[PATCH 1/1] serial: 8250_bcm2835aux: Add support for RTS/CTS auto flow control

The bcm2835aux mini UART can automatically de-assert RTS based on the RX
FIFO fill level and gate the transmitter on CTS, but the driver never
made use of it. Wire it up:

- Enable AUTORTS/AUTOCTS (with the RTS4 threshold and inverted, i.e.
  conventional active-low, polarity) whenever CRTSCTS is set on ports
  declaring the "uart-has-rtscts" property, advertised via
  UPF_HARD_FLOW.

- Gate the AUTORTS function on TIOCM_RTS in set_mctrl() so that
  clearing RTS (port close, TIOCMBIC, B0) actually de-asserts the pin
  while auto flow control is active.

- Implement throttle()/unthrottle() by disabling the RX interrupts and
  letting the FIFO fill up, making the hardware de-assert RTS - the
  same approach as in 8250_omap. Re-sync this state on termios and
  rs485 changes, as those paths may re-enable the RX interrupts (or
  leave them disabled) behind the throttle's back.

- Never enable auto flow control together with rs485, which uses RTS
  as the transceiver direction control. As the software CTS fallback
  of the serial core cannot work without a modem status interrupt,
  strip CRTSCTS whenever it cannot be handled in hardware.

- Restore RXEN when rs485 is disabled, as the emulation may be torn
  down without the rs485_stop_tx() callback ever being called, which
  would leave the receiver disabled.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Jan Čermák <[email protected]>
---
 drivers/tty/serial/8250/8250_bcm2835aux.c | 160 +++++++++++++++++++++-
 1 file changed, 156 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_bcm2835aux.c b/drivers/tty/serial/8250/8250_bcm2835aux.c
index 0609582a62f7..cf9cf08ddb17 100644
--- a/drivers/tty/serial/8250/8250_bcm2835aux.c
+++ b/drivers/tty/serial/8250/8250_bcm2835aux.c
@@ -7,9 +7,9 @@
  * Based on 8250_lpc18xx.c:
  * Copyright (C) 2015 Joachim Eastwood <[email protected]>
  *
- * The bcm2835aux is capable of RTS auto flow-control, but this driver doesn't
- * take advantage of it yet.  When adding support, be sure not to enable it
- * simultaneously to rs485.
+ * The bcm2835aux's RTS/CTS auto flow-control is enabled for ports declaring
+ * the "uart-has-rtscts" property, but never simultaneously to rs485, which
+ * repurposes RTS as the transceiver direction control.
  */
 
 #include <linux/clk.h>
@@ -19,6 +19,7 @@
 #include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/property.h>
+#include <linux/tty.h>
 
 #include "8250.h"
 
@@ -34,6 +35,18 @@
 #define BCM2835_AUX_UART_CNTL_RTSINV	0x40 /* Invert auto RTS polarity */
 #define BCM2835_AUX_UART_CNTL_CTSINV	0x80 /* Invert auto CTS polarity */
 
+/*
+ * Set auto RTS to de-assert with 4 FIFO slots left (RTS4).
+ *
+ * The RTSINV/CTSINV bits select the auto flow assert level, which out of
+ * reset is active-high. Set them to get the conventional active-low RTS/CTS.
+ */
+#define BCM2835_AUX_UART_CNTL_AUTORTS_MASK \
+	(BCM2835_AUX_UART_CNTL_AUTORTS | BCM2835_AUX_UART_CNTL_RTS4 | \
+	 BCM2835_AUX_UART_CNTL_RTSINV)
+#define BCM2835_AUX_UART_CNTL_AUTOCTS_MASK \
+	(BCM2835_AUX_UART_CNTL_AUTOCTS | BCM2835_AUX_UART_CNTL_CTSINV)
+
 /**
  * struct bcm2835aux_data - driver private data of BCM2835 auxiliary UART
  * @clk: clock producer of the port's uartclk
@@ -46,6 +59,139 @@ struct bcm2835aux_data {
 	u32 cntl;
 };
 
