On Thu, 4 Feb 2021, Eric Tremblay wrote:
> The patch introduce the UART_CAP_NOTEMT capability. The capability
> indicate 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@xxxxxxxxxxxxxxxx>
> [moved to use added UART_CAP_TEMT]
> Signed-off-by: Heiko Stuebner <heiko.stuebner@xxxxxxxxxxxxxxxxxxxxx>
> [moved to use added UART_CAP_NOTEMT, improve timeout]
> Signed-off-by: Eric Tremblay <etremblay@xxxxxxxxxxxxxxxxxxxx>
> ---
> @@ -1531,8 +1560,19 @@ static inline void __stop_tx(struct uart_8250_port *p)
> * 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))
> + 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);
Is it ok to start the timer here without first confirming UART_LSR_THRE?
> return;
> + }
>
> __stop_tx_rs485(p);
> }
> @@ -1631,6 +1671,27 @@ 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) {
> + __stop_tx(p);
> + em485->active_timer = NULL;
If BOTH_EMPTY is still not set when calling __stop_tx from here,
__stop_tx ends up just starting the timer again and the timer won't do
anything useful when expiring because active_timer is now NULL.
> + }
> +
> + spin_unlock_irqrestore(&p->port.lock, flags);
> + serial8250_rpm_put(p);
> + return HRTIMER_NORESTART;
> +}
--
i.
