Subject: [PATCH 17/18] serial: 8250: implement write_atomic
From: John Ogness <[email-masked]>
Date: Fri, 4 Feb 2022 16:01:17 +0106

Implement a non-sleeping NMI-safe write_atomic() console function in
order to support atomic console printing during a panic.

Trasmitting data requires disabling interrupts. Since write_atomic()
can be called from any context, it may be called while another CPU
is executing in console code. In order to maintain the correct state
of the IER register, use the global cpu_sync to synchronize all
access to the IER register. This synchronization is only necessary
for serial ports that are being used as consoles.

The global cpu_sync is also used to synchronize between the write()
and write_atomic() callbacks. write() synchronizes per character,
write_atomic() synchronizes per line.

Signed-off-by: John Ogness <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 drivers/tty/serial/8250/8250.h              |   41 ++++++-
 drivers/tty/serial/8250/8250_aspeed_vuart.c |    2 
 drivers/tty/serial/8250/8250_bcm7271.c      |   21 +++
 drivers/tty/serial/8250/8250_core.c         |   24 +++-
 drivers/tty/serial/8250/8250_exar.c         |    4 
 drivers/tty/serial/8250/8250_fsl.c          |    3 
 drivers/tty/serial/8250/8250_ingenic.c      |    3 
 drivers/tty/serial/8250/8250_mtk.c          |   32 +++++
 drivers/tty/serial/8250/8250_omap.c         |   20 +--
 drivers/tty/serial/8250/8250_port.c         |  158 ++++++++++++++++++++++------
 drivers/tty/serial/8250/Kconfig             |    1 
 include/linux/serial_8250.h                 |    5 
 12 files changed, 262 insertions(+), 52 deletions(-)

--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -177,12 +177,49 @@ static inline void serial_dl_write(struc
 	up->dl_write(up, value);
 }
 
+static inline int serial8250_in_IER(struct uart_8250_port *up)
+{
+	struct uart_port *port = &up->port;
+	unsigned long flags;
+	bool is_console;
+	int ier;
+
+	is_console = uart_console(port);
+
+	if (is_console)
+		printk_cpu_sync_get_irqsave(flags);
+
+	ier = serial_in(up, UART_IER);
+
+	if (is_console)
+		printk_cpu_sync_put_irqrestore(flags);
+
+	return ier;
+}
+
+static inline void serial8250_set_IER(struct uart_8250_port *up, int ier)
+{
+	struct uart_port *port = &up->port;
+	unsigned long flags;
+	bool is_console;
+
+	is_console = uart_console(port);
+
+	if (is_console)
+		printk_cpu_sync_get_irqsave(flags);
+
+	serial_out(up, UART_IER, ier);
+
+	if (is_console)
+		printk_cpu_sync_put_irqrestore(flags);
+}
+
 static inline bool serial8250_set_THRI(struct uart_8250_port *up)
 {
 	if (up->ier & UART_IER_THRI)
 		return false;
 	up->ier |= UART_IER_THRI;
-	serial_out(up, UART_IER, up->ier);
+	serial8250_set_IER(up, up->ier);
 	return true;
 }
 
@@ -191,7 +228,7 @@ static inline bool serial8250_clear_THRI
 	if (!(up->ier & UART_IER_THRI))
 		return false;
 	up->ier &= ~UART_IER_THRI;
-	serial_out(up, UART_IER, up->ier);
+	serial8250_set_IER(up, up->ier);
 	return true;
 }
 
