[PATCH v8 4/4] serial: 8250_dw: fix PSLVERR on RX_TIMEOUT

[Yunhui Cui]

The DW UART may trigger the RX_TIMEOUT interrupt without data
present and remain stuck in this state indefinitely. The
dw8250_handle_irq() function detects this condition by checking
if the UART_LSR_DR bit is not set when RX_TIMEOUT occurs. When
detected, it performs a "dummy read" to recover the DW UART from
this state.

When the PSLVERR_RESP_EN parameter is set to 1, reading the UART_RX
while the FIFO is enabled and UART_LSR_DR is not set will generate a
PSLVERR error, which may lead to a system panic. There are two methods
to prevent PSLVERR: one is to check if UART_LSR_DR is set before reading
UART_RX when the FIFO is enabled, and the other is to read UART_RX when
the FIFO is disabled.

Given these two scenarios, the FIFO must be disabled before the
"dummy read" operation and re-enabled afterward to maintain normal
UART functionality.

Fixes: 424d79183af0 ("serial: 8250_dw: Avoid "too much work" from bogus rx timeout interrupt")
Signed-off-by: Yunhui Cui <cuiyunhui@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 drivers/tty/serial/8250/8250_dw.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 8b0018fadccea..686f9117a3339 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -301,9 +301,17 @@ static int dw8250_handle_irq(struct uart_port *p)
 		uart_port_lock_irqsave(p, &flags);
 		status = serial_lsr_in(up);
 
-		if (!(status & (UART_LSR_DR | UART_LSR_BI)))
+		if (!(status & (UART_LSR_DR | UART_LSR_BI))) {
+			/* To avoid PSLVERR, disable the FIFO first. */
+			if (up->fcr & UART_FCR_ENABLE_FIFO)
+				serial_out(up, UART_FCR, 0);
+
 			serial_port_in(p, UART_RX);
 
+			if (up->fcr & UART_FCR_ENABLE_FIFO)
+				serial_out(up, UART_FCR, up->fcr);
+		}
+
 		uart_port_unlock_irqrestore(p, flags);
 	}
 
-- 
2.39.5