linux-kernel - Re: [PATCH v1 19/31] serial: 8250_mxpcie: add break signal support under RS485

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Message-ID: <CAHp75Veb_iu+WiUdJ6anqu3TbkjgwAU2deo8=b8TmcC7COS03A@mail.gmail.com>
Date: Mon, 1 Dec 2025 03:56:38 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Crescent Hsieh <crescentcy.hsieh@...a.com>
Cc: gregkh@...uxfoundation.org, jirislaby@...nel.org, 
	ilpo.jarvinen@...ux.intel.com, linux-kernel@...r.kernel.org, 
	linux-serial@...r.kernel.org
Subject: Re: [PATCH v1 19/31] serial: 8250_mxpcie: add break signal support
 under RS485

On Sun, Nov 30, 2025 at 12:45 PM Crescent Hsieh
<crescentcy.hsieh@...a.com> wrote:
>
> Moxa PCIe boards require a specific sequence to send break signals in
> RS485 mode. This patch implements the required behavior, while falling
> back to the default break mechanism in RS232 and RS422 modes.

...

> +static void mxpcie8250_software_break_ctl(struct uart_port *port, int break_state)
> +{
> +       struct uart_8250_port *up = up_to_u8250p(port);
> +       struct tty_struct *tty = port->state->port.tty;
> +       unsigned char tx_byte = 0x01;
> +       unsigned int baud, quot;
> +       unsigned long flags;
> +       u8 sfr;
> +
> +       uart_port_lock_irqsave(port, &flags);
> +
> +       if (break_state == -1) {
> +               serial_out(up, UART_LCR, up->lcr | UART_LCR_DLAB);
> +               serial_out(up, UART_DLL, 0);
> +               serial_out(up, UART_DLM, 0);
> +               serial_out(up, UART_LCR, up->lcr);

> +               memcpy(port->membase + MOXA_PUART_TX_FIFO_MEM, &tx_byte, 1);

My gosh, why?
It's even different to the way you used in the previous change.

> +               sfr = serial_in(up, MOXA_PUART_SFR);
> +               serial_out(up, MOXA_PUART_SFR, sfr | MOXA_PUART_SFR_FORCE_TX);
> +
> +               up->lcr |= UART_LCR_SBC;
> +               serial_out(up, UART_LCR, up->lcr);
> +       } else {
> +               up->lcr &= ~UART_LCR_SBC;
> +               serial_out(up, UART_LCR, up->lcr);
> +
> +               sfr = serial_in(up, MOXA_PUART_SFR);
> +               serial_out(up, MOXA_PUART_SFR, sfr &= ~MOXA_PUART_SFR_FORCE_TX);
> +
> +               serial_out(up, UART_FCR, UART_FCR_CLEAR_XMIT);
> +
> +               baud = tty_get_baud_rate(tty);
> +               quot = uart_get_divisor(port, baud);
> +               serial8250_do_set_divisor(port, baud, quot);
> +               serial_out(up, UART_LCR, up->lcr);
> +       }
> +       uart_port_unlock_irqrestore(port, flags);
> +}

-- 
With Best Regards,
Andy Shevchenko