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serial: 8250_bcm2835aux: Add support for RTS/CTS auto flow control

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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. Keep it when a CTS
  GPIO is present, as the serial core's mctrl_gpio handling then
  delivers the interrupt and the software fallback keeps working.

- 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 <sairon@sairon.cz>
Link: https://patch.msgid.link/20261001090432.1045270-1-sairon@sairon.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Jan Čermák 2026-10-01 11:04:32 +02:00 committed by Greg Kroah-Hartman
parent e95442c31a
commit ca576923c3
1 changed files with 163 additions and 4 deletions
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167
drivers/tty/serial/8250/8250_bcm2835aux.c
View File

@ -7,9 +7,9 @@
* Based on 8250_lpc18xx.c:
* Copyright (C) 2015 Joachim Eastwood <manabian@gmail.com>
*
* 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 +59,146 @@ struct bcm2835aux_data {
u32 cntl;
};
static bool bcm2835aux_tty_throttled(struct uart_port *port)
{
if (!port->state)
return false;
scoped_guard(tty_port_tty, &port->state->port)
return tty_throttled(scoped_tty());
return false;
}
/* 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)
{
struct uart_8250_port *up = up_to_u8250p(port);
bool hw_flow = (port->flags & UPF_HARD_FLOW) &&
!(port->rs485.flags & SER_RS485_ENABLED);
/*
* 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. A CTS GPIO provides one.
*/
if (!hw_flow && !mctrl_gpio_to_gpiod(up->gpios, UART_GPIO_CTS))
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 +252,11 @@ 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 +291,8 @@ 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;