--- a/drivers/tty/serial/8250/8250_aspeed_vuart.c
+++ b/drivers/tty/serial/8250/8250_aspeed_vuart.c
@@ -278,7 +278,7 @@ static void __aspeed_vuart_set_throttle(
 	up->ier &= ~irqs;
 	if (!throttle)
 		up->ier |= irqs;
-	serial_out(up, UART_IER, up->ier);
+	serial8250_set_IER(up, up->ier);
 }
 static void aspeed_vuart_set_throttle(struct uart_port *port, bool throttle)
 {
--- a/drivers/tty/serial/8250/8250_bcm7271.c
+++ b/drivers/tty/serial/8250/8250_bcm7271.c
@@ -609,7 +609,7 @@ static int brcmuart_startup(struct uart_
 	 * will handle this.
 	 */
 	up->ier &= ~UART_IER_RDI;
-	serial_port_out(port, UART_IER, up->ier);
+	serial8250_set_IER(up, up->ier);
 
 	priv->tx_running = false;
 	priv->dma.rx_dma = NULL;
@@ -775,10 +775,12 @@ static int brcmuart_handle_irq(struct ua
 	unsigned int iir = serial_port_in(p, UART_IIR);
 	struct brcmuart_priv *priv = p->private_data;
 	struct uart_8250_port *up = up_to_u8250p(p);
+	unsigned long cs_flags;
 	unsigned int status;
 	unsigned long flags;
 	unsigned int ier;
 	unsigned int mcr;
+	bool is_console;
 	int handled = 0;
 
 	/*
@@ -789,6 +791,10 @@ static int brcmuart_handle_irq(struct ua
 		spin_lock_irqsave(&p->lock, flags);
 		status = serial_port_in(p, UART_LSR);
 		if ((status & UART_LSR_DR) == 0) {
+			is_console = uart_console(p);
+
+			if (is_console)
+				printk_cpu_sync_get_irqsave(cs_flags);
 
 			ier = serial_port_in(p, UART_IER);
 			/*
@@ -809,6 +815,9 @@ static int brcmuart_handle_irq(struct ua
 				serial_port_in(p, UART_RX);
 			}
 
+			if (is_console)
+				printk_cpu_sync_put_irqrestore(cs_flags);
+
 			handled = 1;
 		}
 		spin_unlock_irqrestore(&p->lock, flags);
@@ -823,8 +832,10 @@ static enum hrtimer_restart brcmuart_hrt
 	struct brcmuart_priv *priv = container_of(t, struct brcmuart_priv, hrt);
 	struct uart_port *p = priv->up;
 	struct uart_8250_port *up = up_to_u8250p(p);
+	unsigned long cs_flags;
 	unsigned int status;
 	unsigned long flags;
+	bool is_console;
 
 	if (priv->shutdown)
 		return HRTIMER_NORESTART;
@@ -846,12 +857,20 @@ static enum hrtimer_restart brcmuart_hrt
 	/* re-enable receive unless upper layer has disabled it */
 	if ((up->ier & (UART_IER_RLSI | UART_IER_RDI)) ==
 	    (UART_IER_RLSI | UART_IER_RDI)) {
+		is_console = uart_console(p);
+
+		if (is_console)
+			printk_cpu_sync_get_irqsave(cs_flags);
+
 		status = serial_port_in(p, UART_IER);
 		status |= (UART_IER_RLSI | UART_IER_RDI);
 		serial_port_out(p, UART_IER, status);
 		status = serial_port_in(p, UART_MCR);
 		status |= UART_MCR_RTS;
 		serial_port_out(p, UART_MCR, status);
+
+		if (is_console)
+			printk_cpu_sync_put_irqrestore(cs_flags);
 	}
 	spin_unlock_irqrestore(&p->lock, flags);
 	return HRTIMER_NORESTART;
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -255,8 +255,11 @@ 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);
+	struct uart_port *port = &up->port;
 	unsigned int iir, ier = 0, lsr;
+	unsigned long cs_flags;
 	unsigned long flags;
+	bool is_console;
 
 	spin_lock_irqsave(&up->port.lock, flags);
 
