RE: [PATCH V2 2/2] tty: serial: uartps: Add rs485 support to uartps driver

[Guntupalli, Manikanta]

Hi,

> -----Original Message-----
> From: m.brock@xxxxxxxxxxxxx <m.brock@xxxxxxxxxxxxx>
> Sent: Friday, October 13, 2023 12:35 AM
> To: Guntupalli, Manikanta <manikanta.guntupalli@xxxxxxx>
> Cc: git (AMD-Xilinx) <git@xxxxxxx>; Simek, Michal
> <michal.simek@xxxxxxx>; gregkh@xxxxxxxxxxxxxxxxxxx;
> robh+dt@xxxxxxxxxx; krzysztof.kozlowski+dt@xxxxxxxxxx;
> conor+dt@xxxxxxxxxx; linux-serial@xxxxxxxxxxxxxxx;
> devicetree@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> jirislaby@xxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; Pandey, Radhey
> Shyam <radhey.shyam.pandey@xxxxxxx>; Goud, Srinivas
> <srinivas.goud@xxxxxxx>; Datta, Shubhrajyoti
> <shubhrajyoti.datta@xxxxxxx>; manion05gk@xxxxxxxxx
> Subject: Re: [PATCH V2 2/2] tty: serial: uartps: Add rs485 support to uartps
> driver
> 
> Manikanta Guntupalli wrote on 2023-10-11 16:56:
> > In RS485 half duplex configuration, DriverEnable and ReceiverEnable
> > shorted to each other, and at a time, any node acts as either a driver
> > or a receiver. Use either xlnx,phy-ctrl-gpios or RTS to control
> > RS485 phy as driver or a receiver.
> >
> > Signed-off-by: Manikanta Guntupalli <manikanta.guntupalli@xxxxxxx>
> > ---
> > Changes for V2:
> > Modify optional gpio name to xlnx,phy-ctrl-gpios.
> > Update commit description.
> > Add support for RTS, delay_rts_before_send and delay_rts_after_send in
> > RS485 mode.
> > ---
> >  drivers/tty/serial/xilinx_uartps.c | 116
> > ++++++++++++++++++++++++++++-
> >  1 file changed, 115 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/serial/xilinx_uartps.c
> > b/drivers/tty/serial/xilinx_uartps.c
> > index 8e521c69a959..abddcf1a8bf4 100644
> > --- a/drivers/tty/serial/xilinx_uartps.c
> > +++ b/drivers/tty/serial/xilinx_uartps.c
> > @@ -23,6 +23,7 @@
> >  #include <linux/module.h>
> >  #include <linux/pm_runtime.h>
> >  #include <linux/iopoll.h>
> > +#include <linux/gpio.h>
> >
> >  #define CDNS_UART_TTY_NAME	"ttyPS"
> >  #define CDNS_UART_NAME		"xuartps"
> > @@ -193,6 +194,7 @@ MODULE_PARM_DESC(rx_timeout, "Rx timeout, 1-
> 255");
> >   * @clk_rate_change_nb:	Notifier block for clock changes
> >   * @quirks:		Flags for RXBS support.
> >   * @cts_override:	Modem control state override
> > + * @gpiod:		Pointer to the gpio descriptor
> >   */
> >  struct cdns_uart {
> >  	struct uart_port	*port;
> > @@ -203,10 +205,19 @@ struct cdns_uart {
> >  	struct notifier_block	clk_rate_change_nb;
> >  	u32			quirks;
> >  	bool cts_override;
> > +	struct gpio_desc	*gpiod;
> >  };
> >  struct cdns_platform_data {
> >  	u32 quirks;
> >  };
> > +
> > +struct serial_rs485 cdns_rs485_supported = {
> > +	.flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND |
> > +		 SER_RS485_RTS_AFTER_SEND,
> 
> You promise here to support both RTS-on-send and RTS-after-send, but...
> 
> > +	.delay_rts_before_send = 1,
> > +	.delay_rts_after_send = 1,
> > +};
> > +
