serial: Drop timeout from uart_port

Since commit 31f6bd7f ("serial: Store character timing information
to uart_port"), per frame timing information is available on uart_port.
Uart port's timeout can be derived from frame_time by multiplying with
fifosize.

Most callers of uart_poll_timeout are not made under port's lock. To be
on the safe side, make sure frame_time is only accessed once. As
fifo_size is effectively a constant, it shouldn't cause any issues.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220613113905.22962-1-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/serial/driver.rst

@@ -422,8 +422,9 @@ Other functions
 ---------------
 
 uart_update_timeout(port,cflag,baud)
-	Update the FIFO drain timeout, port->timeout, according to the
-	number of bits, parity, stop bits and baud rate.
+	Update the frame timing information according to the number of bits,
+	parity, stop bits and baud rate. The FIFO drain timeout is derived
+	from the frame timing information.
 
 	Locking: caller is expected to take port->lock
 

drivers/tty/serial/mux.c

@@ -481,12 +481,6 @@ static int __init mux_probe(struct parisc_device *dev)
 		port->line	= port_cnt;
 		port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_MUX_CONSOLE);
 
-		/* The port->timeout needs to match what is present in
-		 * uart_wait_until_sent in serial_core.c.  Otherwise
-		 * the time spent in msleep_interruptable will be very
-		 * long, causing the appearance of a console hang.
-		 */
-		port->timeout   = HZ / 50;
 		spin_lock_init(&port->lock);
 
 		status = uart_add_one_port(&mux_driver, port);

drivers/tty/serial/serial_core.c

@@ -327,13 +327,14 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state)
 }
 
 /**
- *	uart_update_timeout - update per-port FIFO timeout.
+ *	uart_update_timeout - update per-port frame timing information.
  *	@port:  uart_port structure describing the port
  *	@cflag: termios cflag value
  *	@baud:  speed of the port
  *
- *	Set the port FIFO timeout value.  The @cflag value should
- *	reflect the actual hardware settings.
+ *	Set the port frame timing information from which the FIFO timeout
+ *	value is derived. The @cflag value should reflect the actual hardware
+ *	settings.
  */
 void
 uart_update_timeout(struct uart_port *port, unsigned int cflag,
@@ -343,13 +344,6 @@ uart_update_timeout(struct uart_port *port, unsigned int cflag,
 	u64 frame_time;
 
 	frame_time = (u64)size * NSEC_PER_SEC;
-	size *= port->fifosize;
-
-	/*
-	 * Figure the timeout to send the above number of bits.
-	 * Add .02 seconds of slop
-	 */
-	port->timeout = (HZ * size) / baud + HZ/50;
 	port->frame_time = DIV64_U64_ROUND_UP(frame_time, baud);
 }
 EXPORT_SYMBOL(uart_update_timeout);
@@ -1698,7 +1692,7 @@ static void uart_wait_until_sent(struct tty_struct *tty, int timeout)
 {
 	struct uart_state *state = tty->driver_data;
 	struct uart_port *port;
-	unsigned long char_time, expire;
+	unsigned long char_time, expire, fifo_timeout;
 
 	port = uart_port_ref(state);
 	if (!port)
@@ -1728,12 +1722,13 @@ static void uart_wait_until_sent(struct tty_struct *tty, int timeout)
 		 * amount of time to send the entire FIFO, it probably won't
 		 * ever clear.  This assumes the UART isn't doing flow
 		 * control, which is currently the case.  Hence, if it ever
-		 * takes longer than port->timeout, this is probably due to a
+		 * takes longer than FIFO timeout, this is probably due to a
 		 * UART bug of some kind.  So, we clamp the timeout parameter at
-		 * 2*port->timeout.
+		 * 2 * FIFO timeout.
 		 */
-		if (timeout == 0 || timeout > 2 * port->timeout)
-			timeout = 2 * port->timeout;
+		fifo_timeout = uart_fifo_timeout(port);
+		if (timeout == 0 || timeout > 2 * fifo_timeout)
+			timeout = 2 * fifo_timeout;
 	}
 
 	expire = jiffies + timeout;

include/linux/serial_core.h

@@ -232,7 +232,6 @@ struct uart_port {
 
 	int			hw_stopped;		/* sw-assisted CTS flow state */
 	unsigned int		mctrl;			/* current modem ctrl settings */
-	unsigned int		timeout;		/* character-based timeout */
 	unsigned int		frame_time;		/* frame timing in ns */
 	unsigned int		type;			/* port type */
 	const struct uart_ops	*ops;
@@ -335,10 +334,23 @@ unsigned int uart_get_baud_rate(struct uart_port *port, struct ktermios *termios
 				unsigned int max);
 unsigned int uart_get_divisor(struct uart_port *port, unsigned int baud);
 
+/*
+ * Calculates FIFO drain time.
+ */
+static inline unsigned long uart_fifo_timeout(struct uart_port *port)
+{
+	u64 fifo_timeout = (u64)READ_ONCE(port->frame_time) * port->fifosize;
+
+	/* Add .02 seconds of slop */
+	fifo_timeout += 20 * NSEC_PER_MSEC;
+
+	return max(nsecs_to_jiffies(fifo_timeout), 1UL);
+}
+
 /* Base timer interval for polling */
 static inline int uart_poll_timeout(struct uart_port *port)
 {
-	int timeout = port->timeout;
+	int timeout = uart_fifo_timeout(port);
 
 	return timeout > 6 ? (timeout / 2 - 2) : 1;
 }