* [PATCH RESEND v4 1/2] serial: amba-pl011: finish a transmission still running when RS485 is enabled
2026-11-12 8:08 [PATCH RESEND v4 0/2] serial: amba-pl011: fix RS485 state on enable and reconfiguration Nicolai Buchwitz
@ 2026-11-12 8:08 ` Nicolai Buchwitz
2026-11-12 8:40 ` sashiko-bot
2026-11-12 8:08 ` [PATCH RESEND v4 2/2] serial: amba-pl011: keep RS485 transmission running on reconfiguration Nicolai Buchwitz
1 sibling, 1 reply; 5+ messages in thread
From: Nicolai Buchwitz @ 2026-11-12 8:08 UTC (permalink / raw)
To: Russell King, Greg Kroah-Hartman, Jiri Slaby, Lino Sanfilippo,
Nicolai Buchwitz
Cc: linux-kernel, linux-serial, stable
Disabling RS485 during a transmission keeps it running. If RS485 is
enabled again before it ends, the state is OFF while data is still going
out. The TX path does not end a transmission in state OFF, so nothing
sets the idle RTS level afterwards and the transceiver keeps driving the
bus.
Hand such a transmission back to RS485 when it is enabled on an open
port. While PIO or DMA still feeds the FIFO, assert RTS and mark it as
sending, so that the TX path ends it like any other. With only the FIFO
left, drain it from trigger_stop_tx instead. Data that is queued but not
started yet, for example while output is stopped, is not part of the
transmission.
Fixes: 97f85204275a ("serial: amba-pl011: keep transmitter enabled when disabling RS485")
Cc: stable@vger.kernel.org # 6.18.x: 1b6d660e038b: serial: amba-pl011: drive the bus during break in RS485 mode
Cc: stable@vger.kernel.org # 6.18.x
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
---
drivers/tty/serial/amba-pl011.c | 57 +++++++++++++++++++++++++++++++++++++++++
1 file changed, 57 insertions(+)
diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 79776a882ae35..1edbe75611e63 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -1285,6 +1285,11 @@ static inline bool pl011_dma_tx_queued(struct uart_amba_port *uap)
return uap->dmatx.queued;
}
+static inline bool pl011_dma_tx_active(struct uart_amba_port *uap)
+{
+ return uap->dmacr & UART011_TXDMAE;
+}
+
#else
/* Blank functions if the DMA engine is not available */
static inline void pl011_dma_remove(struct uart_amba_port *uap)
@@ -1341,6 +1346,11 @@ static inline bool pl011_dma_tx_queued(struct uart_amba_port *uap)
return false;
}
+static inline bool pl011_dma_tx_active(struct uart_amba_port *uap)
+{
+ return false;
+}
+
#define pl011_dma_flush_buffer NULL
#endif
@@ -1734,6 +1744,12 @@ static unsigned int pl011_tx_empty(struct uart_port *port)
0 : TIOCSER_TEMT;
}
+/* PIO or DMA still feeds the FIFO */
+static bool pl011_tx_active(struct uart_amba_port *uap)
+{
+ return (uap->im & UART011_TXIM) || pl011_dma_tx_active(uap);
+}
+
static void pl011_maybe_set_bit(bool cond, unsigned int *ptr, unsigned int mask)
{
if (cond)
@@ -2454,6 +2470,42 @@ static int pl011_verify_port(struct uart_port *port, struct serial_struct *ser)
return ret;
}
+/*
+ * Data from before RS485 was enabled is still going out. Finish it as an
+ * RS485 transmission, so that its end sets the idle level.
