5.4-stable review patch. If anyone has any objections, please let me know.
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From: Yunhui Cui cuiyunhui@bytedance.com
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: 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 Reviewed-by: John Ogness john.ogness@linutronix.de Cc: stable stable@kernel.org Link: https://lore.kernel.org/r/20250723023322.464-2-cuiyunhui@bytedance.com [ Applied fix to serial8250_do_startup() instead of serial8250_initialize() ] Signed-off-by: Sasha Levin sashal@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/serial/8250/8250_port.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
--- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2296,9 +2296,8 @@ int serial8250_do_startup(struct uart_po /* * 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;