[PATCH tty v1 8/8] serial: 8250: synchronize and annotate UART_IER access

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

The UART_IER register is modified twice by each console write
(serial8250_console_write()) under the port lock. Any driver code that
accesses UART_IER must do so with the port locked in order to ensure
consistent values, even when for read accesses.

Add locking, lockdep notation, and/or comments everywhere UART_IER is
accessed. The added locking is not fixing a real problem because it
occurs where the console is not active. However, adding the locking
to these non-critical paths greatly simplifies UART_IER access
tracking by establishing a general policy that all UART_IER access
is performed with the port locked.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 drivers/tty/serial/8250/8250.h              |  6 +++
 drivers/tty/serial/8250/8250_aspeed_vuart.c |  3 ++
 drivers/tty/serial/8250/8250_mtk.c          |  9 ++++
 drivers/tty/serial/8250/8250_omap.c         | 14 ++++++
 drivers/tty/serial/8250/8250_port.c         | 53 +++++++++++++++++++++
 5 files changed, 85 insertions(+)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index 5418708f4631..471c6bc5f78f 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -179,6 +179,9 @@ static inline void serial_dl_write(struct uart_8250_port *up, u32 value)
 
 static inline bool serial8250_set_THRI(struct uart_8250_port *up)
 {
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&up->port.lock);
+
 	if (up->ier & UART_IER_THRI)
 		return false;
 	up->ier |= UART_IER_THRI;
@@ -188,6 +191,9 @@ static inline bool serial8250_set_THRI(struct uart_8250_port *up)
 
 static inline bool serial8250_clear_THRI(struct uart_8250_port *up)
 {
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&up->port.lock);
+
 	if (!(up->ier & UART_IER_THRI))
 		return false;
 	up->ier &= ~UART_IER_THRI;
diff --git a/drivers/tty/serial/8250/8250_aspeed_vuart.c b/drivers/tty/serial/8250/8250_aspeed_vuart.c
index 9d2a7856784f..4a9e71b2dbbc 100644
--- a/drivers/tty/serial/8250/8250_aspeed_vuart.c
+++ b/drivers/tty/serial/8250/8250_aspeed_vuart.c
@@ -275,6 +275,9 @@ static void __aspeed_vuart_set_throttle(struct uart_8250_port *up,
 {
 	unsigned char irqs = UART_IER_RLSI | UART_IER_RDI;
 
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&up->port.lock);
+
 	up->ier &= ~irqs;
 	if (!throttle)
 		up->ier |= irqs;
diff --git a/drivers/tty/serial/8250/8250_mtk.c b/drivers/tty/serial/8250/8250_mtk.c
index fb1d5ec0940e..aa8e98164d68 100644
--- a/drivers/tty/serial/8250/8250_mtk.c
+++ b/drivers/tty/serial/8250/8250_mtk.c
@@ -222,11 +222,17 @@ static void mtk8250_shutdown(struct uart_port *port)
 
 static void mtk8250_disable_intrs(struct uart_8250_port *up, int mask)
 {
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&up->port.lock);
+
 	serial_out(up, UART_IER, serial_in(up, UART_IER) & (~mask));
 }
 
 static void mtk8250_enable_intrs(struct uart_8250_port *up, int mask)
 {
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&up->port.lock);
+
 	serial_out(up, UART_IER, serial_in(up, UART_IER) | mask);
 }
 
@@ -235,6 +241,9 @@ static void mtk8250_set_flow_ctrl(struct uart_8250_port *up, int mode)
 	struct uart_port *port = &up->port;
 	int lcr = serial_in(up, UART_LCR);
 
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&port->lock);
+
 	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_B);
 	serial_out(up, MTK_UART_EFR, UART_EFR_ECB);
 	serial_out(up, UART_LCR, lcr);
diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index 3225c95fde1d..0498b9b0e4e9 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -533,6 +533,10 @@ static void omap_8250_pm(struct uart_port *port, unsigned int state,
 	u8 efr;
 
 	pm_runtime_get_sync(port->dev);
+
+	/* Synchronize UART_IER access against the console. */
+	spin_lock_irq(&port->lock);
+
 	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_B);
 	efr = serial_in(up, UART_EFR);
 	serial_out(up, UART_EFR, efr | UART_EFR_ECB);
@@ -543,6 +547,8 @@ static void omap_8250_pm(struct uart_port *port, unsigned int state,
 	serial_out(up, UART_EFR, efr);
 	serial_out(up, UART_LCR, 0);
 