@@ -265,8 +268,16 @@ static void serial8250_backup_timeout(st
 	 * based handler.
 	 */
 	if (up->port.irq) {
+		is_console = uart_console(port);
+
+		if (is_console)
+			printk_cpu_sync_get_irqsave(cs_flags);
+
 		ier = serial_in(up, UART_IER);
 		serial_out(up, UART_IER, 0);
+
+		if (is_console)
+			printk_cpu_sync_put_irqrestore(cs_flags);
 	}
 
 	iir = serial_in(up, UART_IIR);
@@ -289,7 +300,7 @@ static void serial8250_backup_timeout(st
 		serial8250_tx_chars(up);
 
 	if (up->port.irq)
-		serial_out(up, UART_IER, ier);
+		serial8250_set_IER(up, ier);
 
 	spin_unlock_irqrestore(&up->port.lock, flags);
 
@@ -575,6 +586,14 @@ 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)
 {
@@ -668,6 +687,7 @@ 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,
@@ -961,7 +981,7 @@ static void serial_8250_overrun_backoff_
 	spin_lock_irqsave(&port->lock, flags);
 	up->ier |= UART_IER_RLSI | UART_IER_RDI;
 	up->port.read_status_mask |= UART_LSR_DR;
-	serial_out(up, UART_IER, up->ier);
+	serial8250_set_IER(up, up->ier);
 	spin_unlock_irqrestore(&port->lock, flags);
 }
 
--- a/drivers/tty/serial/8250/8250_exar.c
+++ b/drivers/tty/serial/8250/8250_exar.c
@@ -185,6 +185,8 @@ static void xr17v35x_set_divisor(struct
 
 static int xr17v35x_startup(struct uart_port *port)
 {
+	struct uart_8250_port *up = up_to_u8250p(port);
+
 	/*
 	 * First enable access to IER [7:5], ISR [5:4], FCR [5:4],
 	 * MCR [7:5] and MSR [7:0]
@@ -195,7 +197,7 @@ static int xr17v35x_startup(struct uart_
 	 * Make sure all interrups are masked until initialization is
 	 * complete and the FIFOs are cleared
 	 */
-	serial_port_out(port, UART_IER, 0);
+	serial8250_set_IER(up, 0);
 
 	return serial8250_do_startup(port);
 }
--- a/drivers/tty/serial/8250/8250_fsl.c
+++ b/drivers/tty/serial/8250/8250_fsl.c
@@ -58,7 +58,8 @@ int fsl8250_handle_irq(struct uart_port
 	if ((orig_lsr & UART_LSR_OE) && (up->overrun_backoff_time_ms > 0)) {
 		unsigned long delay;
 
-		up->ier = port->serial_in(port, UART_IER);
+		up->ier = serial8250_in_IER(up);
+
 		if (up->ier & (UART_IER_RLSI | UART_IER_RDI)) {
 			port->ops->stop_rx(port);
 		} else {
--- a/drivers/tty/serial/8250/8250_ingenic.c
+++ b/drivers/tty/serial/8250/8250_ingenic.c
@@ -146,6 +146,7 @@ OF_EARLYCON_DECLARE(x1000_uart, "ingenic
 
 static void ingenic_uart_serial_out(struct uart_port *p, int offset, int value)
 {
+	struct uart_8250_port *up = up_to_u8250p(p);
 	int ier;
 
 	switch (offset) {
@@ -167,7 +168,7 @@ static void ingenic_uart_serial_out(stru
 		 * If we have enabled modem status IRQs we should enable
 		 * modem mode.
 		 */
-		ier = p->serial_in(p, UART_IER);
+		ier = serial8250_in_IER(up);
 
 		if (ier & UART_IER_MSI)
 			value |= UART_MCR_MDCE | UART_MCR_FCM;
--- a/drivers/tty/serial/8250/8250_mtk.c
+++ b/drivers/tty/serial/8250/8250_mtk.c
@@ -222,12 +222,40 @@ static void mtk8250_shutdown(struct uart
 
 static void mtk8250_disable_intrs(struct uart_8250_port *up, int mask)
 {
-	serial_out(up, UART_IER, serial_in(up, UART_IER) & (~mask));
+	struct uart_port *port = &up->port;
+	unsigned long flags;
+	bool is_console;
+	int ier;
+
+	is_console = uart_console(port);
+
+	if (is_console)
+		printk_cpu_sync_get_irqsave(flags);
+
+	ier = serial_in(up, UART_IER);
+	serial_out(up, UART_IER, ier & (~mask));
+
+	if (is_console)
+		printk_cpu_sync_put_irqrestore(flags);
 }
 
