staging: comedi: serial2002: Use monotonic clock

Wall time obtained from do_gettimeofday is susceptible to sudden jumps due to
user setting the time or due to NTP.

Monotonic time is constantly increasing time better suited for comparing two
timestamps.
Signed-off-by: NAbhilash Jindal <klock.android@gmail.com>
Reviewed-by: NIan Abbott <abbotti@mev.co.uk>
Reviewed-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/serial2002.c

@@ -32,6 +32,7 @@ Status: in development
 #include <linux/delay.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
+#include <linux/ktime.h>
 
 #include <linux/termios.h>
 #include <asm/ioctls.h>
@@ -121,9 +122,9 @@ static int serial2002_tty_write(struct file *f, unsigned char *buf, int count)
 static void serial2002_tty_read_poll_wait(struct file *f, int timeout)
 {
 	struct poll_wqueues table;
-	struct timeval start, now;
+	ktime_t start, now;
 
-	do_gettimeofday(&start);
+	start = ktime_get();
 	poll_initwait(&table);
 	while (1) {
 		long elapsed;
@@ -134,9 +135,8 @@ static void serial2002_tty_read_poll_wait(struct file *f, int timeout)
 			    POLLHUP | POLLERR)) {
 			break;
 		}
-		do_gettimeofday(&now);
-		elapsed = 1000000 * (now.tv_sec - start.tv_sec) +
-			  now.tv_usec - start.tv_usec;
+		now = ktime_get();
+		elapsed = ktime_us_delta(now, start);
 		if (elapsed > timeout)
 			break;
 		set_current_state(TASK_INTERRUPTIBLE);