[Linux-stm32] [PATCH] tty: serial: Add RS485 RTS GPIO control

[Fabrice Gasnier]

On 7/24/20 12:14 PM, Marek Vasut wrote:
> While the STM32 does support RS485 drive-enable control within the
> UART IP itself, some systems have the drive-enable line connected
> to a pin which cannot be pinmuxed as RTS. Add support for toggling
> the RTS GPIO line using the modem control GPIOs to provide at least
> some sort of emulation.
> 
> Signed-off-by: Marek Vasut <marex at denx.de>
> Cc: Andy Shevchenko <andy.shevchenko at gmail.com>
> Cc: Manivannan Sadhasivam <mani at kernel.org>
> Cc: Fabrice Gasnier <fabrice.gasnier at st.com>
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>

Hi Marek,

You could add stm32 in the commit title.
'git log --oneline' gives "serial: stm32: ..." most of the time here.

Also could you please CC STM32 maintainers and mailing list?

Please find other remark below.

> ---
>  drivers/tty/serial/stm32-usart.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
> index d1c5e536a167..22c8477571ce 100644
> --- a/drivers/tty/serial/stm32-usart.c
> +++ b/drivers/tty/serial/stm32-usart.c
> @@ -34,6 +34,7 @@
>  #include "serial_mctrl_gpio.h"
>  #include "stm32-usart.h"
>  
> +static void stm32_set_mctrl(struct uart_port *port, unsigned int mctrl);
>  static void stm32_stop_tx(struct uart_port *port);
>  static void stm32_transmit_chars(struct uart_port *port);
>  
> @@ -129,9 +130,13 @@ static int stm32_config_rs485(struct uart_port *port,
>  		if (rs485conf->flags & SER_RS485_RTS_ON_SEND) {
>  			cr3 &= ~USART_CR3_DEP;
>  			rs485conf->flags &= ~SER_RS485_RTS_AFTER_SEND;
> +			stm32_set_mctrl(port,
> +					stm32_port->port.mctrl & ~TIOCM_RTS);


Calling stm32_set_mctrl() will also set or clear bit in cr3.
But right after the if() {} else {} statement here there's a:
- writel_relaxed(cr3, port->membase + ofs->cr3);

That's basically overwritten... I guess that's undesired.

So I suggest to simply call mctrl_gpio_set() instead. This is probably
all you need here ?

>  		} else {
>  			cr3 |= USART_CR3_DEP;
>  			rs485conf->flags |= SER_RS485_RTS_AFTER_SEND;
> +			stm32_set_mctrl(port,
> +					stm32_port->port.mctrl | TIOCM_RTS);
>  		}
>  
>  		writel_relaxed(cr3, port->membase + ofs->cr3);
> @@ -847,9 +852,13 @@ static void stm32_set_termios(struct uart_port *port, struct ktermios *termios,
>  		if (rs485conf->flags & SER_RS485_RTS_ON_SEND) {
>  			cr3 &= ~USART_CR3_DEP;
>  			rs485conf->flags &= ~SER_RS485_RTS_AFTER_SEND;
> +			stm32_set_mctrl(port,
> +					stm32_port->port.mctrl & ~TIOCM_RTS);
>  		} else {
>  			cr3 |= USART_CR3_DEP;
>  			rs485conf->flags |= SER_RS485_RTS_AFTER_SEND;
> +			stm32_set_mctrl(port,
> +					stm32_port->port.mctrl | TIOCM_RTS);

Same as above.

Best Regards,
Fabrice

>  		}
>  
>  	} else {
>