linux-kernel - 8250_omap: incorrect RS485 RTS on close during transmission

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Message-ID: <f7cf39a9c152031a09608a68115e32988617f626.camel@ew.tq-group.com>
Date:   Fri, 18 Nov 2022 12:57:49 +0100
From:   Matthias Schiffer <matthias.schiffer@...tq-group.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: 8250_omap: incorrect RS485 RTS on close during transmission

Hi Lukas,

I've discovered a new issue with EM485 in 8250_omap (and probably other
serial drivers as well?): When closing the TTY device while there is
still an ongoing transmission, the RTS pin will get stuck active until
the TTY is opened again. This can be easily reproduced by running `cat
/dev/zero > /dev/ttySx` and then stopping it using Ctrl-C.

The issue exists in current mainline (6.1-rc5+), and applying "serial:
8250: 8250_omap: Avoid RS485 RTS glitch on ->set_termios()" from next
doesn't solve it. I think it should work with "serial: 8250: 8250_omap:
Support native RS485", but we also need RS485 on AM57xx CPUs, which
don't have native RS485.

I intend to look into this myself next week, but as you've worked on
this code a lot lately, maybe you already have an idea how to fix it?

Regards,
Matthias