> >  #define to_cdns_uart(_nb) container_of(_nb, struct cdns_uart, \
> >  		clk_rate_change_nb)
> >
> > @@ -305,6 +316,42 @@ static void cdns_uart_handle_rx(void *dev_id,
> > unsigned int isrstatus)
> >  	tty_flip_buffer_push(&port->state->port);
> >  }
> >
> > +/**
> > + * cdns_rs485_tx_setup - Tx setup specific to rs485
> > + * @cdns_uart: Handle to the cdns_uart  */ static void
> > +cdns_rs485_tx_setup(struct cdns_uart *cdns_uart) {
> > +	u32 val;
> > +
> > +	if (cdns_uart->gpiod) {
> > +		gpiod_set_value(cdns_uart->gpiod, 1);
> > +	} else {
> > +		val = readl(cdns_uart->port->membase +
> CDNS_UART_MODEMCR);
> > +		val &= ~CDNS_UART_MODEMCR_RTS;
> > +		writel(val, cdns_uart->port->membase +
> CDNS_UART_MODEMCR);
> 
> Here you don't care about RTS-on-send or RTS-after-send anymore.
> And neither do you btw. in the if clause.
On Xilinx/AMD Kria SOM KD240 board rs485 connects via TI ISOW1432
Transceiver device, where one GPIO is used for driving DE/RE signals.
With rs485 half duplex configuration, DE and RE shorted to each other,
and at a time, any node acts as either a driver or a receiver.
We recommend using either GPIO or RTS to control RS485 phy as driver or a
receiver. So, we fall back to RTS, if rts-gpios not passed.
> 
> > +	}
> > +}
> > +
> > +/**
> > + * cdns_rs485_rx_setup - Rx setup specific to rs485
> > + * @cdns_uart: Handle to the cdns_uart  */ static void
> > +cdns_rs485_rx_setup(struct cdns_uart *cdns_uart) {
> > +	u32 val;
> > +
> > +	if (cdns_uart->gpiod) {
> > +		gpiod_set_value(cdns_uart->gpiod, 0);
> > +	} else {
> > +		val = readl(cdns_uart->port->membase +
> CDNS_UART_MODEMCR);
> > +		val |= CDNS_UART_MODEMCR_RTS;
> > +		writel(val, cdns_uart->port->membase +
> CDNS_UART_MODEMCR);
> > +	}
> 
> Same here.
> 
> > +}
> > +
> > +static unsigned int cdns_uart_tx_empty(struct uart_port *port);
> > +
> 
> I think it's better to move up the implementation than to use a forward
> declaration.
We will fix.
> 
> >  /**
> >   * cdns_uart_handle_tx - Handle the bytes to be Txed.
> >   * @dev_id: Id of the UART port
> > @@ -313,12 +360,20 @@ static void cdns_uart_handle_rx(void *dev_id,
> > unsigned int isrstatus)  static void cdns_uart_handle_tx(void *dev_id)
> > {
> >  	struct uart_port *port = (struct uart_port *)dev_id;
> > +	struct cdns_uart *cdns_uart = port->private_data;
> >  	struct circ_buf *xmit = &port->state->xmit;
> > +	unsigned long time_out;
> >  	unsigned int numbytes;
> >
> > +	if (cdns_uart->port->rs485.flags & SER_RS485_ENABLED) {
> > +		cdns_rs485_tx_setup(cdns_uart);
> > +		if (cdns_uart->port->rs485.delay_rts_before_send)
> > +			mdelay(cdns_uart->port-
> >rs485.delay_rts_before_send);
> 
> mdelay?
> https://www.kernel.org/doc/html/latest/timers/timers-howto.html
> "In general, use of mdelay is discouraged and code should be refactored to
> allow for the use of msleep."
For refactoring the code will move this code snippet to start_tx, even there,
interrupts are disabled, so can't use msleep.

