Re: [PATCH] serial: amba-pl011: drive the bus during break in RS485 mode

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Sun, Oct 11, 2026 at 10:23:07PM +0200, Nicolai Buchwitz wrote:
> In RS485 mode the transceiver is enabled only while data is sent. A
> break set with TIOCSBRK therefore never reaches the bus, and protocols
> that start each frame with a break, like DMX512, do not work.
> 
> Handle an active break like a transmission. Enable the transceiver
> before setting the break, after delay_rts_before_send if one is
> configured. When the break ends, stop transmitting unless the TX path
> is still sending data.
> 
> Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
> ---
> Noticed while implementing a DMX master on a CM5 with its PL011.
> Tested on that board, driving a DMX512 fixture with RS485 mode left
> enabled.
> 
> The 8250 em485 code probably has the same issue. I have no hardware to
> test it, but I'm happy to send a patch if someone is willing to.
> ---
>  drivers/tty/serial/amba-pl011.c | 69 +++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 67 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> index fcfa9f00aec6..7bef1be5e9b4 100644
> --- a/drivers/tty/serial/amba-pl011.c
> +++ b/drivers/tty/serial/amba-pl011.c
> @@ -327,6 +327,7 @@ struct uart_amba_port {
>  	char			type[12];
>  	ktime_t			rs485_tx_drain_interval; /* nano */
>  	enum pl011_rs485_tx_state	rs485_tx_state;
> +	bool			rs485_break;
>  	struct hrtimer		trigger_start_tx;
>  	struct hrtimer		trigger_stop_tx;
>  	bool			console_line_ended;
> @@ -1279,6 +1280,11 @@ static inline bool pl011_dma_rx_running(struct uart_amba_port *uap)
>  	return uap->using_rx_dma && uap->dmarx.running;
>  }
>  
> +static inline bool pl011_dma_tx_queued(struct uart_amba_port *uap)
> +{
> +	return uap->dmatx.queued;
> +}
> +
>  #else
>  /* Blank functions if the DMA engine is not available */
>  static inline void pl011_dma_remove(struct uart_amba_port *uap)
> @@ -1330,6 +1336,11 @@ static inline bool pl011_dma_rx_running(struct uart_amba_port *uap)
>  	return false;
>  }
>  
> +static inline bool pl011_dma_tx_queued(struct uart_amba_port *uap)
> +{
> +	return false;
> +}
> +
>  #define pl011_dma_flush_buffer	NULL
>  #endif
>  
> @@ -1357,6 +1368,10 @@ static void pl011_rs485_tx_stop(struct uart_amba_port *uap)
>  {
>  	struct uart_port *port = &uap->port;
>  
> +	/* pl011_break_ctl() stops the transmission when the break ends */
> +	if (uap->rs485_break)
> +		return;
> +
>  	if (uap->rs485_tx_state == SEND)
>  		uap->rs485_tx_state = WAIT_AFTER_SEND;
>  
> @@ -1477,8 +1492,12 @@ static enum hrtimer_restart pl011_trigger_start_tx(struct hrtimer *t)
>  	unsigned long flags;
>  
>  	uart_port_lock_irqsave(&uap->port, &flags);
> -	if (uap->rs485_tx_state == WAIT_AFTER_RTS)
> -		pl011_start_tx(&uap->port);
> +	if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
> +		uap->rs485_tx_state = SEND;
> +		/* A break can arm the timer while TX is stopped */
> +		if (!uart_tx_stopped(&uap->port))
> +			pl011_start_tx(&uap->port);
> +	}
>  	uart_port_unlock_irqrestore(&uap->port, flags);
>  
>  	return HRTIMER_NORESTART;
> @@ -1770,16 +1789,60 @@ static void pl011_break_ctl(struct uart_port *port, int break_state)
>  {
>  	struct uart_amba_port *uap =
>  	    container_of(port, struct uart_amba_port, port);
> +	struct tty_port *tport = &port->state->port;
> +	bool rs485 = port->rs485.flags & SER_RS485_ENABLED;
>  	unsigned long flags;
>  	unsigned int lcr_h;
>  
>  	uart_port_lock_irqsave(&uap->port, &flags);
> +
> +	/*
> +	 * In RS485 mode the transceiver only drives the bus while
> +	 * transmitting. Treat a break as a transmission.
> +	 */
> +	if (rs485 && break_state == -1 && !uap->rs485_break) {
> +		uap->rs485_break = true;
> +
> +		/* Pending data already enabled the transceiver */
> +		if (uap->rs485_tx_state != SEND &&
> +		    uap->rs485_tx_state != WAIT_AFTER_RTS)
> +			pl011_rs485_tx_start(uap);
> +
> +		/*
> +		 * Wait out delay_rts_before_send before starting the break.
> +		 * The state stays WAIT_AFTER_RTS, so the TX path behaves as
> +		 * without a break. Start it here if trigger_start_tx is late.
> +		 */
> +		if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
> +			uart_port_unlock_irqrestore(&uap->port, flags);
> +			msleep(port->rs485.delay_rts_before_send);
> +			uart_port_lock_irqsave(&uap->port, &flags);
> +
> +			if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
> +				hrtimer_try_to_cancel(&uap->trigger_start_tx);
> +				uap->rs485_tx_state = SEND;
> +				if (!uart_tx_stopped(port))
> +					pl011_start_tx(port);
> +			}
> +		}
> +	}
> +
>  	lcr_h = pl011_read(uap, REG_LCRH_TX);
>  	if (break_state == -1)
>  		lcr_h |= UART01x_LCRH_BRK;
>  	else
>  		lcr_h &= ~UART01x_LCRH_BRK;
>  	pl011_write(lcr_h, uap, REG_LCRH_TX);
> +
> +	if (break_state == 0 && uap->rs485_break) {
> +		uap->rs485_break = false;
> +		/* An active TX path stops the transmission when it is done */
> +		if (rs485 && (uart_tx_stopped(port) ||
> +			      (!pl011_dma_tx_queued(uap) &&
> +			       kfifo_is_empty(&tport->xmit_fifo))))
> +			pl011_rs485_tx_stop(uap);
> +	}
> +
>  	uart_port_unlock_irqrestore(&uap->port, flags);
>  }
>  
> @@ -2100,6 +2163,7 @@ static void pl011_shutdown(struct uart_port *port)
>  	hrtimer_cancel(&uap->trigger_stop_tx);
>  
>  	uart_port_lock_irq(port);
> +	uap->rs485_break = false;
>  	if (uap->rs485_tx_state != OFF)
>  		pl011_rs485_tx_stop_now(uap);
>  	uart_port_unlock_irq(port);
> @@ -2396,6 +2460,7 @@ static int pl011_rs485_config(struct uart_port *port, struct ktermios *termios,
>  	struct uart_amba_port *uap =
>  		container_of(port, struct uart_amba_port, port);
>  
> +	uap->rs485_break = false;
>  	if (port->rs485.flags & SER_RS485_ENABLED)
>  		pl011_rs485_tx_stop(uap);
>  
> 
> ---
> base-commit: 36844ea19656fb41278799ef3d5cd52120d89149
> change-id: 20261011-devel-pl011-rs485-break-bec3b979a420

Does not apply to the tty-testing branch :(