+static bool bcm2835aux_tty_throttled(struct uart_port *port)
+{
+	struct tty_struct *tty = port->state ? port->state->port.tty : NULL;
+
+	return tty && tty_throttled(tty);
+}
+
+/* Program the CNTL auto flow bits from port->status and mctrl */
+static void bcm2835aux_update_flow(struct uart_port *port, unsigned int mctrl)
+{
+	struct bcm2835aux_data *data = dev_get_drvdata(port->dev);
+	struct uart_8250_port *up = up_to_u8250p(port);
+
+	data->cntl &= ~(BCM2835_AUX_UART_CNTL_AUTORTS_MASK |
+			BCM2835_AUX_UART_CNTL_AUTOCTS_MASK);
+	if ((port->status & UPSTAT_AUTORTS) && (mctrl & TIOCM_RTS))
+		data->cntl |= BCM2835_AUX_UART_CNTL_AUTORTS_MASK;
+	if (port->status & UPSTAT_AUTOCTS)
+		data->cntl |= BCM2835_AUX_UART_CNTL_AUTOCTS_MASK;
+
+	serial_out(up, BCM2835_AUX_UART_CNTL, data->cntl);
+}
+
+static void bcm2835aux_enable_rx_irq(struct uart_port *port)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+
+	up->ier |= UART_IER_RLSI | UART_IER_RDI;
+	serial_out(up, UART_IER, up->ier);
+}
+
+static void bcm2835aux_set_autoflow(struct uart_port *port, tcflag_t cflag,
+				    bool rs485_enabled)
+{
+	bool was_enabled = port->status & UPSTAT_AUTORTS;
+	bool enable;
+
+	/*
+	 * rs485 uses RTS as the transceiver direction control, which is
+	 * mutually exclusive with the auto flow-control of the pin.
+	 */
+	enable = (cflag & CRTSCTS) && (port->flags & UPF_HARD_FLOW) &&
+		 !rs485_enabled;
+
+	if (enable) {
+		port->status |= UPSTAT_AUTORTS | UPSTAT_AUTOCTS;
+		/* Restore the throttle state lost in startup/resume/rs485 paths */
+		if (bcm2835aux_tty_throttled(port)) {
+			/* let the FIFO fill up and de-assert RTS in hardware */
+			port->ops->stop_rx(port);
+			/* reclaim an RTS lowered by software flow control */
+			if ((cflag & CBAUD) != B0)
+				port->mctrl |= TIOCM_RTS;
+		}
+	} else {
+		port->status &= ~(UPSTAT_AUTORTS | UPSTAT_AUTOCTS);
+		/* unthrottle() will no longer be called after leaving auto-RTS mode */
+		if (was_enabled)
+			bcm2835aux_enable_rx_irq(port);
+	}
+
+	bcm2835aux_update_flow(port, port->mctrl);
+}
+
+static void bcm2835aux_set_mctrl(struct uart_port *port, unsigned int mctrl)
+{
+	serial8250_do_set_mctrl(port, mctrl);
+	bcm2835aux_update_flow(port, mctrl);
+}
+
+static void bcm2835aux_set_termios(struct uart_port *port,
+				   struct ktermios *termios,
+				   const struct ktermios *old)
+{
+	/*
+	 * Strip CRTSCTS when the hardware auto flow-control cannot be
+	 * used, as the serial core's software CTS fallback cannot work
+	 * without a modem status interrupt.
+	 */
+	if (!(port->flags & UPF_HARD_FLOW) ||
+	    (port->rs485.flags & SER_RS485_ENABLED))
+		termios->c_cflag &= ~CRTSCTS;
+
+	serial8250_do_set_termios(port, termios, old);
+
+	guard(uart_port_lock_irqsave)(port);
+
+	bcm2835aux_set_autoflow(port, termios->c_cflag,
+				port->rs485.flags & SER_RS485_ENABLED);
+}
+
+static void bcm2835aux_throttle(struct uart_port *port)
+{
+	guard(uart_port_lock_irqsave)(port);
+
+	port->ops->stop_rx(port);
+}
+
+static void bcm2835aux_unthrottle(struct uart_port *port)
+{
+	guard(uart_port_lock_irqsave)(port);
+
+	bcm2835aux_enable_rx_irq(port);
+}
+
+static int bcm2835aux_rs485_config(struct uart_port *port,
+				   struct ktermios *termios,
+				   struct serial_rs485 *rs485)
+{
+	tcflag_t cflag = termios ? termios->c_cflag : 0;
+	bool rs485_enabled;
+	int ret;
+
+	ret = serial8250_em485_config(port, termios, rs485);
+	if (ret)
+		return ret;
+
+	rs485_enabled = rs485->flags & SER_RS485_ENABLED;
+	if (!rs485_enabled) {
+		struct bcm2835aux_data *data = dev_get_drvdata(port->dev);
+
+		data->cntl |= BCM2835_AUX_UART_CNTL_RXEN;
+	}
+
+	/*
+	 * termios is NULL when rs485 is set up from firmware properties at
+	 * port registration, before the port has been opened.
+	 */
+	bcm2835aux_set_autoflow(port, cflag, rs485_enabled);
+
+	return 0;
+}
+
 static void bcm2835aux_rs485_start_tx(struct uart_8250_port *up, bool toggle_ier)
 {
 	if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX)) {
@@ -99,7 +245,11 @@ static int bcm2835aux_serial_probe(struct platform_device *pdev)
 	up.port.dev = &pdev->dev;
 	up.port.type = PORT_16550;
 	up.port.flags = UPF_FIXED_PORT | UPF_FIXED_TYPE | UPF_SKIP_TEST | UPF_IOREMAP;
-	up.port.rs485_config = serial8250_em485_config;
+	up.port.set_termios = bcm2835aux_set_termios;
+	up.port.set_mctrl = bcm2835aux_set_mctrl;
+	up.port.throttle = bcm2835aux_throttle;
+	up.port.unthrottle = bcm2835aux_unthrottle;
+	up.port.rs485_config = bcm2835aux_rs485_config;
 	up.port.rs485_supported = serial8250_em485_supported;
 	up.rs485_start_tx = bcm2835aux_rs485_start_tx;
 	up.rs485_stop_tx = bcm2835aux_rs485_stop_tx;
@@ -134,6 +284,8 @@ static int bcm2835aux_serial_probe(struct platform_device *pdev)
 	ret = uart_read_port_properties(&up.port);
 	if (ret)
 		goto rm_swnode;
+	if (device_property_read_bool(&pdev->dev, "uart-has-rtscts"))
+		up.port.flags |= UPF_HARD_FLOW;
 
 	up.port.regshift = 2;
 	up.port.fifosize = 8;
-- 
2.55.0