serial: core: Use usec resolution for timeout

Signed-off-by: Lukas Wunner <lukas@wunner.de>
[p.rosenberger: port to 5.10]
Signed-off-by: Philipp Rosenberger <p.rosenberger@kunbus.com>

drivers/tty/serial/mux.c

@@ -486,7 +486,7 @@ static int __init mux_probe(struct parisc_device *dev)
 		 * the time spent in msleep_interruptable will be very
 		 * long, causing the appearance of a console hang.
 		 */
-		port->timeout   = HZ / 50;
+		port->timeout   = 20 * MSEC_PER_SEC;
 		spin_lock_init(&port->lock);
 
 		status = uart_add_one_port(&mux_driver, port);

drivers/tty/serial/serial_core.c

@@ -25,6 +25,7 @@
 #include <linux/delay.h>
 #include <linux/mutex.h>
 #include <linux/security.h>
+#include <linux/math64.h>
 
 #include <linux/irq.h>
 #include <linux/uaccess.h>
@@ -366,7 +367,7 @@ uart_update_timeout(struct uart_port *port, unsigned int cflag,
 	 * Figure the timeout to send the above number of bits.
 	 * Add .02 seconds of slop
 	 */
-	port->timeout = (HZ * bits) / baud + HZ/50;
+	port->timeout = div_u64(USEC_PER_SEC * bits, baud) + 20 * MSEC_PER_SEC;
 }
 
 EXPORT_SYMBOL(uart_update_timeout);
@@ -1596,7 +1597,8 @@ 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;
+	ktime_t expire;
 
 	port = uart_port_ref(state);
 	if (!port)
@@ -1607,18 +1609,19 @@ static void uart_wait_until_sent(struct tty_struct *tty, int timeout)
 		return;
 	}
 
+	/* tty layer only uses jiffies resolution, but serial core uses usec */
+	timeout = jiffies_to_usecs(timeout);
+
 	/*
 	 * Set the check interval to be 1/5 of the estimated time to
-	 * send a single character, and make it at least 1.  The check
-	 * interval should also be less than the timeout.
+	 * send a single character.  The check interval should also be
+	 * less than the timeout.
 	 *
 	 * Note: we have to use pretty tight timings here to satisfy
 	 * the NIST-PCTS.
 	 */
-	char_time = (port->timeout - HZ/50) / port->fifosize;
+	char_time = (port->timeout - 20 * MSEC_PER_SEC) / port->fifosize;
 	char_time = char_time / 5;
-	if (char_time == 0)
-		char_time = 1;
 	if (timeout && timeout < char_time)
 		char_time = timeout;
 
@@ -1634,21 +1637,21 @@ static void uart_wait_until_sent(struct tty_struct *tty, int timeout)
 	if (timeout == 0 || timeout > 2 * port->timeout)
 		timeout = 2 * port->timeout;
 
-	expire = jiffies + timeout;
+	expire = ktime_add_us(ktime_get(), timeout);
 
-	pr_debug("uart_wait_until_sent(%d), jiffies=%lu, expire=%lu...\n",
-		port->line, jiffies, expire);
+	pr_debug("uart_wait_until_sent(%d), now=%llu, expire=%llu...\n",
+		 port->line, ktime_to_us(ktime_get()), ktime_to_us(expire));
 
 	/*
 	 * Check whether the transmitter is empty every 'char_time'.
 	 * 'timeout' / 'expire' give us the maximum amount of time
 	 * we wait.
 	 */
 	while (!port->ops->tx_empty(port)) {
-		msleep_interruptible(jiffies_to_msecs(char_time));
+		usleep_range(char_time, char_time + 50);
 		if (signal_pending(current))
 			break;
-		if (time_after(jiffies, expire))
+		if (ktime_after(ktime_get(), expire))
 			break;
 	}
 	uart_port_deref(port);

include/linux/serial_core.h

@@ -231,7 +231,7 @@ 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		timeout;		/* timespan to transmit full FIFO (usec) */
 	unsigned int		type;			/* port type */
 	const struct uart_ops	*ops;
 	unsigned int		custom_divisor;
@@ -335,7 +335,7 @@ unsigned int uart_get_divisor(struct uart_port *port, unsigned int baud);
 /* Base timer interval for polling */
 static inline int uart_poll_timeout(struct uart_port *port)
 {
-	int timeout = port->timeout;
+	int timeout = usecs_to_jiffies(port->timeout);
 
 	return timeout > 6 ? (timeout / 2 - 2) : 1;
 }