Revert "USB: cdc-acm: fix rounding error in TIOCSSERIAL"

This reverts commit b401f8c4.

The offending commit claimed that trying to set the values reported back
by TIOCGSERIAL as a regular user could result in an -EPERM error when HZ
is 250, but that was never the case.

With HZ=250, the default 0.5 second value of close_delay is converted to
125 jiffies when set and is converted back to 50 centiseconds by
TIOCGSERIAL as expected (not 12 cs as was claimed, even if that was the
case before an earlier fix).

Comparing the internal current and new jiffies values is just fine to
determine if the value is about to change so drop the bogus workaround
(which was also backported to stable).

For completeness: With different default values for these parameters or
with a HZ value not divisible by two, the lack of rounding when setting
the default values in tty_port_init() could result in an -EPERM being
returned, but this is hardly something we need to worry about.

Cc: Anthony Mallet <anthony.mallet@laas.fr>
Cc: stable@vger.kernel.org
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210408131602.27956-2-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -942,7 +942,6 @@ static int set_serial_info(struct tty_struct *tty, struct serial_struct *ss)
 {
 	struct acm *acm = tty->driver_data;
 	unsigned int closing_wait, close_delay;
-	unsigned int old_closing_wait, old_close_delay;
 	int retval = 0;
 
 	close_delay = msecs_to_jiffies(ss->close_delay * 10);
@@ -950,17 +949,11 @@ static int set_serial_info(struct tty_struct *tty, struct serial_struct *ss)
 			ASYNC_CLOSING_WAIT_NONE :
 			msecs_to_jiffies(ss->closing_wait * 10);
 
-	/* we must redo the rounding here, so that the values match */
-	old_close_delay	= jiffies_to_msecs(acm->port.close_delay) / 10;
-	old_closing_wait = acm->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
-				ASYNC_CLOSING_WAIT_NONE :
-				jiffies_to_msecs(acm->port.closing_wait) / 10;
-
 	mutex_lock(&acm->port.mutex);
 
 	if (!capable(CAP_SYS_ADMIN)) {
-		if ((ss->close_delay != old_close_delay) ||
-		    (ss->closing_wait != old_closing_wait))
+		if ((close_delay != acm->port.close_delay) ||
+		    (closing_wait != acm->port.closing_wait))
 			retval = -EPERM;
 		else
 			retval = -EOPNOTSUPP;