console_device,
 	.setup		= univ8250_console_setup,
diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index bfa1a857f3ff..2fb394ddc420 100644
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -36,7 +36,7 @@ static void __dma_tx_complete(void *param)
 	ret = serial8250_tx_dma(p);
 	if (ret) {
 		p->ier |= UART_IER_THRI;
-		serial_port_out(&p->port, UART_IER, p->ier);
+		set_ier(p, p->ier);
 	}
 
 	spin_unlock_irqrestore(&p->port.lock, flags);
@@ -101,8 +101,7 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 	if (dma->tx_err) {
 		dma->tx_err = 0;
 		if (p->ier & UART_IER_THRI) {
-			p->ier &= ~UART_IER_THRI;
-			serial_out(p, UART_IER, p->ier);
+			set_ier(p, p->ier);
 		}
 	}
 	return 0;
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index d2f3310abe54..c1f308c1aaec 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -731,7 +731,7 @@ static void serial8250_set_sleep(struct uart_8250_port *p, int sleep)
 			serial_out(p, UART_EFR, UART_EFR_ECB);
 			serial_out(p, UART_LCR, 0);
 		}
-		serial_out(p, UART_IER, sleep ? UART_IERX_SLEEP : 0);
+		set_ier(p, sleep ? UART_IERX_SLEEP : 0);
 		if (p->capabilities & UART_CAP_EFR) {
 			serial_out(p, UART_LCR, UART_LCR_CONF_MODE_B);
 			serial_out(p, UART_EFR, efr);
@@ -1433,7 +1433,7 @@ static void serial8250_stop_rx(struct uart_port *port)
 
 	up->ier &= ~(UART_IER_RLSI | UART_IER_RDI);
 	up->port.read_status_mask &= ~UART_LSR_DR;
-	serial_port_out(port, UART_IER, up->ier);
+	set_ier(up, up->ier);
 
 	serial8250_rpm_put(up);
 }
@@ -1451,7 +1451,7 @@ static void __do_stop_tx_rs485(struct uart_8250_port *p)
 		serial8250_clear_and_reinit_fifos(p);
 
 		p->ier |= UART_IER_RLSI | UART_IER_RDI;
-		serial_port_out(&p->port, UART_IER, p->ier);
+		set_ier(p, p->ier);
 	}
 }
 static enum hrtimer_restart serial8250_em485_handle_stop_tx(struct hrtimer *t)
@@ -1504,7 +1504,7 @@ static inline void __do_stop_tx(struct uart_8250_port *p)
 {
 	if (p->ier & UART_IER_THRI) {
 		p->ier &= ~UART_IER_THRI;
-		serial_out(p, UART_IER, p->ier);
+		set_ier(p, p->ier);
 		serial8250_rpm_put_tx(p);
 	}
 }
@@ -1557,7 +1557,7 @@ static inline void __start_tx(struct uart_port *port)
 
 	if (!(up->ier & UART_IER_THRI)) {
 		up->ier |= UART_IER_THRI;
-		serial_port_out(port, UART_IER, up->ier);
+		set_ier(up, up->ier);
 
 		if (up->bugs & UART_BUG_TXEN) {
 			unsigned char lsr;
@@ -1663,7 +1663,7 @@ static void serial8250_disable_ms(struct uart_port *port)
 		return;
 
 	up->ier &= ~UART_IER_MSI;
-	serial_port_out(port, UART_IER, up->ier);
+	set_ier(up, up->ier);
 }
 
 static void serial8250_enable_ms(struct uart_port *port)
@@ -1677,7 +1677,7 @@ static void serial8250_enable_ms(struct uart_port *port)
 	up->ier |= UART_IER_MSI;
 
 	serial8250_rpm_get(up);
-	serial_port_out(port, UART_IER, up->ier);
+	set_ier(up, up->ier);
 	serial8250_rpm_put(up);
 }
 
@@ -2050,6 +2050,52 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits)
 	}
 }
 