+	spin_unlock_irq(&port->lock);
+
 	pm_runtime_mark_last_busy(port->dev);
 	pm_runtime_put_autosuspend(port->dev);
 }
@@ -760,8 +766,11 @@ static void omap_8250_shutdown(struct uart_port *port)
 	if (priv->habit & UART_HAS_EFR2)
 		serial_out(up, UART_OMAP_EFR2, 0x0);
 
+	/* Synchronize UART_IER access against the console. */
+	spin_lock_irq(&port->lock);
 	up->ier = 0;
 	serial_out(up, UART_IER, 0);
+	spin_unlock_irq(&port->lock);
 	disable_irq_nosync(up->port.irq);
 	dev_pm_clear_wake_irq(port->dev);
 
@@ -803,6 +812,7 @@ static void omap_8250_unthrottle(struct uart_port *port)
 
 	pm_runtime_get_sync(port->dev);
 
+	/* Synchronize UART_IER access against the console. */
 	spin_lock_irqsave(&port->lock, flags);
 	priv->throttled = false;
 	if (up->dma)
@@ -953,6 +963,7 @@ static void __dma_rx_complete(void *param)
 	struct dma_tx_state     state;
 	unsigned long flags;
 
+	/* Synchronize UART_IER access against the console. */
 	spin_lock_irqsave(&p->port.lock, flags);
 
 	/*
@@ -1227,6 +1238,9 @@ static u16 omap_8250_handle_rx_dma(struct uart_8250_port *up, u8 iir, u16 status
 static void am654_8250_handle_rx_dma(struct uart_8250_port *up, u8 iir,
 				     u16 status)
 {
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&up->port.lock);
+
 	/*
 	 * Queue a new transfer if FIFO has data.
 	 */
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index b3971302d8e5..4b7bbd8b3305 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -539,6 +539,9 @@ EXPORT_SYMBOL_GPL(serial8250_rpm_put);
  */
 static int serial8250_em485_init(struct uart_8250_port *p)
 {
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&p->port.lock);
+
 	if (p->em485)
 		goto deassert_rts;
 
@@ -676,6 +679,8 @@ static void serial8250_set_sleep(struct uart_8250_port *p, int sleep)
 	serial8250_rpm_get(p);
 
 	if (p->capabilities & UART_CAP_SLEEP) {
+		/* Synchronize UART_IER access against the console. */
+		spin_lock_irq(&p->port.lock);
 		if (p->capabilities & UART_CAP_EFR) {
 			lcr = serial_in(p, UART_LCR);
 			efr = serial_in(p, UART_EFR);
@@ -689,6 +694,7 @@ static void serial8250_set_sleep(struct uart_8250_port *p, int sleep)
 			serial_out(p, UART_EFR, efr);
 			serial_out(p, UART_LCR, lcr);
 		}
+		spin_unlock_irq(&p->port.lock);
 	}
 
 	serial8250_rpm_put(p);
@@ -696,6 +702,9 @@ static void serial8250_set_sleep(struct uart_8250_port *p, int sleep)
 
 static 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);
+
 	if (up->capabilities & UART_CAP_UUE)
 		serial_out(up, UART_IER, UART_IER_UUE);
 	else
@@ -968,6 +977,9 @@ static void autoconfig_16550a(struct uart_8250_port *up)
 	unsigned char status1, status2;
 	unsigned int iersave;
 
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&up->port.lock);
+
 	up->port.type = PORT_16550A;
 	up->capabilities |= UART_CAP_FIFO;
 
@@ -1151,6 +1163,8 @@ static void autoconfig(struct uart_8250_port *up)
 	/*
 	 * We really do need global IRQs disabled here - we're going to
 	 * be frobbing the chips IRQ enable register to see if it exists.
+	 *
+	 * Synchronize UART_IER access against the console.
 	 */
 	spin_lock_irqsave(&port->lock, flags);
 
