serial: 8250: fix possible ISR soft lockup

There are rare cases in which the host gets stuck in the ISR because it
is flooded with messages during the startup phase.

The reason for the soft lockup in the ISR is the missing FIFO error IRQ
(FIFOE) handling. Not handling it and reporting IRQ_HANDLED triggers
the IRQ immediately again.

Fix this by adding a check for the FIFOE status and clearing the FIFO
if no data is ready (DR).

This behavior was observed on an AM62L device which uses the OMAP 8250
driver. Fix it for all 8250 drivers, since the OMAP driver's special
IRQ setup handling may trigger this behavior more frequently, but it
is not ensured that other 8250 drivers aren't affected.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Link: https://patch.msgid.link/20260519-v7-1-topic-serial-8250-v1-1-56b04293a246@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index ef245114105bc2a352145a003907a98d89ca45d3..141229b9bc17e9dfdfc3cd39ceb4ce3dd92de80b 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1799,6 +1799,13 @@ void serial8250_handle_irq_locked(struct uart_port *port, unsigned int iir)
 
        status = serial_lsr_in(up);
 
+       /*
+        * Recover from no-data-ready and FIFO error condition to avoid getting
+        * stuck in the ISR.
+        */
+       if (!(status & UART_LSR_DR) && (status & UART_LSR_FIFOE))
+               serial8250_clear_and_reinit_fifos(up);
+
        /*
         * 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