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[v5,1/4] serial: 8250: fix panic due to PSLVERR

Message ID 20250506112321.61710-1-cuiyunhui@bytedance.com
State New
Headers show
Series [v5,1/4] serial: 8250: fix panic due to PSLVERR | expand

Commit Message

yunhui cui May 6, 2025, 11:23 a.m. UTC 
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().

Put serial_port_out(port, UART_LCR, UART_LCR_WLEN8) under the port->lock
to fix this issue.

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 | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
diff mbox series

Patch

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 3f256e96c722..a913135d5217 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2380,9 +2380,10 @@  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;