[PATCH v1] serial: 8250: clear stuck RX-timeout interrupt on LPC32xx (PORT_LPC3220)

[Ryan Wilbur <rwilbur633@gmail.com>]

The LPC32xx UART 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().

Fix this by doing one throwaway RHR read to clear the timeout. It is gated
on PORT_LPC3220 and only fires when the FIFO is empty (LSR.DR clear), so
no real received data is ever discarded, and it is a no-op on healthy
UARTs which never report a timeout with DR==0.

This is the same class of bug already worked around for other 8250 cores;
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
Signed-off-by: Ryan Wilbur <rwilbur633@gmail.com>
---
 drivers/tty/serial/8250/8250_port.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 8c241ec7f4f2..20014f54637b 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1803,6 +1803,20 @@ void serial8250_handle_irq_locked(struct uart_port *port, unsigned int iir)
 	if (!(status & UART_LSR_DR) && (status & UART_LSR_FIFOE))
 		serial8250_clear_and_reinit_fifos(up);
 
+	/*
+	 * On PORT_LPC3220 the UART can raise an RX character-timeout
+	 * interrupt with an empty RX FIFO (IIR reports RX_TIMEOUT but
+	 * LSR.DR is clear). The timeout is only cleared by reading RHR,
+	 * but the RX path below is skipped when the FIFO is empty, so
+	 * nothing clears it. IRQ then re-fires immediately and livelocks
+	 * this single-core. Do one throwaway RHR read to clear it.
+	 * Same bug worked around for other 8250 cores (8250_dw, 8250_omap, 8250_bcm7271).
+	 */
+	if (port->type == PORT_LPC3220 &&
+	    (iir & UART_IIR_RX_TIMEOUT) == UART_IIR_RX_TIMEOUT &&
+	    !(status & UART_LSR_DR))
+		serial_in(up, UART_RX);
+
 	/*
 	 * If port is stopped and there are no error conditions in the
 	 * FIFO, then don't drain the FIFO, as this may lead to TTY buffer

base-commit: da7b5fd4e17f8e44c5590f2d603c01d499f056e6
-- 
2.25.1