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

Introduce the UART_CAP_NOTEMT capability. The capability indicates that
the UART doesn't have an interrupt available on TEMT.

In the case where the device does not support it, we calculate the
maximum time it could take for the transmitter to empty the
shift register. When we get in the situation where we get the
THRE interrupt, we check if the TEMT bit is set. If it's not, we start
the a timer and recall __stop_tx() after the delay.

The transmit sequence is a bit modified when the capability is set. The
new timer is used between the last interrupt(THRE) and a potential
stop_tx timer.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
[moved to use added UART_CAP_TEMT]
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
[moved to use added UART_CAP_NOTEMT, improve timeout]
Signed-off-by: Eric Tremblay <etremblay@distech-controls.com>
[rebased to v5.17, making use of tty_get_frame_size]
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220330104642.229507-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index db784ace25d83bef20f591ce26287eefece705bc..39ffeb37786ff7cba0d703b795fba30e3619b963 100644 (file)
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -83,6 +83,7 @@ struct serial8250_config {
 #define UART_CAP_MINI  BIT(17) /* Mini UART on BCM283X family lacks:
                                         * STOP PARITY EPAR SPAR WLEN5 WLEN6
                                         */
+#define UART_CAP_NOTEMT        BIT(18) /* UART without interrupt on TEMT available */
 
 #define UART_BUG_QUOT  BIT(0)  /* UART has buggy quot LSB */
 #define UART_BUG_TXEN  BIT(1)  /* UART has buggy TX IIR status */

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 318af6f1360504b6222cd3483b02fd83f3790e13..31545cd8276efa71f8fd9ec02978b862e3751ef7 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -571,8 +571,21 @@ 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)
 {
@@ -631,6 +644,16 @@ 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;
@@ -662,6 +685,7 @@ 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;
@@ -1504,6 +1528,11 @@ 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;
@@ -1535,14 +1564,33 @@ 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 timeing and allow FIFO transfer,
+                * To provide required timing 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 ((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);
                        return;
+               }
 
                __stop_tx_rs485(p);
        }
@@ -1653,6 +1701,27 @@ 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,
@@ -2858,6 +2927,9 @@ 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.

diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index ff84a3ed10ea911c742978ba01863052d8d6a4ab..de135852107cb57bdf86b1d31bc1a0be362713f7 100644 (file)
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -79,7 +79,9 @@ 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 */
 };