 static void mtk8250_enable_intrs(struct uart_8250_port *up, int mask)
 {
-	serial_out(up, UART_IER, serial_in(up, UART_IER) | mask);
+	struct uart_port *port = &up->port;
+	unsigned long flags;
+	bool is_console;
+	int ier;
+
+	is_console = uart_console(port);
+
+	if (is_console)
+		printk_cpu_sync_get_irqsave(flags);
+
+	ier = serial_in(up, UART_IER);
+	serial_out(up, UART_IER, ier | mask);
+
+	if (is_console)
+		printk_cpu_sync_put_irqrestore(flags);
 }
 
 static void mtk8250_set_flow_ctrl(struct uart_8250_port *up, int mode)
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -328,7 +328,7 @@ static void omap8250_restore_regs(struct
 	/* drop TCR + TLR access, we setup XON/XOFF later */
 	serial8250_out_MCR(up, mcr);
 
-	serial_out(up, UART_IER, up->ier);
+	serial8250_set_IER(up, up->ier);
 
 	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_B);
 	serial_dl_write(up, priv->quot);
@@ -518,7 +518,7 @@ static void omap_8250_pm(struct uart_por
 	serial_out(up, UART_EFR, efr | UART_EFR_ECB);
 	serial_out(up, UART_LCR, 0);
 
-	serial_out(up, UART_IER, (state != 0) ? UART_IERX_SLEEP : 0);
+	serial8250_set_IER(up, (state != 0) ? UART_IERX_SLEEP : 0);
 	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_B);
 	serial_out(up, UART_EFR, efr);
 	serial_out(up, UART_LCR, 0);
@@ -639,7 +639,7 @@ static irqreturn_t omap8250_irq(int irq,
 	if ((lsr & UART_LSR_OE) && up->overrun_backoff_time_ms > 0) {
 		unsigned long delay;
 
-		up->ier = port->serial_in(port, UART_IER);
+		up->ier = serial8250_in_IER(up);
 		if (up->ier & (UART_IER_RLSI | UART_IER_RDI)) {
 			port->ops->stop_rx(port);
 		} else {
@@ -698,7 +698,7 @@ static int omap_8250_startup(struct uart
 		goto err;
 
 	up->ier = UART_IER_RLSI | UART_IER_RDI;
-	serial_out(up, UART_IER, up->ier);
+	serial8250_set_IER(up, up->ier);
 
 #ifdef CONFIG_PM
 	up->capabilities |= UART_CAP_RPM;
@@ -739,7 +739,7 @@ static void omap_8250_shutdown(struct ua
 		serial_out(up, UART_OMAP_EFR2, 0x0);
 
 	up->ier = 0;
-	serial_out(up, UART_IER, 0);
+	serial8250_set_IER(up, 0);
 
 	if (up->dma)
 		serial8250_release_dma(up);
@@ -787,7 +787,7 @@ static void omap_8250_unthrottle(struct
 		up->dma->rx_dma(up);
 	up->ier |= UART_IER_RLSI | UART_IER_RDI;
 	port->read_status_mask |= UART_LSR_DR;
-	serial_out(up, UART_IER, up->ier);
+	serial8250_set_IER(up, up->ier);
 	spin_unlock_irqrestore(&port->lock, flags);
 
