serial: stm32: re-introduce an irq flag condition in usart_receive_chars

Re-introduce an irq flag condition in usart_receive_chars.
This condition has been deleted by commit 75f4e83 ("serial: do not
restore interrupt state in sysrq helper").
This code was present to handle threaded case, and has been removed
because it is no more needed in this case. Nevertheless an irq safe lock
is still needed in some cases, when DMA should be stopped to receive errors
or breaks in PIO mode.
This patch is a precursor to the complete rework or stm32 serial driver
DMA implementation.

Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20211020150332.10214-2-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -209,19 +209,22 @@ static unsigned long stm32_usart_get_char(struct uart_port *port, u32 *sr,
 	return c;
 }
 
-static void stm32_usart_receive_chars(struct uart_port *port, bool threaded)
+static void stm32_usart_receive_chars(struct uart_port *port, bool irqflag)
 {
 	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;
 
-	spin_lock(&port->lock);
+	if (irqflag)
+		spin_lock_irqsave(&port->lock, flags);
+	else
+		spin_lock(&port->lock);
 
 	while (stm32_usart_pending_rx(port, &sr, &stm32_port->last_res,
-				      threaded)) {
+				      irqflag)) {
 		sr |= USART_SR_DUMMY_RX;
 		flag = TTY_NORMAL;
 
@@ -275,7 +278,10 @@ static void stm32_usart_receive_chars(struct uart_port *port, bool threaded)
 		uart_insert_char(port, sr, USART_SR_ORE, c, flag);
 	}
 
-	uart_unlock_and_check_sysrq(port);
+	if (irqflag)
+		uart_unlock_and_check_sysrq_irqrestore(port, irqflag);
+	else
+		uart_unlock_and_check_sysrq(port);
 
 	tty_flip_buffer_push(tport);
 }
@@ -496,10 +502,9 @@ static irqreturn_t stm32_usart_interrupt(int irq, void *ptr)
 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);
 
-	if (stm32_port->rx_ch)
-		stm32_usart_receive_chars(port, true);
+	/* Receiver timeout irq for DMA RX */
+	stm32_usart_receive_chars(port, false);
 
 	return IRQ_HANDLED;
 }