+static atomic_t ier_counter = ATOMIC_INIT(0);
+static atomic_t ier_value = ATOMIC_INIT(0);
+
+void set_ier(struct uart_8250_port *up, unsigned char ier)
+{
+	struct uart_port *port = &up->port;
+	unsigned int flags;
+
+	console_atomic_lock(&flags);
+	if (atomic_read(&ier_counter) > 0)
+		atomic_set(&ier_value, ier);
+	else
+		serial_port_out(port, UART_IER, ier);
+	console_atomic_unlock(flags);
+}
+
+void clear_ier(struct uart_8250_port *up)
+{
+	struct uart_port *port = &up->port;
+	unsigned int ier_cleared = 0;
+	unsigned int flags;
+	unsigned int ier;
+
+	console_atomic_lock(&flags);
+	atomic_inc(&ier_counter);
+	ier = serial_port_in(port, UART_IER);
+	if (up->capabilities & UART_CAP_UUE)
+		ier_cleared = UART_IER_UUE;
+	if (ier != ier_cleared) {
+		serial_port_out(port, UART_IER, ier_cleared);
+		atomic_set(&ier_value, ier);
+	}
+	console_atomic_unlock(flags);
+}
+
+void restore_ier(struct uart_8250_port *up)
+{
+	struct uart_port *port = &up->port;
+	unsigned int flags;
+
+	console_atomic_lock(&flags);
+	if (atomic_fetch_dec(&ier_counter) == 1)
+		serial_port_out(port, UART_IER, atomic_read(&ier_value));
+	console_atomic_unlock(flags);
+}
+
 #ifdef CONFIG_CONSOLE_POLL
 /*
  * Console polling routines for writing and reading from the uart while
@@ -2081,18 +2127,10 @@ static int serial8250_get_poll_char(struct uart_port *port)
 static void serial8250_put_poll_char(struct uart_port *port,
 			 unsigned char c)
 {
-	unsigned int ier;
 	struct uart_8250_port *up = up_to_u8250p(port);
 
 	serial8250_rpm_get(up);
-	/*
-	 *	First save the IER then disable the interrupts
-	 */
-	ier = serial_port_in(port, UART_IER);
-	if (up->capabilities & UART_CAP_UUE)
-		serial_port_out(port, UART_IER, UART_IER_UUE);
-	else
-		serial_port_out(port, UART_IER, 0);
+	clear_ier(up);
 
 	wait_for_xmitr(up, BOTH_EMPTY);
 	/*
@@ -2105,7 +2143,7 @@ static void serial8250_put_poll_char(struct uart_port *port,
 	 *	and restore the IER
 	 */
 	wait_for_xmitr(up, BOTH_EMPTY);
-	serial_port_out(port, UART_IER, ier);
+	restore_ier(up);
 	serial8250_rpm_put(up);
 }
 
@@ -2417,7 +2455,7 @@ void serial8250_do_shutdown(struct uart_port *port)
 	 */
 	spin_lock_irqsave(&port->lock, flags);
 	up->ier = 0;
-	serial_port_out(port, UART_IER, 0);
+	set_ier(up, 0);
 	spin_unlock_irqrestore(&port->lock, flags);
 
 	synchronize_irq(port->irq);
@@ -2728,7 +2766,7 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
 	if (up->capabilities & UART_CAP_RTOIE)
 		up->ier |= UART_IER_RTOIE;
 
-	serial_port_out(port, UART_IER, up->ier);
+	set_ier(up, up->ier);
 
 	if (up->capabilities & UART_CAP_EFR) {
 		unsigned char efr = 0;
@@ -3192,7 +3230,7 @@ EXPORT_SYMBOL_GPL(serial8250_set_defaults);
 
 #ifdef CONFIG_SERIAL_8250_CONSOLE
 
-static void serial8250_console_putchar(struct uart_port *port, int ch)
+static void serial8250_console_putchar_locked(struct uart_port *port, int ch)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 
@@ -3200,6 +3238,18 @@ static void serial8250_console_putchar(struct uart_port *port, int ch)
 	serial_port_out(port, UART_TX, ch);
 }
 
+static void serial8250_console_putchar(struct uart_port *port, int ch)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+	unsigned int flags;
+
+	wait_for_xmitr(up, UART_LSR_THRE);
+
+	console_atomic_lock(&flags);
+	serial8250_console_putchar_locked(port, ch);
+	console_atomic_unlock(flags);
+}
+
 /*
  *	Restore serial console when h/w power-off detected
  */
@@ -3221,6 +3271,42 @@ static void serial8250_console_restore(struct uart_8250_port *up)
 	serial8250_out_MCR(up, UART_MCR_DTR | UART_MCR_RTS);
 }
 