 	pm_runtime_mark_last_busy(port->dev);
@@ -878,7 +878,7 @@ static void __dma_rx_complete(void *para
 	__dma_rx_do_complete(p);
 	if (!priv->throttled) {
 		p->ier |= UART_IER_RLSI | UART_IER_RDI;
-		serial_out(p, UART_IER, p->ier);
+		serial8250_set_IER(p, p->ier);
 		if (!(priv->habit & UART_HAS_EFR2))
 			omap_8250_rx_dma(p);
 	}
@@ -935,7 +935,7 @@ static int omap_8250_rx_dma(struct uart_
 			 * callback to run.
 			 */
 			p->ier &= ~(UART_IER_RLSI | UART_IER_RDI);
-			serial_out(p, UART_IER, p->ier);
+			serial8250_set_IER(p, p->ier);
 		}
 		goto out;
 	}
@@ -1148,12 +1148,12 @@ static void am654_8250_handle_rx_dma(str
 		 * periodic timeouts, re-enable interrupts.
 		 */
 		up->ier &= ~(UART_IER_RLSI | UART_IER_RDI);
-		serial_out(up, UART_IER, up->ier);
+		serial8250_set_IER(up, up->ier);
 		omap_8250_rx_dma_flush(up);
 		serial_in(up, UART_IIR);
 		serial_out(up, UART_OMAP_EFR2, 0x0);
 		up->ier |= UART_IER_RLSI | UART_IER_RDI;
-		serial_out(up, UART_IER, up->ier);
+		serial8250_set_IER(up, up->ier);
 	}
 }
 
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -743,7 +743,7 @@ 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);
+		serial8250_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);
@@ -754,12 +754,29 @@ static void serial8250_set_sleep(struct
 	serial8250_rpm_put(p);
 }
 
-static void serial8250_clear_IER(struct uart_8250_port *up)
+static unsigned int serial8250_clear_IER(struct uart_8250_port *up)
 {
+	struct uart_port *port = &up->port;
+	unsigned int clearval = 0;
+	unsigned long flags;
+	bool is_console;
+	unsigned int prior;
+
+	is_console = uart_console(port);
+
 	if (up->capabilities & UART_CAP_UUE)
-		serial_out(up, UART_IER, UART_IER_UUE);
-	else
-		serial_out(up, UART_IER, 0);
+		clearval = UART_IER_UUE;
+
+	if (is_console)
+		printk_cpu_sync_get_irqsave(flags);
+
+	prior = serial_in(up, UART_IER);
+	serial_out(up, UART_IER, clearval);
+
+	if (is_console)
+		printk_cpu_sync_put_irqrestore(flags);
+
+	return prior;
 }
 
 #ifdef CONFIG_SERIAL_8250_RSA
@@ -1025,8 +1042,11 @@ static int broken_efr(struct uart_8250_p
  */
 static void autoconfig_16550a(struct uart_8250_port *up)
 {
+	struct uart_port *port = &up->port;
 	unsigned char status1, status2;
 	unsigned int iersave;
+	unsigned long flags;
+	bool is_console;
 
 	up->port.type = PORT_16550A;
 	up->capabilities |= UART_CAP_FIFO;
@@ -1138,6 +1158,11 @@ static void autoconfig_16550a(struct uar
 		return;
 	}
 
+	is_console = uart_console(port);
+
+	if (is_console)
+		printk_cpu_sync_get_irqsave(flags);
+
 	/*
 	 * Try writing and reading the UART_IER_UUE bit (b6).
 	 * If it works, this is probably one of the Xscale platform's
@@ -1173,6 +1198,9 @@ static void autoconfig_16550a(struct uar
 	}
 	serial_out(up, UART_IER, iersave);
 
+	if (is_console)
+		printk_cpu_sync_put_irqrestore(flags);
+
 	/*
 	 * We distinguish between 16550A and U6 16550A by counting
 	 * how many bytes are in the FIFO.
@@ -1195,8 +1223,10 @@ static void autoconfig(struct uart_8250_
 	unsigned char status1, scratch, scratch2, scratch3;
 	unsigned char save_lcr, save_mcr;
 	struct uart_port *port = &up->port;
+	unsigned long cs_flags;
 	unsigned long flags;
 	unsigned int old_capabilities;
+	bool is_console;
 
 	if (!port->iobase && !port->mapbase && !port->membase)
 		return;
@@ -1214,6 +1244,11 @@ static void autoconfig(struct uart_8250_
 	up->bugs = 0;
 
