linux-kernel - Re: [PATCH v3 6/6] tty: serial: 8250: Add support for MOXA PCIe boards to switch interface between RS422/RS485

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Message-ID: <3332a86c-1a1d-4b78-bbfa-8ac3e2e642a1@kernel.org>
Date:   Thu, 19 Oct 2023 08:06:38 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Crescent CY Hsieh <crescentcy.hsieh@...a.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v3 6/6] tty: serial: 8250: Add support for MOXA PCIe
 boards to switch interface between RS422/RS485

On 18. 10. 23, 11:17, Crescent CY Hsieh wrote:
> MOXA PCIe boards have 4 serial interfaces and don't require additional
> stuff to switch between interfaces:
> 
> - RS232
> - RS422
> - RS485_2W (half-duplex)
> - RS485_4W (full-duplex)
> 
> By using ioctl command "TIOCRS485", it can switch between default
> interface and RS485 if supported.
> 
> That means, for RS422/RS485 board, it can switch between RS422 and
> RS485 by setting the flags within struct serial_rs485.
> 
> However, for the RS232/RS422/RS485 board, it can only switch between
> RS232 and RS485, there's no flag for switching interface into RS422.
> 
> This patch uses "SER_RS485_TERMINATE_BUS" to represent RS422 as a
> workaround solution:
> 
> - RS232                   = (no flags are set)
> - RS422                   = SER_RS485_ENABLED | SER_RS485_TERMINATE_BUS
> - RS485_2W (half-duplex)  = SER_RS485_ENABLED
> - RS485_4W (full-duplex)  = SER_RS485_ENABLED | SER_RS485_RX_DURING_TX
> 
> Signed-off-by: Crescent CY Hsieh <crescentcy.hsieh@...a.com>
> ---
>   drivers/tty/serial/8250/8250_pci.c | 58 ++++++++++++++++++++++++++++++
>   1 file changed, 58 insertions(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_pci.c b/drivers/tty/serial/8250/8250_pci.c
> index 29a28e72b..098ac466b 100644
> --- a/drivers/tty/serial/8250/8250_pci.c
> +++ b/drivers/tty/serial/8250/8250_pci.c
> @@ -1974,6 +1974,10 @@ pci_sunix_setup(struct serial_private *priv,
>   #define MOXA_RS485_2W	0x0F
>   #define MOXA_UIR_OFFSET	0x04
>   
> +static const struct serial_rs485 pci_moxa_rs485_supported = {
> +	.flags = SER_RS485_ENABLED | SER_RS485_RX_DURING_TX | SER_RS485_TERMINATE_BUS,
> +};
> +
>   static bool pci_moxa_is_mini_pcie(unsigned short device)
>   {
>   	if (device == PCI_DEVICE_ID_MOXA_CP102N	||
> @@ -2024,6 +2028,46 @@ static int pci_moxa_set_interface(const struct pci_dev *dev,
>   	return 0;
>   }
>   
> +/*
> + * MOXA PCIe boards support switching the serial interface using the ioctl
> + * command "TIOCSRS485", but there is currently no dedicated flag for switching
> + * to RS422. As a workaround, we utilize the "SER_RS485_TERMINATE_BUS" flag to
> + * represent RS422.
> + *
> + *	RS232			= (no flags are set)
> + *	RS422			= SER_RS485_ENABLED | SER_RS485_TERMINATE_BUS

Oh, I noticed only now. Can we implement this properly? I mean by 
defining e.g. SER_RS422_ENABLED? And add checks to 
uart_check_rs485_flags() that only one of 485/422 is set and whatever 
else makes sense.

> + *	RS485_2W (half-duplex)	= SER_RS485_ENABLED
> + *	RS485_4W (full-duplex)	= SER_RS485_ENABLED | SER_RS485_RX_DURING_TX
> + */

thanks,
-- 
js
suse labs