path: root/debian/patches-rt/0020-serial-8250-implement-write_atomic.patch

From: John Ogness <john.ogness@linutronix.de>
Date: Tue, 12 Feb 2019 15:29:58 +0100
Subject: [PATCH 20/25] serial: 8250: implement write_atomic
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.4/older/patches-5.4.3-rt1.tar.xz

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>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 drivers/tty/serial/8250/8250.h      |   22 +++++
 drivers/tty/serial/8250/8250_core.c |   19 +++-
 drivers/tty/serial/8250/8250_dma.c  |    4 
 drivers/tty/serial/8250/8250_port.c |  154 ++++++++++++++++++++++++++----------
 include/linux/serial_8250.h         |    5 +
 5 files changed, 157 insertions(+), 47 deletions(-)

--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -96,6 +96,10 @@ struct serial8250_config {
 #define SERIAL8250_SHARE_IRQS 0
 #endif
 
+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);
+
 #define SERIAL8250_PORT_FLAGS(_base, _irq, _flags)		\
 	{							\
 		.iobase		= _base,			\
@@ -139,6 +143,15 @@ static inline bool serial8250_set_THRI(s
 	return true;
 }
 
+static inline bool serial8250_set_THRI_sier(struct uart_8250_port *up)
+{
+	if (up->ier & UART_IER_THRI)
+		return false;
+	up->ier |= UART_IER_THRI;
+	set_ier(up, up->ier);
+	return true;
+}
+
 static inline bool serial8250_clear_THRI(struct uart_8250_port *up)
 {
 	if (!(up->ier & UART_IER_THRI))
@@ -148,6 +161,15 @@ static inline bool serial8250_clear_THRI
 	return true;
 }
 
+static inline bool serial8250_clear_THRI_sier(struct uart_8250_port *up)
+{
+	if (!(up->ier & UART_IER_THRI))
+		return false;
+	up->ier &= ~UART_IER_THRI;
+	set_ier(up, up->ier);
+	return true;
+}
+
 struct uart_8250_port *serial8250_get_port(int line);
 
 void serial8250_rpm_get(struct uart_8250_port *p);
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -266,7 +266,7 @@ static void serial8250_timeout(struct ti
 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);
@@ -275,10 +275,8 @@ static void serial8250_backup_timeout(st
 	 * 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);
 
@@ -301,7 +299,7 @@ static void serial8250_backup_timeout(st
 		serial8250_tx_chars(up);
 
 	if (up->port.irq)
-		serial_out(up, UART_IER, ier);
+		restore_ier(up);
 
 	spin_unlock_irqrestore(&up->port.lock, flags);
 
@@ -579,6 +577,14 @@ serial8250_register_ports(struct uart_dr
 
 #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)
 {
@@ -664,6 +670,7 @@ static int univ8250_console_match(struct
 
 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,
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -35,7 +35,7 @@ static void __dma_tx_complete(void *para
 
 	ret = serial8250_tx_dma(p);
 	if (ret)
-		serial8250_set_THRI(p);
+		serial8250_set_THRI_sier(p);
 
 	spin_unlock_irqrestore(&p->port.lock, flags);
 }
@@ -98,7 +98,7 @@ int serial8250_tx_dma(struct uart_8250_p
 	dma_async_issue_pending(dma->txchan);
 	if (dma->tx_err) {
 		dma->tx_err = 0;
-		serial8250_clear_THRI(p);
+		serial8250_clear_THRI_sier(p);
 	}
 	return 0;
 err:
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -721,7 +721,7 @@ static void serial8250_set_sleep(struct
 			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);
@@ -1390,7 +1390,7 @@ static void serial8250_stop_rx(struct ua
 
 	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);
 }
@@ -1408,7 +1408,7 @@ static void __do_stop_tx_rs485(struct ua
 		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)
@@ -1459,7 +1459,7 @@ static void __stop_tx_rs485(struct uart_
 
 static inline void __do_stop_tx(struct uart_8250_port *p)
 {
-	if (serial8250_clear_THRI(p))
+	if (serial8250_clear_THRI_sier(p))
 		serial8250_rpm_put_tx(p);
 }
 
@@ -1509,7 +1509,7 @@ static inline void __start_tx(struct uar
 	if (up->dma && !up->dma->tx_dma(up))
 		return;
 
-	if (serial8250_set_THRI(up)) {
+	if (serial8250_set_THRI_sier(up)) {
 		if (up->bugs & UART_BUG_TXEN) {
 			unsigned char lsr;
 
@@ -1616,7 +1616,7 @@ static void serial8250_disable_ms(struct
 	mctrl_gpio_disable_ms(up->gpios);
 
 	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)
@@ -1632,7 +1632,7 @@ static void serial8250_enable_ms(struct
 	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);
 }
 
@@ -1991,6 +1991,52 @@ static void wait_for_xmitr(struct uart_8
 	}
 }
 
+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
@@ -2022,18 +2068,10 @@ static int serial8250_get_poll_char(stru
 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);
 	/*
@@ -2046,7 +2084,7 @@ static void serial8250_put_poll_char(str
 	 *	and restore the IER
 	 */
 	wait_for_xmitr(up, BOTH_EMPTY);
-	serial_port_out(port, UART_IER, ier);
+	restore_ier(up);
 	serial8250_rpm_put(up);
 }
 
@@ -2354,7 +2392,7 @@ void serial8250_do_shutdown(struct uart_
 	 */
 	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);
@@ -2639,7 +2677,7 @@ serial8250_do_set_termios(struct uart_po
 	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;
@@ -3103,7 +3141,7 @@ EXPORT_SYMBOL_GPL(serial8250_set_default
 
 #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);
 
@@ -3111,6 +3149,18 @@ static void serial8250_console_putchar(s
 	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
  */
@@ -3132,6 +3182,42 @@ static void serial8250_console_restore(s
 	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...
@@ -3143,27 +3229,13 @@ void serial8250_console_write(struct uar
 {
 	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))) {
@@ -3171,14 +3243,16 @@ void serial8250_console_write(struct uar
 		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
@@ -3190,8 +3264,7 @@ void serial8250_console_write(struct uar
 	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);
 }
 
@@ -3212,6 +3285,7 @@ static unsigned int probe_baud(struct ua
 
 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';
@@ -3220,6 +3294,8 @@ int serial8250_console_setup(struct uart
 	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)
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -7,6 +7,7 @@
 #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>
@@ -123,6 +124,8 @@ 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;
 
@@ -174,6 +177,8 @@ void serial8250_init_port(struct uart_82
 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)