serial: Store character timing information to uart_port

Struct uart_port currently stores FIFO timeout. Having character timing
information readily available is useful. Even serial core itself
determines char_time from port->timeout using inverse calculation.

Store frame_time directly into uart_port. Character time is stored in
nanoseconds to have reasonable precision with high rates. To avoid
overflow, 64-bit math is necessary.

It might be possible to determine timeout from frame_time by
multiplying it with fifosize as needed but only part of the users seem
to be protected by a lock. Thus, this patch does not pursue storing
only frame_time in uart_port.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220425143410.12703-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -24,6 +24,7 @@
 #include <linux/sysrq.h>
 #include <linux/delay.h>
 #include <linux/mutex.h>
+#include <linux/math64.h>
 #include <linux/security.h>
 
 #include <linux/irq.h>
@@ -338,15 +339,18 @@ void
 uart_update_timeout(struct uart_port *port, unsigned int cflag,
 		    unsigned int baud)
 {
-	unsigned int size;
+	unsigned int size = tty_get_frame_size(cflag);
+	u64 frame_time;
 
-	size = tty_get_frame_size(cflag) * port->fifosize;
+	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);
 
@@ -1643,10 +1647,8 @@ static void uart_wait_until_sent(struct tty_struct *tty, int 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 = char_time / 5;
-	if (char_time == 0)
-		char_time = 1;
+	char_time = max(nsecs_to_jiffies(port->frame_time / 5), 1UL);
+
 	if (timeout && timeout < char_time)
 		char_time = timeout;
 

include/linux/serial_core.h

@@ -232,6 +232,7 @@ 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		frame_time;		/* frame timing in ns */
 	unsigned int		type;			/* port type */
 	const struct uart_ops	*ops;
 	unsigned int		custom_divisor;