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Message-Id: <20241221-amba-rts-v3-1-d3d444681419@faster.cz>
Date: Sat, 21 Dec 2024 20:11:04 +0100
From: Miroslav Ondra via B4 Relay <devnull+ondra.faster.cz@...nel.org>
To: Russell King <linux@...linux.org.uk>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jirislaby@...nel.org>,
Lino Sanfilippo <l.sanfilippo@...bus.com>
Cc: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
Miroslav Ondra <ondra@...ter.cz>
Subject: [PATCH v3] serial: amba-pl011: Fix RTS handling in RS485 mode
From: Miroslav Ondra <ondra@...ter.cz>
Data loss on serial line was observed during communication through
serial ports ttyAMA1 and ttyAMA2 interconnected via RS485 transcievers.
Both ports are in one BCM2711 (Compute Module CM40) and they share
the same interrupt line.
The problem is caused by long waiting for tx queue flush in the function
pl011_rs485_tx_stop. Udelay or mdelay are used to wait.
The function is called from the interrupt handler. If multiple devices
share a single interrupt line, late processing of pending interrupts
and data loss may occur. When operation of both devices are synchronous,
collisions are quite often.
This rework is based on the method used in tty/serial/imx.c
Use hrtimer instead of udelay and mdelay calls.
Replace simple bool variable rs485_tx_started by 4-state variable
rs485_tx_state.
Tested-by: Lino Sanfilippo <l.sanfilippo@...bus.com>
Signed-off-by: Miroslav Ondra <ondra@...ter.cz>
---
Thanks Lino for testing.
Thanks Greg for the advice.
---
Changes in v3:
- Collect tags from tester
- Link to v2: https://lore.kernel.org/r/20241206-master-v2-1-02628dc036c9@faster.cz
Changes in v2:
- Reword commit message to explain the problem.
- Link to v1: https://lore.kernel.org/r/20241123-master-v1-1-260194426ea3@faster.cz
---
drivers/tty/serial/amba-pl011.c | 126 ++++++++++++++++++++++++++++++----------
1 file changed, 96 insertions(+), 30 deletions(-)
diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 69b7a3e1e418e200944b0b0429b6341d71ecff26..04212c823a91d5a343327292d8ffd95be1465473 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -248,6 +248,13 @@ struct pl011_dmatx_data {
bool queued;
};
+enum pl011_rs485_tx_state {
+ OFF,
+ WAIT_AFTER_RTS,
+ SEND,
+ WAIT_AFTER_SEND,
+};
+
/*
* We wrap our port structure around the generic uart_port.
*/
@@ -261,8 +268,10 @@ struct uart_amba_port {
unsigned int fifosize; /* vendor-specific */
unsigned int fixed_baud; /* vendor-set fixed baud rate */
char type[12];
- bool rs485_tx_started;
- unsigned int rs485_tx_drain_interval; /* usecs */
+ ktime_t rs485_tx_drain_interval; /* nano */
+ enum pl011_rs485_tx_state rs485_tx_state;
+ struct hrtimer trigger_start_tx;
+ struct hrtimer trigger_stop_tx;
#ifdef CONFIG_DMA_ENGINE
/* DMA stuff */
unsigned int dmacr; /* dma control reg */
@@ -1260,30 +1269,31 @@ static inline bool pl011_dma_rx_running(struct uart_amba_port *uap)
static void pl011_rs485_tx_stop(struct uart_amba_port *uap)
{
- /*
- * To be on the safe side only time out after twice as many iterations
- * as fifo size.
- */
- const int MAX_TX_DRAIN_ITERS = uap->port.fifosize * 2;
struct uart_port *port = &uap->port;
- int i = 0;
u32 cr;
- /* Wait until hardware tx queue is empty */
- while (!pl011_tx_empty(port)) {
- if (i > MAX_TX_DRAIN_ITERS) {
- dev_warn(port->dev,
- "timeout while draining hardware tx queue\n");
- break;
- }
+ if (uap->rs485_tx_state == SEND)
+ uap->rs485_tx_state = WAIT_AFTER_SEND;
- udelay(uap->rs485_tx_drain_interval);
- i++;
+ if (uap->rs485_tx_state == WAIT_AFTER_SEND) {
+ /* Schedule hrtimer if tx queue not empty */
+ if (!pl011_tx_empty(port)) {
+ hrtimer_start(&uap->trigger_stop_tx,
+ uap->rs485_tx_drain_interval,
+ HRTIMER_MODE_REL);
+ return;
+ }
+ if (port->rs485.delay_rts_after_send > 0) {
+ hrtimer_start(&uap->trigger_stop_tx,
+ ms_to_ktime(port->rs485.delay_rts_after_send),
+ HRTIMER_MODE_REL);
+ return;
+ }
+ /* Continue without any delay */
+ } else if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
+ hrtimer_try_to_cancel(&uap->trigger_start_tx);
}
- if (port->rs485.delay_rts_after_send)
- mdelay(port->rs485.delay_rts_after_send);
-
cr = pl011_read(uap, REG_CR);
if (port->rs485.flags & SER_RS485_RTS_AFTER_SEND)
@@ -1296,7 +1306,7 @@ static void pl011_rs485_tx_stop(struct uart_amba_port *uap)
cr |= UART011_CR_RXE;
pl011_write(cr, uap, REG_CR);
- uap->rs485_tx_started = false;