+ */
+static void pl011_rs485_tx_resume(struct uart_amba_port *uap,
+ const struct serial_rs485 *rs485)
+{
+ u32 cr = pl011_read(uap, REG_CR);
+
+ cr |= UART011_CR_TXE;
+ if (!(rs485->flags & SER_RS485_RX_DURING_TX))
+ cr &= ~UART011_CR_RXE;
+ if (rs485->flags & SER_RS485_RTS_ON_SEND)
+ cr &= ~UART011_CR_RTS;
+ else
+ cr |= UART011_CR_RTS;
+ pl011_write(cr, uap, REG_CR);
+
+ /* pl011_break_ctl() ends the transmission when the break ends */
+ if (pl011_read(uap, REG_LCRH_TX) & UART01x_LCRH_BRK) {
+ uap->rs485_break = true;
+ uap->rs485_tx_state = SEND;
+ return;
+ }
+
+ if (pl011_tx_active(uap)) {
+ uap->rs485_tx_state = SEND;
+ return;
+ }
+
+ /* Only the FIFO is left. The timer drains it with the new settings */
+ uap->rs485_tx_state = WAIT_AFTER_SEND;
+ hrtimer_start(&uap->trigger_stop_tx, uap->rs485_tx_drain_interval,
+ HRTIMER_MODE_REL);
+}
+
static int pl011_rs485_config(struct uart_port *port, struct ktermios *termios,
struct serial_rs485 *rs485)
{
@@ -2483,6 +2535,11 @@ static int pl011_rs485_config(struct uart_port *port, struct ktermios *termios,
if (tx_pending && !uart_tx_stopped(port) &&
!pl011_dma_tx_start(uap))
pl011_start_tx_pio(uap);
+ } else if ((rs485->flags & SER_RS485_ENABLED) &&
+ uap->rs485_tx_state == OFF &&
+ tty_port_initialized(&port->state->port) &&
+ (pl011_tx_active(uap) || !pl011_tx_empty(port))) {
+ pl011_rs485_tx_resume(uap, rs485);
} else if (port->rs485.flags & SER_RS485_ENABLED) {
pl011_rs485_tx_stop(uap);
}
--
2.53.0
^ permalink raw reply related [flat|nested] 5+ messages in thread* [PATCH RESEND v4 2/2] serial: amba-pl011: keep RS485 transmission running on reconfiguration
2026-11-12 8:08 [PATCH RESEND v4 0/2] serial: amba-pl011: fix RS485 state on enable and reconfiguration Nicolai Buchwitz
2026-11-12 8:08 ` [PATCH RESEND v4 1/2] serial: amba-pl011: finish a transmission still running when RS485 is enabled Nicolai Buchwitz
@ 2026-11-12 8:08 ` Nicolai Buchwitz
2026-11-12 8:28 ` sashiko-bot
1 sibling, 1 reply; 5+ messages in thread
From: Nicolai Buchwitz @ 2026-11-12 8:08 UTC (permalink / raw)
To: Russell King, Greg Kroah-Hartman, Jiri Slaby, Lino Sanfilippo,
Nicolai Buchwitz
Cc: linux-kernel, linux-serial, stable
Changing the RS485 settings while RS485 stays enabled calls
pl011_rs485_tx_stop(), also while a transmission is in progress. Unlike
pl011_stop_tx() it leaves the TX interrupt enabled, so the remaining
data is still pushed into the FIFO after the drain turned off the
transmitter. The FIFO never empties, the port stalls, and closing it
hangs in pl011_dma_shutdown() waiting for the transmitter to go idle.
A start still waiting for delay_rts_before_send is dropped as well.
Only stop the transmitter when no transmission is in progress. The
call is still needed there, it sets the idle RTS level when the port
is configured or resumed. A transmission in progress sets that level
when it ends.
An active break counts as a transmission too. Forget it only when
leaving RS485 mode, so that pl011_break_ctl() still ends the
transmission when the break ends.
Fixes: 8d479237727c ("serial: amba-pl011: add RS485 support")
Cc: stable@vger.kernel.org # 6.18.x
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
---
drivers/tty/serial/amba-pl011.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 1edbe75611e63..e4ac9225cdcba 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -2512,12 +2512,13 @@ 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;
u32 cr = pl011_read(uap, REG_CR);
+ uap->rs485_break = false;
+
/*
* Leaving RS485 mode, which only enables the transmitter while
* sending. Don't stop it, as that stalls a DMA transfer still
@@ -2540,7 +2541,9 @@ static int pl011_rs485_config(struct uart_port *port, struct ktermios *termios,
tty_port_initialized(&port->state->port) &&
(pl011_tx_active(uap) || !pl011_tx_empty(port))) {
pl011_rs485_tx_resume(uap, rs485);
- } else if (port->rs485.flags & SER_RS485_ENABLED) {
+ } else if ((port->rs485.flags & SER_RS485_ENABLED) &&
+ uap->rs485_tx_state == OFF) {
+ /* Set the idle level now, a transmission sets it when done */
pl011_rs485_tx_stop(uap);
}
--
2.53.0
^ permalink raw reply related [flat|nested] 5+ messages in thread