[PATCH] [RFC] serial: stm32: Move stm32_usart_transmit_chars() to interrupt thread

[Marek Vasut]

Avoid locking in hard interrupt context, move the stm32_usart_transmit_chars()
into the threaded IRQ handler. This fixes the following splat with preempt-rt:

 BUG: scheduling while atomic: (mount)/1289/0x00010001
 Modules linked in:
 Preemption disabled at:
 [<c0119127>] irq_enter_rcu+0xb/0x42
 CPU: 0 PID: 1289 Comm: (mount) Not tainted 6.1.0-rc7-rt5-stable-standard-00006-gd70aeccb9f0f #17
 Hardware name: STM32 (Device Tree Support)
  unwind_backtrace from show_stack+0xb/0xc
  show_stack from dump_stack_lvl+0x2b/0x34
  dump_stack_lvl from __schedule_bug+0x53/0x80
  __schedule_bug from __schedule+0x47/0x404
  __schedule from schedule_rtlock+0x15/0x34
  schedule_rtlock from rtlock_slowlock_locked+0x1d7/0x57e
  rtlock_slowlock_locked from rt_spin_lock+0x29/0x3c
  rt_spin_lock from stm32_usart_interrupt+0xa9/0x110
  stm32_usart_interrupt from __handle_irq_event_percpu+0x73/0x14e
  __handle_irq_event_percpu from handle_irq_event_percpu+0x9/0x22
  handle_irq_event_percpu from handle_irq_event+0x53/0x76
  handle_irq_event from handle_fasteoi_irq+0x65/0xa8
  handle_fasteoi_irq from handle_irq_desc+0xf/0x18
  handle_irq_desc from gic_handle_irq+0x45/0x54
  gic_handle_irq from generic_handle_arch_irq+0x19/0x2c
  generic_handle_arch_irq from call_with_stack+0xd/0x10

Signed-off-by: Marek Vasut <marex at denx.de>
---
Cc: Alexandre Torgue <alexandre.torgue at foss.st.com>
Cc: Erwan Le Ray <erwan.leray at foss.st.com>
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: Jean Philippe Romain <jean-philippe.romain at foss.st.com>
Cc: Jiri Slaby <jirislaby at kernel.org>
Cc: Maxime Coquelin <mcoquelin.stm32 at gmail.com>
Cc: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Valentin Caron <valentin.caron at foss.st.com>
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-stm32 at st-md-mailman.stormreply.com
To: linux-serial at vger.kernel.org
---
 drivers/tty/serial/stm32-usart.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index a1490033aa164..56357a7962edc 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -791,11 +791,8 @@ static irqreturn_t stm32_usart_interrupt(int irq, void *ptr)
 		}
 	}
 
-	if ((sr & USART_SR_TXE) && !(stm32_port->tx_ch)) {
-		spin_lock(&port->lock);
-		stm32_usart_transmit_chars(port);
-		spin_unlock(&port->lock);
-	}
+	if ((sr & USART_SR_TXE) && !(stm32_port->tx_ch))
+		return IRQ_WAKE_THREAD;
 
 	if (stm32_usart_rx_dma_enabled(port))
 		return IRQ_WAKE_THREAD;
@@ -808,8 +805,18 @@ static irqreturn_t stm32_usart_threaded_interrupt(int irq, void *ptr)
 	struct uart_port *port = ptr;
 	struct tty_port *tport = &port->state->port;
 	struct stm32_port *stm32_port = to_stm32_port(port);
-	unsigned int size;
+	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
 	unsigned long flags;
+	unsigned int size;
+	u32 sr;
+
+	sr = readl_relaxed(port->membase + ofs->isr);
+
+	if ((sr & USART_SR_TXE) && !(stm32_port->tx_ch)) {
+		spin_lock_irqsave(&port->lock, flags);
+		stm32_usart_transmit_chars(port);
+		spin_unlock_irqrestore(&port->lock, flags);
+	}
 
 	/* Receiver timeout irq for DMA RX */
 	if (!stm32_port->throttled) {
-- 
2.35.1