linux-kernel - Re: [REGRESSION][PATCH] serial: 8250_pci: Fix broken RS485 for F81504/508/512

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Message-ID: <0ab2436f-5c0d-966f-7095-d281292c494c@linux.intel.com>
Date: Tue, 30 Sep 2025 14:32:45 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Marnix Rijnart <marnix.rijnart@...ll.eu>
cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
    Jiri Slaby <jirislaby@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
    linux-serial <linux-serial@...r.kernel.org>, regressions@...ts.linux.dev
Subject: Re: [REGRESSION][PATCH] serial: 8250_pci: Fix broken RS485 for
 F81504/508/512

On Wed, 24 Sep 2025, Marnix Rijnart wrote:

> Commit 4afeced ("serial: core: fix sanitizing check for RTS settings")
> introduced a regression making it impossible to unset
> SER_RS485_RTS_ON_SEND from userspace if SER_RS485_RTS_AFTER_SEND is
> unsupported. Because these devices need RTS to be low on TX (fecf27a)
> they are effectively broken.
> 
> The hardware supports both RTS_ON_SEND and RTS_AFTER_SEND,
> so fix this by announcing support for SER_RS485_RTS_AFTER_SEND,
> similar to commit 068d35a.
> 
> Signed-off-by: Marnix Rijnart <marnix.rijnart@...ll.eu>

The Fixes tag is missing?

(And Cc stable tag will be necessary as well).

> ---
>  drivers/tty/serial/8250/8250_pci.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_pci.c b/drivers/tty/serial/8250/8250_pci.c
> index 152f914c599d..a9da222bd174 100644
> --- a/drivers/tty/serial/8250/8250_pci.c
> +++ b/drivers/tty/serial/8250/8250_pci.c
> @@ -1645,7 +1645,7 @@ static int pci_fintek_rs485_config(struct uart_port *port, struct ktermios *term
>  }
>  
>  static const struct serial_rs485 pci_fintek_rs485_supported = {
> -	.flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND,
> +	.flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND | SER_RS485_RTS_AFTER_SEND,
>  	/* F81504/508/512 does not support RTS delay before or after send */
>  };
>  
> 

-- 
 i.