RE: [PATCH V3 2/2] tty: serial: uartps: Add rs485 support to uartps driver

["Guntupalli, Manikanta" <manikanta.guntupalli@amd.com>]

Hi,

> -----Original Message-----
> From: Lino Sanfilippo <LinoSanfilippo@gmx.de>
> Sent: Monday, November 13, 2023 1:08 AM
> To: Guntupalli, Manikanta <manikanta.guntupalli@amd.com>; git (AMD-
> Xilinx) <git@amd.com>; Simek, Michal <michal.simek@amd.com>;
> gregkh@linuxfoundation.org; robh+dt@kernel.org;
> krzysztof.kozlowski+dt@linaro.org; conor+dt@kernel.org; linux-
> serial@vger.kernel.org; devicetree@vger.kernel.org; linux-
> kernel@vger.kernel.org; jirislaby@kernel.org; linux-arm-
> kernel@lists.infradead.org
> Cc: Pandey, Radhey Shyam <radhey.shyam.pandey@amd.com>; Goud,
> Srinivas <srinivas.goud@amd.com>; Datta, Shubhrajyoti
> <shubhrajyoti.datta@amd.com>; manion05gk@gmail.com
> Subject: Re: [PATCH V3 2/2] tty: serial: uartps: Add rs485 support to uartps
> driver
> 
> Hi,
> 
> On 24.10.23 16:48, Manikanta Guntupalli wrote:
> > Add rs485 support to uartps driver. Use either rts-gpios or RTS to
> > control RS485 phy as driver or a receiver.
> >
> > Signed-off-by: Manikanta Guntupalli <manikanta.guntupalli@amd.com>
> > ---
> > 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.
> > Changes for V3:
> > Modify optional gpio name to rts-gpios.
> > Update commit description.
> > Move cdns_uart_tx_empty function to avoid prototype statement.
> > Remove assignment of struct serial_rs485 to port->rs485 as serial core
> > performs that.
> > Switch to native RTS in non GPIO case.
> > Handle rs485 during stop tx.
> > Remove explicit calls to configure gpio direction and value, as
> > devm_gpiod_get_optional performs that by using GPIOD_OUT_LOW
> argument.
> > Update implementation to support configuration of GPIO/RTS value based
> > on user configuration of SER_RS485_RTS_ON_SEND and
> > SER_RS485_RTS_AFTER_SEND. Move implementation to start_tx from
> handle_tx.
> > ---
> >  drivers/tty/serial/xilinx_uartps.c | 180
> > ++++++++++++++++++++++++++---
> >  1 file changed, 165 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/tty/serial/xilinx_uartps.c
> > b/drivers/tty/serial/xilinx_uartps.c
> > index 9c13dac1d4d1..32229cf5c508 100644
> > --- a/drivers/tty/serial/xilinx_uartps.c
> > +++ b/drivers/tty/serial/xilinx_uartps.c
> > @@ -23,6 +23,9 @@
> >  #include <linux/module.h>
> >  #include <linux/pm_runtime.h>
> >  #include <linux/iopoll.h>
> > +#include <linux/gpio.h>
> > +#include <linux/gpio/consumer.h>
> > +#include <linux/delay.h>
> >
> >  #define CDNS_UART_TTY_NAME	"ttyPS"
> >  #define CDNS_UART_NAME		"xuartps"
> > @@ -193,6 +196,7 @@ 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 +207,19 @@ 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,
> > +	.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 +318,79 @@ static void cdns_uart_handle_rx(void *dev_id,
> unsigned int isrstatus)
> >  	tty_flip_buffer_push(&port->state->port);
> >  }
> >
> > +/**
> > + * cdns_rs485_config_gpio_rts_high - Configure GPIO/RTS to high
> > + * @cdns_uart: Handle to the cdns_uart  */ static void
