serial: 8250: fix panic due to PSLVERR

commit 7f8fdd4dbffc05982b96caf586f77a014b2a9353 upstream.

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 serial_port_in(port, UART_RX).
This satisfies the PSLVERR trigger condition.

When 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) in dw8250_check_lcr(), 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: c49436b ("serial: 8250_dw: Improve unwritable LCR workaround")
Link: https://lore.kernel.org/all/[email protected]/T/
Signed-off-by: Yunhui Cui <[email protected]>
Reviewed-by: John Ogness <[email protected]>
Cc: stable <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Applied fix to serial8250_do_startup() instead of serial8250_initialize() ]
Signed-off-by: Sasha Levin <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
(cherry picked from commit cb7b3633ed749db8e56f475f43c960652cbd6882)

Author: cuiyunhui

drivers/tty/serial/8250/8250_port.c

@@ -2375,9 +2375,8 @@ int serial8250_do_startup(struct uart_port *port)
 	/*
 	 * Now, initialize the UART
 	 */
-	serial_port_out(port, UART_LCR, UART_LCR_WLEN8);
-
 	spin_lock_irqsave(&port->lock, 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;