[PATCH v2] serial: 8250: fix panic due to PSLVERR

Yunhui Cui
2025-04-08T06:12:12Z

When the PSLVERR_RESP_EN parameter is set to 1, the device generates
an error response if an attempt is made to read an empty RBR (Receive
Buffer Register) while the FIFO is enabled.

In serial8250_do_startup(), calling serial_port_out(port, UART_LCR,
UART_LCR_WLEN8) triggers dw8250_check_lcr(), which invokes
dw8250_force_idle() and serial8250_clear_and_reinit_fifos(). The latter
function enables the FIFO via serial_out(p, UART_FCR, p->fcr).
Execution proceeds to the dont_test_tx_en label:
...
serial_port_in(port, UART_RX);
This satisfies the PSLVERR trigger condition.

Because another CPU(e.g., using printk()) is accessing the UART (UART
is busy), the current CPU fails the check (value & ~UART_LCR_SPAR) ==
(lcr & ~UART_LCR_SPAR), causing it to enter dw8250_force_idle().

To resolve this issue, relevant serial_port_out() operations should be
placed in a critical section, and UART_RX data should only be read
when the UART_LSR DR bit is set.

Panic backtrace:
[ 0.442336] Oops - unknown exception [#1]
[ 0.442343] epc : dw8250_serial_in32+0x1e/0x4a
[ 0.442351] ra : serial8250_do_startup+0x2c8/0x88e
...
[ 0.442416] console_on_rootfs+0x26/0x70

Fixes: c49436b657d0 ("serial: 8250_dw: Improve unwritable LCR workaround")
Link: https://lore.kernel.org/all/84cydt5peu.fsf@jogness.linutronix.de/T/
Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
---
drivers/tty/serial/8250/8250_port.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 3f256e96c722..3df358008489 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2264,13 +2264,16 @@ int serial8250_do_startup(struct uart_port *port)
* Clear the FIFO buffers and disable them.
* (they will be reenabled in set_termios())
*/
+ uart_port_lock_irqsave(port, &flags);
serial8250_clear_fifos(up);
+ uart_port_unlock_irqrestore(port, flags);

/*
* Clear the interrupt registers.
*/
- serial_port_in(port, UART_LSR);
- serial_port_in(port, UART_RX);
+ lsr = serial_port_in(port, UART_LSR);
+ if (lsr & UART_LSR_DR)
+ serial_port_in(port, UART_RX);
serial_port_in(port, UART_IIR);
serial_port_in(port, UART_MSR);

@@ -2380,9 +2383,10 @@ int serial8250_do_startup(struct uart_port *port)
/*
* Now, initialize the UART
*/
- serial_port_out(port, UART_LCR, UART_LCR_WLEN8);

uart_port_lock_irqsave(port, &flags);
+ serial_port_out(port, UART_LCR, UART_LCR_WLEN8);
+
if (up->port.flags & UPF_FOURPORT) {
if (!up->port.irq)
up->port.mctrl |= TIOCM_OUT1;
@@ -2435,8 +2439,9 @@ int serial8250_do_startup(struct uart_port *port)
* saved flags to avoid getting false values from polling
* routines or the previous session.
*/
- serial_port_in(port, UART_LSR);
- serial_port_in(port, UART_RX);
+ lsr = serial_port_in(port, UART_LSR);
+ if (lsr & UART_LSR_DR)
+ serial_port_in(port, UART_RX);
serial_port_in(port, UART_IIR);
serial_port_in(port, UART_MSR);
up->lsr_saved_flags = 0;
@@ -2492,6 +2497,7 @@ void serial8250_do_shutdown(struct uart_port *port)
{
struct uart_8250_port *up = up_to_u8250p(port);
unsigned long flags;
+ u16 lsr;

serial8250_rpm_get(up);
/*
@@ -2538,7 +2544,9 @@ void serial8250_do_shutdown(struct uart_port *port)
* Read data port to reset things, and then unlink from
* the IRQ chain.
*/
- serial_port_in(port, UART_RX);
+ lsr = serial_port_in(port, UART_LSR);
+ if (lsr & UART_LSR_DR)
+ serial_port_in(port, UART_RX);
serial8250_rpm_put(up);

up->ops->release_irq(up);
--
2.39.2