 	if (!(port->flags & UPF_BUGGY_UART)) {
+		is_console = uart_console(port);
+
+		if (is_console)
+			printk_cpu_sync_get_irqsave(cs_flags);
+
 		/*
 		 * Do a simple existence test first; if we fail this,
 		 * there's no point trying anything else.
@@ -1243,6 +1278,10 @@ static void autoconfig(struct uart_8250_
 #endif
 		scratch3 = serial_in(up, UART_IER) & 0x0f;
 		serial_out(up, UART_IER, scratch);
+
+		if (is_console)
+			printk_cpu_sync_put_irqrestore(cs_flags);
+
 		if (scratch2 != 0 || scratch3 != 0x0F) {
 			/*
 			 * We failed; there's nothing here
@@ -1366,7 +1405,9 @@ static void autoconfig_irq(struct uart_8
 	unsigned char save_mcr, save_ier;
 	unsigned char save_ICP = 0;
 	unsigned int ICP = 0;
+	unsigned long flags;
 	unsigned long irqs;
+	bool is_console;
 	int irq;
 
 	if (port->flags & UPF_FOURPORT) {
@@ -1376,8 +1417,12 @@ static void autoconfig_irq(struct uart_8
 		inb_p(ICP);
 	}
 
-	if (uart_console(port))
+	is_console = uart_console(port);
+
+	if (is_console) {
 		console_lock();
+		printk_cpu_sync_get_irqsave(flags);
+	}
 
 	/* forget possible initially masked and pending IRQ */
 	probe_irq_off(probe_irq_on());
@@ -1409,8 +1454,10 @@ static void autoconfig_irq(struct uart_8
 	if (port->flags & UPF_FOURPORT)
 		outb_p(save_ICP, ICP);
 
-	if (uart_console(port))
+	if (is_console) {
+		printk_cpu_sync_put_irqrestore(flags);
 		console_unlock();
+	}
 
 	port->irq = (irq > 0) ? irq : 0;
 }
@@ -1423,7 +1470,7 @@ 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);
+	serial8250_set_IER(up, up->ier);
 
 	serial8250_rpm_put(up);
 }
@@ -1453,7 +1500,7 @@ void serial8250_em485_stop_tx(struct uar
 		serial8250_clear_and_reinit_fifos(p);
 
 		p->ier |= UART_IER_RLSI | UART_IER_RDI;
-		serial_port_out(&p->port, UART_IER, p->ier);
+		serial8250_set_IER(p, p->ier);
 	}
 }
 EXPORT_SYMBOL_GPL(serial8250_em485_stop_tx);
@@ -1702,7 +1749,7 @@ 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);
+	serial8250_set_IER(up, up->ier);
 }
 
 static void serial8250_enable_ms(struct uart_port *port)
@@ -1718,7 +1765,7 @@ static void serial8250_enable_ms(struct
 	up->ier |= UART_IER_MSI;
 
 	serial8250_rpm_get(up);
-	serial_port_out(port, UART_IER, up->ier);
+	serial8250_set_IER(up, up->ier);
 	serial8250_rpm_put(up);
 }
 
@@ -2155,8 +2202,7 @@ static void serial8250_put_poll_char(str
 	/*
 	 *	First save the IER then disable the interrupts
 	 */
-	ier = serial_port_in(port, UART_IER);
-	serial8250_clear_IER(up);
+	ier = serial8250_clear_IER(up);
 
 	wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
 	/*
@@ -2169,7 +2215,7 @@ static void serial8250_put_poll_char(str
 	 *	and restore the IER
 	 */
 	wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
-	serial_port_out(port, UART_IER, ier);
+	serial8250_set_IER(up, ier);
 	serial8250_rpm_put(up);
 }
 
@@ -2178,8 +2224,10 @@ static void serial8250_put_poll_char(str
 int serial8250_do_startup(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
+	unsigned long cs_flags;
 	unsigned long flags;
 	unsigned char iir;
+	bool is_console;
 	int retval;
 	u16 lsr;
 
