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serial: amba-pl011: drive the bus during break in RS485 mode

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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>
Link: https://patch.msgid.link/20261013-devel-pl011-rs485-break-v2-1-31c9f80d3e3d@tipi-net.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Nicolai Buchwitz 2026-10-13 15:28:44 +02:00 committed by Greg Kroah-Hartman
parent 1b60b18a01
commit 1b6d660e03
1 changed files with 64 additions and 1 deletions
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65
drivers/tty/serial/amba-pl011.c
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@ -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;
@ -1479,7 +1494,9 @@ static enum hrtimer_restart pl011_trigger_start_tx(struct hrtimer *t)
uart_port_lock_irqsave(&uap->port, &flags);
if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
uap->rs485_tx_state = SEND;
pl011_start_tx(&uap->port);
/* 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);
@ -1772,16 +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);
}
@ -2102,6 +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);
@ -2398,6 +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) &&
!(rs485->flags & SER_RS485_ENABLED)) {
bool tx_pending = uap->rs485_tx_state == WAIT_AFTER_RTS;