@@ -1323,7 +1337,10 @@ static void autoconfig_irq(struct uart_8250_port *up)
 	/* forget possible initially masked and pending IRQ */
 	probe_irq_off(probe_irq_on());
 	save_mcr = serial8250_in_MCR(up);
+	/* Synchronize UART_IER access against the console. */
+	spin_lock_irq(&port->lock);
 	save_ier = serial_in(up, UART_IER);
+	spin_unlock_irq(&port->lock);
 	serial8250_out_MCR(up, UART_MCR_OUT1 | UART_MCR_OUT2);
 
 	irqs = probe_irq_on();
@@ -1335,7 +1352,10 @@ static void autoconfig_irq(struct uart_8250_port *up)
 		serial8250_out_MCR(up,
 			UART_MCR_DTR | UART_MCR_RTS | UART_MCR_OUT2);
 	}
+	/* Synchronize UART_IER access against the console. */
+	spin_lock_irq(&port->lock);
 	serial_out(up, UART_IER, UART_IER_ALL_INTR);
+	spin_unlock_irq(&port->lock);
 	serial_in(up, UART_LSR);
 	serial_in(up, UART_RX);
 	serial_in(up, UART_IIR);
@@ -1345,7 +1365,10 @@ static void autoconfig_irq(struct uart_8250_port *up)
 	irq = probe_irq_off(irqs);
 
 	serial8250_out_MCR(up, save_mcr);
+	/* Synchronize UART_IER access against the console. */
+	spin_lock_irq(&port->lock);
 	serial_out(up, UART_IER, save_ier);
+	spin_unlock_irq(&port->lock);
 
 	if (port->flags & UPF_FOURPORT)
 		outb_p(save_ICP, ICP);
@@ -1360,6 +1383,9 @@ static void serial8250_stop_rx(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&port->lock);
+
 	serial8250_rpm_get(up);
 
 	up->ier &= ~(UART_IER_RLSI | UART_IER_RDI);
@@ -1379,6 +1405,9 @@ void serial8250_em485_stop_tx(struct uart_8250_port *p)
 {
 	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
@@ -1428,6 +1457,9 @@ static void __stop_tx_rs485(struct uart_8250_port *p, u64 stop_delay)
 {
 	struct uart_8250_em485 *em485 = p->em485;
 
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&p->port.lock);
+
 	stop_delay += (u64)p->port.rs485.delay_rts_after_send * NSEC_PER_MSEC;
 
 	/*
@@ -1607,6 +1639,9 @@ static void serial8250_start_tx(struct uart_port *port)
 	struct uart_8250_port *up = up_to_u8250p(port);
 	struct uart_8250_em485 *em485 = up->em485;
 
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&port->lock);
+
 	if (!port->x_char && uart_circ_empty(&port->state->xmit))
 		return;
 
@@ -1634,6 +1669,9 @@ static void serial8250_disable_ms(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&port->lock);
+
 	/* no MSR capabilities */
 	if (up->bugs & UART_BUG_NOMSR)
 		return;
@@ -1648,6 +1686,9 @@ static void serial8250_enable_ms(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 
+	/* Port locked to synchronize UART_IER access against the console. */
+	lockdep_assert_held_once(&port->lock);
+
 	/* no MSR capabilities */
 	if (up->bugs & UART_BUG_NOMSR)
 		return;
@@ -2104,6 +2145,14 @@ static void serial8250_put_poll_char(struct uart_port *port,
 	unsigned int ier;
 	struct uart_8250_port *up = up_to_u8250p(port);
 
+	/*
+	 * Normally the port is locked to synchronize UART_IER access
+	 * against the console. However, this function is only used by
+	 * KDB/KGDB, where it may not be possible to acquire the port
+	 * lock because all other CPUs are quiesced. The quiescence
+	 * should allow safe lockless usage here.
+	 */
+
 	serial8250_rpm_get(up);
 	/*
 	 *	First save the IER then disable the interrupts
@@ -2439,6 +2488,8 @@ void serial8250_do_shutdown(struct uart_port *port)
 	serial8250_rpm_get(up);
 	/*
 	 * Disable interrupts from this port
+	 *
+	 * Synchronize UART_IER access against the console.
 	 */
 	spin_lock_irqsave(&port->lock, flags);
 	up->ier = 0;
@@ -2738,6 +2789,8 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
 	/*
 	 * Ok, we're now changing the port state.  Do it with
 	 * interrupts disabled.
+	 *
+	 * Synchronize UART_IER access against the console.
 	 */
 	serial8250_rpm_get(up);
 	spin_lock_irqsave(&port->lock, flags);
-- 
2.30.2