@@ -2200,7 +2248,7 @@ int serial8250_do_startup(struct uart_po
 		up->acr = 0;
 		serial_port_out(port, UART_LCR, UART_LCR_CONF_MODE_B);
 		serial_port_out(port, UART_EFR, UART_EFR_ECB);
-		serial_port_out(port, UART_IER, 0);
+		serial8250_set_IER(up, 0);
 		serial_port_out(port, UART_LCR, 0);
 		serial_icr_write(up, UART_CSR, 0); /* Reset the UART */
 		serial_port_out(port, UART_LCR, UART_LCR_CONF_MODE_B);
@@ -2210,7 +2258,7 @@ int serial8250_do_startup(struct uart_po
 
 	if (port->type == PORT_DA830) {
 		/* Reset the port */
-		serial_port_out(port, UART_IER, 0);
+		serial8250_set_IER(up, 0);
 		serial_port_out(port, UART_DA830_PWREMU_MGMT, 0);
 		mdelay(10);
 
@@ -2309,6 +2357,8 @@ int serial8250_do_startup(struct uart_po
 	if (retval)
 		goto out;
 
+	is_console = uart_console(port);
+
 	if (port->irq && !(up->port.flags & UPF_NO_THRE_TEST)) {
 		unsigned char iir1;
 
@@ -2325,6 +2375,9 @@ int serial8250_do_startup(struct uart_po
 		 */
 		spin_lock_irqsave(&port->lock, flags);
 
+		if (is_console)
+			printk_cpu_sync_get_irqsave(cs_flags);
+
 		wait_for_xmitr(up, UART_LSR_THRE);
 		serial_port_out_sync(port, UART_IER, UART_IER_THRI);
 		udelay(1); /* allow THRE to set */
@@ -2335,6 +2388,9 @@ int serial8250_do_startup(struct uart_po
 		iir = serial_port_in(port, UART_IIR);
 		serial_port_out(port, UART_IER, 0);
 
+		if (is_console)
+			printk_cpu_sync_put_irqrestore(cs_flags);
+
 		spin_unlock_irqrestore(&port->lock, flags);
 
 		if (port->irqflags & IRQF_SHARED)
@@ -2389,10 +2445,14 @@ int serial8250_do_startup(struct uart_po
 	 * Do a quick test to see if we receive an interrupt when we enable
 	 * the TX irq.
 	 */
+	if (is_console)
+		printk_cpu_sync_get_irqsave(cs_flags);
 	serial_port_out(port, UART_IER, UART_IER_THRI);
 	lsr = serial_port_in(port, UART_LSR);
 	iir = serial_port_in(port, UART_IIR);
 	serial_port_out(port, UART_IER, 0);
+	if (is_console)
+		printk_cpu_sync_put_irqrestore(cs_flags);
 
 	if (lsr & UART_LSR_TEMT && iir & UART_IIR_NO_INT) {
 		if (!(up->bugs & UART_BUG_TXEN)) {
@@ -2424,7 +2484,7 @@ int serial8250_do_startup(struct uart_po
 	if (up->dma) {
 		const char *msg = NULL;
 
-		if (uart_console(port))
+		if (is_console)
 			msg = "forbid DMA for kernel console";
 		else if (serial8250_request_dma(up))
 			msg = "failed to request DMA";
@@ -2475,7 +2535,7 @@ void serial8250_do_shutdown(struct uart_
 	 */
 	spin_lock_irqsave(&port->lock, flags);
 	up->ier = 0;
-	serial_port_out(port, UART_IER, 0);
+	serial8250_set_IER(up, 0);
 	spin_unlock_irqrestore(&port->lock, flags);
 
 	synchronize_irq(port->irq);
@@ -2841,7 +2901,7 @@ 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);
+	serial8250_set_IER(up, up->ier);
 
 	if (up->capabilities & UART_CAP_EFR) {
 		unsigned char efr = 0;
@@ -3306,7 +3366,7 @@ EXPORT_SYMBOL_GPL(serial8250_set_default
 
