[PATCH v8 2/4] serial: 8250: avoid potential PSLVERR issue

[Yunhui Cui <cuiyunhui@bytedance.com>]

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