> > +cdns_rs485_config_gpio_rts_high(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);
> > +	}
> > +}
> > +
> > +/**
> > + * cdns_rs485_config_gpio_rts_low - Configure GPIO/RTS to low
> > + * @cdns_uart: Handle to the cdns_uart  */ static void
> > +cdns_rs485_config_gpio_rts_low(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);
> > +	}
> > +}
> > +
> > +/**
> > + * 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) {
> > +	if (cdns_uart->port->rs485.flags & SER_RS485_RTS_ON_SEND)
> > +		cdns_rs485_config_gpio_rts_high(cdns_uart);
> > +	else
> > +		cdns_rs485_config_gpio_rts_low(cdns_uart);
> > +}
> > +
> > +/**
> > + * 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) {
> > +	if (cdns_uart->port->rs485.flags & SER_RS485_RTS_AFTER_SEND)
> > +		cdns_rs485_config_gpio_rts_high(cdns_uart);
> > +	else
> > +		cdns_rs485_config_gpio_rts_low(cdns_uart);
> > +}
> > +
> > +/**
> > + * cdns_uart_tx_empty -  Check whether TX is empty
> > + * @port: Handle to the uart port structure
> > + *
> > + * Return: TIOCSER_TEMT on success, 0 otherwise  */ static unsigned
> > +int cdns_uart_tx_empty(struct uart_port *port) {
> > +	unsigned int status;
> > +
> > +	status = readl(port->membase + CDNS_UART_SR) &
> > +		       (CDNS_UART_SR_TXEMPTY | CDNS_UART_SR_TACTIVE);
> > +	return (status == CDNS_UART_SR_TXEMPTY) ? TIOCSER_TEMT : 0; }
> > +
> >  /**
> >   * cdns_uart_handle_tx - Handle the bytes to be Txed.
> >   * @dev_id: Id of the UART port
> > @@ -571,6 +657,8 @@ static int cdns_uart_clk_notifier_cb(struct
> > notifier_block *nb,  static void cdns_uart_start_tx(struct uart_port
> > *port)  {
> >  	unsigned int status;
> > +	unsigned long time_out;
> > +	struct cdns_uart *cdns_uart = port->private_data;
> >
> >  	if (uart_tx_stopped(port))
> >  		return;
> > @@ -589,8 +677,31 @@ static void cdns_uart_start_tx(struct uart_port
> > *port)
> >
> >  	writel(CDNS_UART_IXR_TXEMPTY, port->membase +
> CDNS_UART_ISR);
> >
> > +	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);
> 
> So, this will be executed each time (including the rts_before_send delay) the
> core wants to send data? This is not how it is supposed to work: The tx setup
> (and the delay before send) has to be done once when transmission starts.
> Note that when sending a bulk of data the core may call cdns_uart_start_tx()
> several times before it eventually calls cdns_uart_stop_tx() to stop the
> transmission.
We have tested bulk transfer (5000 bytes) and observed that cdns_uart_start_tx() is getting called only once. Can you please suggest how to reproduce the case where the core may call cdns_uart_start_tx() several times before it eventually calls cdns_uart_stop_tx().
> 
> 
> > +	}
> > +
> >  	cdns_uart_handle_tx(port);
> >
> > +	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();
> > +
> > +		if (cdns_uart->port->rs485.delay_rts_after_send)
> > +			mdelay(cdns_uart->port-
> >rs485.delay_rts_after_send);
> > +
> > +		/*
> > +		 * Default Rx should be setup, because RX signaling path
> > +		 * need to enable to receive data.
