Subject: FAILED: patch "[PATCH] serial: 8250_of: clear stuck empty-FIFO RX-timeout on LPC32xx" failed to apply to 6.12-stable tree
To: rwilbur633@gmail.com,gregkh@linuxfoundation.org,stable@kernel.org
Cc: <stable@vger.kernel.org>
From: <gregkh@linuxfoundation.org>
Date: Mon, 17 Aug 2026 14:33:05 +0200
Message-ID: <2026081705-tug-surrender-1607@gregkh>
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The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 1423415471274abda87024967d7fe2206ceee0ea
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026081705-tug-surrender-1607@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Possible dependencies:


thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 1423415471274abda87024967d7fe2206ceee0ea Mon Sep 17 00:00:00 2001
From: Ryan Wilbur <rwilbur633@gmail.com>
Date: Thu, 30 Jul 2026 16:39:20 -0300
Subject: [PATCH] serial: 8250_of: clear stuck empty-FIFO RX-timeout on LPC32xx

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 <stable@kernel.org>
Assisted-by: Claude:Opus4.8
Signed-off-by: Ryan Wilbur <rwilbur633@gmail.com>
Link: https://patch.msgid.link/20260730193920.28954-1-rwilbur633@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_of.c b/drivers/tty/serial/8250/8250_of.c
index 81644d40b09a..160f4c71de06 100644
--- a/drivers/tty/serial/8250/8250_of.c
+++ b/drivers/tty/serial/8250/8250_of.c
@@ -81,6 +81,40 @@ 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
  */
@@ -173,6 +207,9 @@ 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;
@@ -369,6 +406,7 @@ static struct platform_driver of_platform_serial_driver = {
 
 module_platform_driver(of_platform_serial_driver);
 
+MODULE_IMPORT_NS("SERIAL_8250");
 MODULE_AUTHOR("Arnd Bergmann <arnd@arndb.de>");
 MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("Serial Port driver for Open Firmware platform devices");