Revert "serial: 8250: Handle UART without interrupt on TEMT using em485"

This partially reverts commit f6f58610. The code added by
that commit containted math overflow for 32-bit archs. In
addition, the approach used in it is unnecessarily complicated
requiring a dedicated timer just for notemt. A simpler approach
for providing UART_CAP_NOTEMT already exists (patches 1-2):
  https://lore.kernel.org/linux-serial/20220411083321.9131-3-ilpo.jarvinen@linux.intel.com/T/#u
Thus, simply revert the UART_CAP_NOTEMT change for now.

There were two driver changes within the patch series adding
UART_CAP_NOTEMT taking advantage of the newly added flag.
This does not revert the driver changes and therefore also
UART_CAP_NOTEMT define has to remain. UART_CAP_NOTEMT remains
no-op until support is again added.

Fixes: f6f58610 ("serial: 8250: Handle UART without interrupt on TEMT using em485")
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/5f874142-fb1f-bff7-f33-fac823e65e2e@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -571,21 +571,8 @@ static void serial8250_clear_fifos(struct uart_8250_port *p)
 	}
 }
 
-static inline void serial8250_em485_update_temt_delay(struct uart_8250_port *p,
-			unsigned int cflag, unsigned int baud)
-{
-	unsigned int bits;
-
-	if (!p->em485)
-		return;
-
-	bits = tty_get_frame_size(cflag);
-	p->em485->no_temt_delay = DIV_ROUND_UP(bits * NSEC_PER_SEC, baud);
-}
-
 static enum hrtimer_restart serial8250_em485_handle_start_tx(struct hrtimer *t);
 static enum hrtimer_restart serial8250_em485_handle_stop_tx(struct hrtimer *t);
-static enum hrtimer_restart serial8250_em485_handle_no_temt(struct hrtimer *t);
 
 void serial8250_clear_and_reinit_fifos(struct uart_8250_port *p)
 {
@@ -644,16 +631,6 @@ static int serial8250_em485_init(struct uart_8250_port *p)
 		     HRTIMER_MODE_REL);
 	hrtimer_init(&p->em485->start_tx_timer, CLOCK_MONOTONIC,
 		     HRTIMER_MODE_REL);
-
-	if (p->capabilities & UART_CAP_NOTEMT) {
-		struct tty_struct *tty = p->port.state->port.tty;
-
-		serial8250_em485_update_temt_delay(p, tty->termios.c_cflag,
-						   tty_get_baud_rate(tty));
-		hrtimer_init(&p->em485->no_temt_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
-		p->em485->no_temt_timer.function = &serial8250_em485_handle_no_temt;
-	}
-
 	p->em485->stop_tx_timer.function = &serial8250_em485_handle_stop_tx;
 	p->em485->start_tx_timer.function = &serial8250_em485_handle_start_tx;
 	p->em485->port = p;
@@ -685,7 +662,6 @@ void serial8250_em485_destroy(struct uart_8250_port *p)
 
 	hrtimer_cancel(&p->em485->start_tx_timer);
 	hrtimer_cancel(&p->em485->stop_tx_timer);
-	hrtimer_cancel(&p->em485->no_temt_timer);
 
 	kfree(p->em485);
 	p->em485 = NULL;
@@ -1528,11 +1504,6 @@ static void start_hrtimer_ms(struct hrtimer *hrt, unsigned long msec)
 	hrtimer_start(hrt, ms_to_ktime(msec), HRTIMER_MODE_REL);
 }
 
-static void start_hrtimer_ns(struct hrtimer *hrt, unsigned long nsec)
-{
-	hrtimer_start(hrt, ns_to_ktime(nsec), HRTIMER_MODE_REL);
-}
-
 static void __stop_tx_rs485(struct uart_8250_port *p)
 {
 	struct uart_8250_em485 *em485 = p->em485;
@@ -1564,33 +1535,14 @@ static inline void __stop_tx(struct uart_8250_port *p)
 
 	if (em485) {
 		unsigned char lsr = serial_in(p, UART_LSR);
-
-		p->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
-
 		/*
-		 * To provide required timing and allow FIFO transfer,
+		 * To provide required timeing and allow FIFO transfer,
 		 * __stop_tx_rs485() must be called only when both FIFO and
 		 * shift register are empty. It is for device driver to enable
 		 * interrupt on TEMT.
 		 */
-		if ((lsr & BOTH_EMPTY) != BOTH_EMPTY) {
-			if (!(p->capabilities & UART_CAP_NOTEMT))
-				/* __stop_tx will be called again once TEMT triggers */
-				return;
-
-			if (!(lsr & UART_LSR_THRE))
-				/* __stop_tx will be called again once THRE triggers */
-				return;
-
-			/*
-			 * On devices with no TEMT interrupt available, start
-			 * a timer for a byte time. The timer will recall
-			 * __stop_tx().
-			 */
-			em485->active_timer = &em485->no_temt_timer;
-			start_hrtimer_ns(&em485->no_temt_timer, em485->no_temt_delay);
+		if ((lsr & BOTH_EMPTY) != BOTH_EMPTY)
 			return;
-		}
 
 		__stop_tx_rs485(p);
 	}
@@ -1701,27 +1653,6 @@ static inline void start_tx_rs485(struct uart_port *port)
 	__start_tx(port);
 }
 
-static enum hrtimer_restart serial8250_em485_handle_no_temt(struct hrtimer *t)
-{
-	struct uart_8250_em485 *em485;
-	struct uart_8250_port *p;
-	unsigned long flags;
-
-	em485 = container_of(t, struct uart_8250_em485, no_temt_timer);
-	p = em485->port;
-
-	serial8250_rpm_get(p);
-	spin_lock_irqsave(&p->port.lock, flags);
-	if (em485->active_timer == &em485->no_temt_timer) {
-		em485->active_timer = NULL;
-		__stop_tx(p);
-	}
-
-	spin_unlock_irqrestore(&p->port.lock, flags);
-	serial8250_rpm_put(p);
-	return HRTIMER_NORESTART;
-}
-
 static enum hrtimer_restart serial8250_em485_handle_start_tx(struct hrtimer *t)
 {
 	struct uart_8250_em485 *em485 = container_of(t, struct uart_8250_em485,
@@ -2927,9 +2858,6 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
 
 	serial8250_set_divisor(port, baud, quot, frac);
 
-	if (up->capabilities & UART_CAP_NOTEMT)
-		serial8250_em485_update_temt_delay(up, termios->c_cflag, baud);
-
 	/*
 	 * LCR DLAB must be set to enable 64-byte FIFO mode. If the FCR
 	 * is written without DLAB set, this mode will be disabled.

include/linux/serial_8250.h

@@ -79,9 +79,7 @@ struct uart_8250_ops {
 struct uart_8250_em485 {
 	struct hrtimer		start_tx_timer; /* "rs485 start tx" timer */
 	struct hrtimer		stop_tx_timer;  /* "rs485 stop tx" timer */
-	struct hrtimer		no_temt_timer;  /* "rs485 no TEMT interrupt" timer */
 	struct hrtimer		*active_timer;  /* pointer to active timer */
-	unsigned long		no_temt_delay;  /* Delay for no_temt_timer */
 	struct uart_8250_port	*port;          /* for hrtimer callbacks */
 	unsigned int		tx_stopped:1;	/* tx is currently stopped */
 };