serial: stm32: fix a deadlock condition with wakeup event

Deadlock issue is seen when enabling CONFIG_PROVE_LOCKING=Y, and uart
console as wakeup source. Deadlock occurs when resuming from low power
mode if system is waked up via usart console.
The deadlock is triggered 100% when also disabling console suspend prior
to go to suspend.

Simplified call stack, deadlock condition:
- stm32_console_write <-- spin_lock already held
- print_circular_bug
- pm_wakeup_dev_event <-- triggers lockdep as seen above
- stm32_receive_chars
- stm32_interrupt <-- wakeup via uart console, takes the lock

So, revisit spin_lock in stm32-usart driver:
- there is no need to hold the lock to access ICR (atomic clear of status
  flags)
- only hold the lock inside stm32_receive_chars() routine (no need to
  call pm_wakeup_dev_event with lock held)
- keep stm32_transmit_chars() routine called with lock held

Fixes: 48a6092 ("serial: stm32-usart: Add STM32 USART Driver")
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20210304162308.8984-6-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -214,13 +214,18 @@ static void stm32_usart_receive_chars(struct uart_port *port, bool threaded)
 	struct tty_port *tport = &port->state->port;
 	struct stm32_port *stm32_port = to_stm32_port(port);
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
-	unsigned long c;
+	unsigned long c, flags;
 	u32 sr;
 	char flag;
 
 	if (irqd_is_wakeup_set(irq_get_irq_data(port->irq)))
 		pm_wakeup_event(tport->tty->dev, 0);
 
+	if (threaded)
+		spin_lock_irqsave(&port->lock, flags);
+	else
+		spin_lock(&port->lock);
+
 	while (stm32_usart_pending_rx(port, &sr, &stm32_port->last_res,
 				      threaded)) {
 		sr |= USART_SR_DUMMY_RX;
@@ -276,9 +281,12 @@ static void stm32_usart_receive_chars(struct uart_port *port, bool threaded)
 		uart_insert_char(port, sr, USART_SR_ORE, c, flag);
 	}
 
-	spin_unlock(&port->lock);
+	if (threaded)
+		spin_unlock_irqrestore(&port->lock, flags);
+	else
+		spin_unlock(&port->lock);
+
 	tty_flip_buffer_push(tport);
-	spin_lock(&port->lock);
 }
 
 static void stm32_usart_tx_dma_complete(void *arg)
@@ -459,8 +467,6 @@ static irqreturn_t stm32_usart_interrupt(int irq, void *ptr)
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
 	u32 sr;
 
-	spin_lock(&port->lock);
-
 	sr = readl_relaxed(port->membase + ofs->isr);
 
 	if ((sr & USART_SR_RTOF) && ofs->icr != UNDEF_REG)
@@ -474,10 +480,11 @@ static irqreturn_t stm32_usart_interrupt(int irq, void *ptr)
 	if ((sr & USART_SR_RXNE) && !(stm32_port->rx_ch))
 		stm32_usart_receive_chars(port, false);
 
-	if ((sr & USART_SR_TXE) && !(stm32_port->tx_ch))
+	if ((sr & USART_SR_TXE) && !(stm32_port->tx_ch)) {
+		spin_lock(&port->lock);
 		stm32_usart_transmit_chars(port);
-
-	spin_unlock(&port->lock);
+		spin_unlock(&port->lock);
+	}
 
 	if (stm32_port->rx_ch)
 		return IRQ_WAKE_THREAD;
@@ -490,13 +497,9 @@ static irqreturn_t stm32_usart_threaded_interrupt(int irq, void *ptr)
 	struct uart_port *port = ptr;
 	struct stm32_port *stm32_port = to_stm32_port(port);
 
-	spin_lock(&port->lock);
-
 	if (stm32_port->rx_ch)
 		stm32_usart_receive_chars(port, true);
 
-	spin_unlock(&port->lock);
-
 	return IRQ_HANDLED;
 }