+void serial8250_console_write_atomic(struct uart_8250_port *up,
+				     const char *s, unsigned int count)
+{
+	struct uart_port *port = &up->port;
+	unsigned int flags;
+	bool locked;
+
+	console_atomic_lock(&flags);
+
+	/*
+	 * If possible, keep any other CPUs from working with the
+	 * UART until the atomic message is completed. This helps
+	 * to keep the output more orderly.
+	 */
+	locked = spin_trylock(&port->lock);
+
+	touch_nmi_watchdog();
+
+	clear_ier(up);
+
+	if (atomic_fetch_inc(&up->console_printing)) {
+		uart_console_write(port, "\n", 1,
+				   serial8250_console_putchar_locked);
+	}
+	uart_console_write(port, s, count, serial8250_console_putchar_locked);
+	atomic_dec(&up->console_printing);
+
+	wait_for_xmitr(up, BOTH_EMPTY);
+	restore_ier(up);
+
+	if (locked)
+		spin_unlock(&port->lock);
+
+	console_atomic_unlock(flags);
+}
+
 /*
  *	Print a string to the serial port trying not to disturb
  *	any possible real use of the port...
@@ -3232,27 +3318,13 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
 {
 	struct uart_port *port = &up->port;
 	unsigned long flags;
-	unsigned int ier;
-	int locked = 1;
 
 	touch_nmi_watchdog();
 
 	serial8250_rpm_get(up);
+	spin_lock_irqsave(&port->lock, flags);
 
-	if (oops_in_progress)
-		locked = spin_trylock_irqsave(&port->lock, flags);
-	else
-		spin_lock_irqsave(&port->lock, flags);
-
-	/*
-	 *	First save the IER then disable the interrupts
-	 */
-	ier = serial_port_in(port, UART_IER);
-
-	if (up->capabilities & UART_CAP_UUE)
-		serial_port_out(port, UART_IER, UART_IER_UUE);
-	else
-		serial_port_out(port, UART_IER, 0);
+	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))) {
@@ -3260,14 +3332,16 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
 		up->canary = 0;
 	}
 
+	atomic_inc(&up->console_printing);
 	uart_console_write(port, s, count, serial8250_console_putchar);
+	atomic_dec(&up->console_printing);
 
 	/*
 	 *	Finally, wait for transmitter to become empty
 	 *	and restore the IER
 	 */
 	wait_for_xmitr(up, BOTH_EMPTY);
-	serial_port_out(port, UART_IER, ier);
+	restore_ier(up);
 
 	/*
 	 *	The receive handling will happen properly because the
@@ -3279,8 +3353,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
 	if (up->msr_saved_flags)
 		serial8250_modem_status(up);
 
-	if (locked)
-		spin_unlock_irqrestore(&port->lock, flags);
+	spin_unlock_irqrestore(&port->lock, flags);
 	serial8250_rpm_put(up);
 }
 
@@ -3301,6 +3374,7 @@ static unsigned int probe_baud(struct uart_port *port)
 
 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';
@@ -3309,6 +3383,8 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
 	if (!port->iobase && !port->membase)
 		return -ENODEV;
 
+	atomic_set(&up->console_printing, 0);
+
 	if (options)
 		uart_parse_options(options, &baud, &parity, &bits, &flow);
 	else if (probe)
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index 5a655ba8d273..a2dbeb0bc005 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -11,6 +11,7 @@
 #ifndef _LINUX_SERIAL_8250_H
 #define _LINUX_SERIAL_8250_H
 
+#include <linux/atomic.h>
 #include <linux/serial_core.h>
 #include <linux/serial_reg.h>
 #include <linux/platform_device.h>
@@ -126,6 +127,8 @@ struct uart_8250_port {
 #define MSR_SAVE_FLAGS UART_MSR_ANY_DELTA
 	unsigned char		msr_saved_flags;
 
+	atomic_t		console_printing;
+
 	struct uart_8250_dma	*dma;
 	const struct uart_8250_ops *ops;
 
@@ -177,6 +180,8 @@ 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_atomic(struct uart_8250_port *up, const char *s,
+				     unsigned int count);
 int serial8250_console_setup(struct uart_port *port, char *options, bool probe);
 
 extern void serial8250_set_isa_configurator(void (*v)
-- 
2.11.0