+ uap->rs485_tx_state = OFF;
}
static void pl011_stop_tx(struct uart_port *port)
@@ -1304,11 +1314,18 @@ static void pl011_stop_tx(struct uart_port *port)
struct uart_amba_port *uap =
container_of(port, struct uart_amba_port, port);
+ if (port->rs485.flags & SER_RS485_ENABLED &&
+ uap->rs485_tx_state == WAIT_AFTER_RTS) {
+ pl011_rs485_tx_stop(uap);
+ return;
+ }
+
uap->im &= ~UART011_TXIM;
pl011_write(uap->im, uap, REG_IMSC);
pl011_dma_tx_stop(uap);
- if ((port->rs485.flags & SER_RS485_ENABLED) && uap->rs485_tx_started)
+ if (port->rs485.flags & SER_RS485_ENABLED &&
+ uap->rs485_tx_state != OFF)
pl011_rs485_tx_stop(uap);
}
@@ -1328,10 +1345,19 @@ static void pl011_rs485_tx_start(struct uart_amba_port *uap)
struct uart_port *port = &uap->port;
u32 cr;
+ if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
+ uap->rs485_tx_state = SEND;
+ return;
+ }
+ if (uap->rs485_tx_state == WAIT_AFTER_SEND) {
+ hrtimer_try_to_cancel(&uap->trigger_stop_tx);
+ uap->rs485_tx_state = SEND;
+ return;
+ }
+ /* uap->rs485_tx_state == OFF */
/* Enable transmitter */
cr = pl011_read(uap, REG_CR);
cr |= UART011_CR_TXE;
-
/* Disable receiver if half-duplex */
if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
cr &= ~UART011_CR_RXE;
@@ -1343,10 +1369,14 @@ static void pl011_rs485_tx_start(struct uart_amba_port *uap)
pl011_write(cr, uap, REG_CR);
- if (port->rs485.delay_rts_before_send)
- mdelay(port->rs485.delay_rts_before_send);
-
- uap->rs485_tx_started = true;
+ if (port->rs485.delay_rts_before_send > 0) {
+ uap->rs485_tx_state = WAIT_AFTER_RTS;
+ hrtimer_start(&uap->trigger_start_tx,
+ ms_to_ktime(port->rs485.delay_rts_before_send),
+ HRTIMER_MODE_REL);
+ } else {
+ uap->rs485_tx_state = SEND;
+ }
}
static void pl011_start_tx(struct uart_port *port)
@@ -1355,13 +1385,44 @@ static void pl011_start_tx(struct uart_port *port)
container_of(port, struct uart_amba_port, port);
if ((uap->port.rs485.flags & SER_RS485_ENABLED) &&
- !uap->rs485_tx_started)
+ uap->rs485_tx_state != SEND) {
pl011_rs485_tx_start(uap);
+ if (uap->rs485_tx_state == WAIT_AFTER_RTS)
+ return;
+ }
if (!pl011_dma_tx_start(uap))
pl011_start_tx_pio(uap);
}
+static enum hrtimer_restart pl011_trigger_start_tx(struct hrtimer *t)
+{
+ struct uart_amba_port *uap =
+ container_of(t, struct uart_amba_port, trigger_start_tx);
+ unsigned long flags;
+
+ uart_port_lock_irqsave(&uap->port, &flags);
+ if (uap->rs485_tx_state == WAIT_AFTER_RTS)
+ pl011_start_tx(&uap->port);
+ uart_port_unlock_irqrestore(&uap->port, flags);
+
+ return HRTIMER_NORESTART;
+}
+
+static enum hrtimer_restart pl011_trigger_stop_tx(struct hrtimer *t)
+{
+ struct uart_amba_port *uap =
+ container_of(t, struct uart_amba_port, trigger_stop_tx);
+ unsigned long flags;
+
+ uart_port_lock_irqsave(&uap->port, &flags);
+ if (uap->rs485_tx_state == WAIT_AFTER_SEND)
+ pl011_rs485_tx_stop(uap);
+ uart_port_unlock_irqrestore(&uap->port, flags);
+
+ return HRTIMER_NORESTART;
+}
+
static void pl011_stop_rx(struct uart_port *port)
{
struct uart_amba_port *uap =
@@ -1953,7 +2014,7 @@ static void pl011_shutdown(struct uart_port *port)
pl011_dma_shutdown(uap);
- if ((port->rs485.flags & SER_RS485_ENABLED) && uap->rs485_tx_started)
+ if ((port->rs485.flags & SER_RS485_ENABLED && uap->rs485_tx_state != OFF))
pl011_rs485_tx_stop(uap);
free_irq(uap->port.irq, uap);
@@ -2098,7 +2159,7 @@ pl011_set_termios(struct uart_port *port, struct ktermios *termios,
* with the given baud rate. We use this as the poll interval when we
* wait for the tx queue to empty.
*/
- uap->rs485_tx_drain_interval = DIV_ROUND_UP(bits * 1000 * 1000, baud);
+ uap->rs485_tx_drain_interval = ns_to_ktime(DIV_ROUND_UP(bits * NSEC_PER_SEC, baud));
pl011_setup_status_masks(port, termios);
@@ -2807,6 +2868,11 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
}
}
+ hrtimer_init(&uap->trigger_start_tx, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+ hrtimer_init(&uap->trigger_stop_tx, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+ uap->trigger_start_tx.function = pl011_trigger_start_tx;
+ uap->trigger_stop_tx.function = pl011_trigger_stop_tx;
+
ret = pl011_setup_port(&dev->dev, uap, &dev->res, portnr);
if (ret)
return ret;
---
base-commit: 8155b4ef3466f0e289e8fcc9e6e62f3f4dceeac2
change-id: 20241221-amba-rts-c56641eb667b
Best regards,
--
Miroslav Ondra <ondra@...ter.cz>
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