> > +		 */
> > +		cdns_rs485_rx_setup(cdns_uart);
> > +	}
> > +
> >  	/* Enable the TX Empty interrupt */
> >  	writel(CDNS_UART_IXR_TXEMPTY, port->membase +
> CDNS_UART_IER);  } @@
> > -602,6 +713,14 @@ static void cdns_uart_start_tx(struct uart_port
> > *port)  static void cdns_uart_stop_tx(struct uart_port *port)  {
> >  	unsigned int regval;
> > +	struct cdns_uart *cdns_uart = port->private_data;
> > +
> > +	if (cdns_uart->port->rs485.flags & SER_RS485_ENABLED) {
> > +		if (cdns_uart->port->rs485.delay_rts_after_send)
> > +			mdelay(cdns_uart->port-
> >rs485.delay_rts_after_send);
> > +
> > +		cdns_rs485_rx_setup(cdns_uart);
> > +	}
> >
> >  	regval = readl(port->membase + CDNS_UART_CR);
> >  	regval |= CDNS_UART_CR_TX_DIS;
> > @@ -626,21 +745,6 @@ static void cdns_uart_stop_rx(struct uart_port
> *port)
> >  	writel(regval, port->membase + CDNS_UART_CR);  }
> >
> > -/**
> > - * cdns_uart_tx_empty -  Check whether TX is empty
> > - * @port: Handle to the uart port structure
> > - *
> > - * Return: TIOCSER_TEMT on success, 0 otherwise
> > - */
> > -static unsigned int cdns_uart_tx_empty(struct uart_port *port) -{
> > -	unsigned int status;
> > -
> > -	status = readl(port->membase + CDNS_UART_SR) &
> > -		       (CDNS_UART_SR_TXEMPTY | CDNS_UART_SR_TACTIVE);
> > -	return (status == CDNS_UART_SR_TXEMPTY) ? TIOCSER_TEMT : 0;
> > -}
> > -
> >  /**
> >   * cdns_uart_break_ctl - Based on the input ctl we have to start or stop
> >   *			transmitting char breaks
> > @@ -829,6 +933,9 @@ 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 +1562,23 @@ 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)
> > +{
> > +	if (rs485->flags & SER_RS485_ENABLED)
> > +		dev_dbg(port->dev, "Setting UART to RS485\n");
> > +
> > +	return 0;
> > +}
> 
> So what if userspace changes the RS485 configuration? When does it take
> effect?
We will disable auto RTS control and call cdns_uart_stop_tx() which will perform both stop tx and cdns_rs485_rx_setup()
> 
> > +
> >  /**
> >   * cdns_uart_probe - Platform driver probe
> >   * @pdev: Pointer to the platform device structure @@ -1463,6 +1587,7
> > @@ static int instances;
> >   */
> >  static int cdns_uart_probe(struct platform_device *pdev)  {
> > +	u32 val;
> >  	int rc, id, irq;
> >  	struct uart_port *port;
> >  	struct resource *res;
> > @@ -1597,9 +1722,23 @@ 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,
> "rts",
> > +							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");
> 
> Why bail out with an error if having cdns_uart_data->gpiod is only optional?
We are using devm_gpiod_get_optional(), it will return NULL when gpio not passed, so IS_ERR() check will be false and will not  bail out.
> 
> > +		goto err_out_clk_notifier;
> > +	}
> > +
> >  	pm_runtime_use_autosuspend(&pdev->dev);
> >  	pm_runtime_set_autosuspend_delay(&pdev->dev,
> UART_AUTOSUSPEND_TIMEOUT);
> >  	pm_runtime_set_active(&pdev->dev);
> > @@ -1638,6 +1777,16 @@ static int cdns_uart_probe(struct
> platform_device *pdev)
> >  	cdns_uart_data->cts_override = of_property_read_bool(pdev-
> >dev.of_node,
> >  							     "cts-override");
> >
> > +	if (cdns_uart_data->port->rs485.flags & SER_RS485_ENABLED) {
> > +		if (!cdns_uart_data->gpiod) {
> > +			val = readl(cdns_uart_data->port->membase
> > +				    + CDNS_UART_MODEMCR);
> > +			val |= CDNS_UART_MODEMCR_RTS;
> > +			writel(val, cdns_uart_data->port->membase
> > +			       + CDNS_UART_MODEMCR);
> > +		}
> > +	}
> 
> This covers the RTS_AFTER_SEND mode. What if SER_RS485_RTS_ON_SEND is
> configured instead (as it is the default set by uart_get_rs485_mode())? What if
> cdns_uart_data->gpiod exists?
> Why not simply call cdns_rs485_rx_setup() which covers all these cases?
> Note that uart_add_one_port() will call into the serial core and eventually
> result in an initial call to the ports rs485_config function (see
> uart_rs485_config()). So maybe put the initial configuration into that function
> and remove the above code. However in this case
> 
> cdns_uart_data->cts_override = of_property_read_bool(pdev->dev.of_node,
> 							     "cts-override");
> should be called before uart_add_one_port().
We will fix.

Thanks,
Manikanta.