[PATCH v7 2/4] serial: 8250: avoid potential PSLVERR issue

Yunhui Cui
2025-05-28T06:26:28Z

When the PSLVERR_RESP_EN parameter is set to 1, reading UART_RX while
the FIFO is enabled and UART_LSR_DR is not set will generate a PSLVERR
error.

Failure to check the UART_LSR_DR before reading UART_RX, or the non-
atomic nature of clearing the FIFO and reading UART_RX, poses
potential risks that could lead to PSLVERR.

PSLVERR is addressed through two methods. One is to introduce
serial8250_discard_data() to check whether UART_LSR_DR is set before
reading UART_RX, thus solving the PSLVERR issue when the FIFO is
enabled. The other is to place FIFO clearing and reading of UART_RX
under port->lock.

Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
---
drivers/tty/serial/8250/8250.h | 13 +++++++++++++
drivers/tty/serial/8250/8250_port.c | 26 +++++++++++++++-----------
2 files changed, 28 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index 18530c31a5981..b3fb8a550db35 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -162,6 +162,19 @@ static inline u16 serial_lsr_in(struct uart_8250_port *up)
return lsr;
}

+/*
+ * To avoid PSLVERR, check UART_LSR_DR in UART_LSR before
+ * reading UART_RX.
+ */
+static inline void serial8250_discard_data(struct uart_8250_port *up)
+{
+ u16 lsr;
+
+ lsr = serial_in(up, UART_LSR);
+ if (lsr & UART_LSR_DR)
+ serial_in(up, UART_RX);
+}
+
/*
* For the 16C950
*/
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 07fe818dffa34..0560df9b064f9 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -764,8 +764,6 @@ static void disable_rsa(struct uart_8250_port *up)

if (up->port.type == PORT_RSA &&
up->port.uartclk == SERIAL_RSA_BAUD_BASE * 16) {
- uart_port_lock_irq(&up->port);
-
mode = serial_in(up, UART_RSA_MSR);
result = !(mode & UART_RSA_MSR_FIFO);

@@ -777,7 +775,6 @@ static void disable_rsa(struct uart_8250_port *up)

if (result)
up->port.uartclk = SERIAL_RSA_BAUD_BASE_LO * 16;
- uart_port_unlock_irq(&up->port);
}
}
#endif /* CONFIG_SERIAL_8250_RSA */
@@ -1353,9 +1350,8 @@ static void autoconfig_irq(struct uart_8250_port *up)
/* Synchronize UART_IER access against the console. */
uart_port_lock_irq(port);
serial_out(up, UART_IER, UART_IER_ALL_INTR);
+ serial8250_discard_data(up);
uart_port_unlock_irq(port);
- serial_in(up, UART_LSR);
- serial_in(up, UART_RX);
serial_in(up, UART_IIR);
serial_in(up, UART_MSR);
serial_out(up, UART_TX, 0xFF);
@@ -2260,13 +2256,20 @@ 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);

/*
- * Clear the interrupt registers.
+ * Read UART_RX to clear interrupts (e.g., Character Timeout).
+ * To prevent PSLVERR, we can either disable the FIFO before reading
+ * UART_RX or read UART_RX only when UART_LSR_DR is set while the FIFO
+ * remains enabled. If using the latter approach to avoid PSLVERR, it
+ * creates a contradiction with the interrupt-clearing (see the
+ * rx_timeout handling in dw8250_handle_irq()).
*/
serial_port_in(port, UART_LSR);
serial_port_in(port, UART_RX);
+ uart_port_unlock_irqrestore(port, flags);
serial_port_in(port, UART_IIR);
serial_port_in(port, UART_MSR);

@@ -2423,15 +2426,13 @@ int serial8250_do_startup(struct uart_port *port)
}
}

- uart_port_unlock_irqrestore(port, flags);
-
/*
* Clear the interrupt registers again for luck, and clear the
* 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);
+ serial8250_discard_data(up);
+ uart_port_unlock_irqrestore(port, flags);
serial_port_in(port, UART_IIR);
serial_port_in(port, UART_MSR);
up->lsr_saved_flags = 0;
@@ -2513,7 +2514,6 @@ void serial8250_do_shutdown(struct uart_port *port)
port->mctrl &= ~TIOCM_OUT2;

serial8250_set_mctrl(port, port->mctrl);
- uart_port_unlock_irqrestore(port, flags);

/*
* Disable break condition and FIFOs
@@ -2532,8 +2532,12 @@ void serial8250_do_shutdown(struct uart_port *port)
/*
* Read data port to reset things, and then unlink from
* the IRQ chain.
+ *
+ * Since reading UART_RX clears interrupts, doing so with
+ * FIFO disabled won't trigger PSLVERR.
*/
serial_port_in(port, UART_RX);
+ uart_port_unlock_irqrestore(port, flags);
serial8250_rpm_put(up);

up->ops->release_irq(up);
--
2.39.5