 #ifdef CONFIG_SERIAL_8250_CONSOLE
 
-static void serial8250_console_putchar(struct uart_port *port, unsigned char ch)
+static void serial8250_console_putchar_locked(struct uart_port *port, unsigned char ch)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 
@@ -3314,6 +3374,18 @@ static void serial8250_console_putchar(s
 	serial_port_out(port, UART_TX, ch);
 }
 
+static void serial8250_console_putchar(struct uart_port *port, unsigned char ch)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+	unsigned long flags;
+
+	wait_for_xmitr(up, UART_LSR_THRE);
+
+	printk_cpu_sync_get_irqsave(flags);
+	serial8250_console_putchar_locked(port, ch);
+	printk_cpu_sync_put_irqrestore(flags);
+}
+
 /*
  *	Restore serial console when h/w power-off detected
  */
@@ -3340,6 +3412,32 @@ static void serial8250_console_restore(s
 	serial8250_out_MCR(up, up->mcr | 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 long flags;
+	unsigned int ier;
+
+	printk_cpu_sync_get_irqsave(flags);
+
+	touch_nmi_watchdog();
+
+	ier = serial8250_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, UART_LSR_BOTH_EMPTY);
+	serial8250_set_IER(up, ier);
+
+	printk_cpu_sync_put_irqrestore(flags);
+}
+
 /*
  * Print a string to the serial port using the device FIFO
  *
@@ -3385,20 +3483,15 @@ void serial8250_console_write(struct uar
 	struct uart_port *port = &up->port;
 	unsigned long flags;
 	unsigned int ier, use_fifo;
-	int locked = 1;
 
 	touch_nmi_watchdog();
 
-	if (oops_in_progress)
-		locked = spin_trylock_irqsave(&port->lock, flags);
-	else
-		spin_lock_irqsave(&port->lock, flags);
+	spin_lock_irqsave(&port->lock, flags);
 
 	/*
 	 *	First save the IER then disable the interrupts
 	 */
-	ier = serial_port_in(port, UART_IER);
-	serial8250_clear_IER(up);
+	ier = 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))) {
@@ -3432,10 +3525,12 @@ void serial8250_console_write(struct uar
 		 */
 		!(up->port.flags & UPF_CONS_FLOW);
 
+	atomic_inc(&up->console_printing);
 	if (likely(use_fifo))
 		serial8250_console_fifo_write(up, s, count);
 	else
 		uart_console_write(port, s, count, serial8250_console_putchar);
+	atomic_dec(&up->console_printing);
 
 	/*
 	 *	Finally, wait for transmitter to become empty
@@ -3448,8 +3543,7 @@ void serial8250_console_write(struct uar
 		if (em485->tx_stopped)
 			up->rs485_stop_tx(up);
 	}
-
-	serial_port_out(port, UART_IER, ier);
+	serial8250_set_IER(up, ier);
 
 	/*
 	 *	The receive handling will happen properly because the
@@ -3461,8 +3555,7 @@ 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);
 }
 
 static unsigned int probe_baud(struct uart_port *port)
@@ -3482,6 +3575,7 @@ 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';
@@ -3491,6 +3585,8 @@ 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/drivers/tty/serial/8250/Kconfig
+++ b/drivers/tty/serial/8250/Kconfig
@@ -9,6 +9,7 @@ config SERIAL_8250
 	depends on !S390
 	select SERIAL_CORE
 	select SERIAL_MCTRL_GPIO if GPIOLIB
+	select HAVE_ATOMIC_CONSOLE
 	help
 	  This selects whether you want to include the driver for the standard
 	  serial ports.  The standard answer is Y.  People who might say N
--- 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>
@@ -125,6 +126,8 @@ 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;
 
@@ -180,6 +183,8 @@ 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);
 int serial8250_console_exit(struct uart_port *port);