https://docs.kernel.org/driver-api/serial/driver.html
Documentation recommends,
start_tx
void ()(struct uart_port *port)
Start transmitting characters.
Locking: port->lock taken. Interrupts: locally disabled. This call must not sleep.

> 
> Furthermore, you're delaying before every burst of bytes here!
> Every TXEMPTY interrupt!
We will move code to start_tx.
> 
> > +	}
> > +
> >  	if (uart_circ_empty(xmit)) {
> >  		writel(CDNS_UART_IXR_TXEMPTY, port->membase +
> CDNS_UART_IDR);
> > -		return;
> > +		goto rs485_rx_setup;
> 
> And when there was nothing more to send you waited for nothing.
We will fix.
> 
> >  	}
> >
> >  	numbytes = port->fifosize;
> > @@ -332,6 +387,23 @@ static void cdns_uart_handle_tx(void *dev_id)
> >
> >  	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
> >  		uart_write_wakeup(port);
> > +
> > +rs485_rx_setup:
> > +	if (cdns_uart->port->rs485.flags & SER_RS485_ENABLED) {
> > +		time_out = jiffies + usecs_to_jiffies(TX_TIMEOUT);
> > +		/* Wait for tx completion */
> > +		while ((cdns_uart_tx_empty(cdns_uart->port) !=
> TIOCSER_TEMT) &&
> > +		       time_before(jiffies, time_out))
> > +			cpu_relax();
> > +
> > +		/*
> > +		 * Default Rx should be setup, because RX signaling path
> > +		 * need to enable to receive data.
> > +		 */
> > +		cdns_rs485_rx_setup(cdns_uart);
> > +		if (cdns_uart->port->rs485.delay_rts_after_send)
> > +			mdelay(cdns_uart->port-
> >rs485.delay_rts_after_send);
> 
> This is not delaying rts after send. You must keep RTS aka DE active for a little
> longer so even the last stop bit(s) are transmitted correctly. So this delay must
> happen before cdns_rs485_rx_setup().
We will fix.
> 
> > +	}
> >  }
> >
> >  /**
> > @@ -829,6 +901,9 @@ static int cdns_uart_startup(struct uart_port
> > *port)
> >  		(CDNS_UART_CR_TXRST | CDNS_UART_CR_RXRST))
> >  		cpu_relax();
> >
> > +	if (cdns_uart->port->rs485.flags & SER_RS485_ENABLED)
> > +		cdns_rs485_rx_setup(cdns_uart);
> > +
> >  	/*
> >  	 * Clear the RX disable bit and then set the RX enable bit to enable
> >  	 * the receiver.
> > @@ -1455,6 +1530,25 @@ MODULE_DEVICE_TABLE(of,
> cdns_uart_of_match);
> >  /* Temporary variable for storing number of instances */  static int
> > instances;
> >
> > +/**
> > + * cdns_rs485_config - Called when an application calls TIOCSRS485
> > ioctl.
> > + * @port: Pointer to the uart_port structure
> > + * @termios: Pointer to the ktermios structure
> > + * @rs485: Pointer to the serial_rs485 structure
> > + *
> > + * Return: 0
> > + */
> > +static int cdns_rs485_config(struct uart_port *port, struct ktermios
> > *termios,
> > +			     struct serial_rs485 *rs485)
> > +{
> > +	port->rs485 = *rs485;
> > +
> > +	if (rs485->flags & SER_RS485_ENABLED)
> > +		dev_dbg(port->dev, "Setting UART to RS485\n");
> > +
> > +	return 0;
> > +}
> > +
> >  /**
> >   * cdns_uart_probe - Platform driver probe
> >   * @pdev: Pointer to the platform device structure @@ -1597,9
> > +1691,28 @@ static int cdns_uart_probe(struct platform_device *pdev)
> >  	port->private_data = cdns_uart_data;
> >  	port->read_status_mask = CDNS_UART_IXR_TXEMPTY |
> > CDNS_UART_IXR_RXTRIG
> > |
> >  			CDNS_UART_IXR_OVERRUN | CDNS_UART_IXR_TOUT;
> > +	port->rs485_config = cdns_rs485_config;
> > +	port->rs485_supported = cdns_rs485_supported;
> >  	cdns_uart_data->port = port;
> >  	platform_set_drvdata(pdev, port);
> >
> > +	rc = uart_get_rs485_mode(port);
> > +	if (rc)
> > +		goto err_out_clk_notifier;
> > +
> > +	cdns_uart_data->gpiod = devm_gpiod_get_optional(&pdev->dev,
> > "xlnx,phy-ctrl",
> > +							GPIOD_OUT_LOW);
> > +	if (IS_ERR(cdns_uart_data->gpiod)) {
> > +		rc = PTR_ERR(cdns_uart_data->gpiod);
> > +		dev_err(port->dev, "xuartps: devm_gpiod_get_optional
> failed\n");
> > +		goto err_out_clk_notifier;
> > +	}
> > +
> > +	if (cdns_uart_data->gpiod) {
> > +		gpiod_direction_output(cdns_uart_data->gpiod,
> GPIOD_OUT_LOW);
> > +		gpiod_set_value(cdns_uart_data->gpiod, 0);
> > +	}
> > +
> >  	pm_runtime_use_autosuspend(&pdev->dev);
> >  	pm_runtime_set_autosuspend_delay(&pdev->dev,
> > UART_AUTOSUSPEND_TIMEOUT);
> >  	pm_runtime_set_active(&pdev->dev);
> > @@ -1646,6 +1759,7 @@ static int cdns_uart_probe(struct
> > platform_device
> > *pdev)
> >  	pm_runtime_disable(&pdev->dev);
> >  	pm_runtime_set_suspended(&pdev->dev);
> >  	pm_runtime_dont_use_autosuspend(&pdev->dev);
> > +err_out_clk_notifier:
> >  #ifdef CONFIG_COMMON_CLK
> >  	clk_notifier_unregister(cdns_uart_data->uartclk,
> >  			&cdns_uart_data->clk_rate_change_nb);
> 
> Maarten


Thanks,
Manikanta.