[PATCH v3] serial: 8250_of: clear stuck empty-FIFO RX-timeout on LPC32xx

[Ryan Wilbur <rwilbur633@gmail.com>]

The NXP LPC32xx UART (PORT_LPC3220) can latch an RX character-timeout
interrupt while the RX FIFO is empty: IIR reports UART_IIR_RX_TIMEOUT
(0x0c) but LSR.DR is clear. A character timeout is only cleared by
reading RHR, but serial8250_rx_chars() reads RHR only when LSR.DR is
set, so nothing ever clears the condition. The interrupt is
level-triggered and re-fires immediately, so on a single-core ARM926
the resulting interrupt storm livelocks the CPU.

It is reproducible when userspace repeatedly opens the front-panel port
(ttyS1): serial8250_do_set_termios() re-enables interrupts on unlock and
the handler then spins forever with iir=0xcc lsr=0x60 ier=0x05, tripping
the soft-lockup detector in serial8250_handle_irq_locked().

LPC32xx has no dedicated 8250 glue driver, it's driven by the generic
8250_of. Add a hardware specific handle_irq for PORT_LPC3220, wired up
in of_platform_serial_setup() the same way fsl8250_handle_irq is
installed. The handler follows dw8250_handle_irq(): on an RX timeout
with an empty FIFO (LSR.DR and LSR.BI clear) it does one throwaway RHR
read to clear the condition, then calls serial8250_handle_irq_locked().
No real received data is ever discarded, and it is a no-op on healthy
UARTs which never report a timeout with DR clear.

This is the same class of bug already worked around in other 8250 drivers;
see commit 424d79183af0 ("serial: 8250_dw: Avoid "too much work" from bogus rx timeout interrupt")
which reports the identical iir=0xcc/lsr=0x60. See also
UART_RX_TIMEOUT_QUIRK in 8250_omap, and the note in 8250_bcm7271.

Cc: stable@vger.kernel.org
Assisted-by: Claude:Opus4.8
Signed-off-by: Ryan Wilbur <rwilbur633@gmail.com>
---
Changes in v3:
- Move the workaround out of the generic serial8250_handle_irq_locked()
  into an LPC32xx-specific ->handle_irq in 8250_of.c
- Skip the dummy read when LSR.BI is set, matching dw8250_handle_irq

Changes in v2:
- Drop the PORT_LPC3220 gate and handle the spurious RX timeout generically

v2: https://lore.kernel.org/linux-serial/20260720131123.29840-1-rwilbur633@gmail.com/
v1: https://lore.kernel.org/linux-serial/20260717123530.481021-1-rwilbur633@gmail.com/

 drivers/tty/serial/8250/8250_of.c | 37 +++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_of.c b/drivers/tty/serial/8250/8250_of.c
index f0537fb6ef4f..93db28a48dea 100644
--- a/drivers/tty/serial/8250/8250_of.c
+++ b/drivers/tty/serial/8250/8250_of.c
@@ -82,6 +82,40 @@ static int of_platform_serial_clk_notifier_cb(struct notifier_block *nb, unsigne
 	return NOTIFY_DONE;
 }
 
+static int lpc32xx_handle_irq(struct uart_port *port)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+	unsigned int iir;
+	u16 status;
+
+	guard(serial8250_rpm)(up);
+
+	iir = serial_port_in(port, UART_IIR);
+	if (iir & UART_IIR_NO_INT)
+		return 0;
+
+	guard(uart_port_lock_check_sysrq_irqsave)(port);
+
+	/*
+	 * The LPC32xx UART can assert an RX character-timeout interrupt while
+	 * the RX FIFO is empty: IIR reports UART_IIR_RX_TIMEOUT but LSR.DR is
+	 * clear. The timeout is only cleared by reading RHR, but the core RX
+	 * path skips that read when the FIFO is empty, so the level-triggered
+	 * IRQ re-fires forever and livelocks this single-core SoC. Do one
+	 * throwaway RHR read to clear it; a healthy UART never reports a
+	 * timeout with DR/BI clear, so no received data is ever discarded.
+	 */
+	if ((iir & 0x3f) == UART_IIR_RX_TIMEOUT) {
+		status = serial_lsr_in(up);
+		if (!(status & (UART_LSR_DR | UART_LSR_BI)))
+			serial_port_in(port, UART_RX);
+	}
+
+	serial8250_handle_irq_locked(port, iir);
+
+	return 1;
+}
+
 /*
  * Fill a struct uart_port for a given device node
  */
@@ -185,6 +219,9 @@ static int of_platform_serial_setup(struct platform_device *ofdev,
 	case PORT_NPCM:
 		ret = npcm_setup(port);
 		break;
+	case PORT_LPC3220:
+		port->handle_irq = lpc32xx_handle_irq;
+		break;
 	default:
 		/* Nothing to do */
 		ret = 0;

base-commit: da7b5fd4e17f8e44c5590f2d603c01d499f056e6
-- 
2.25.1