linux-kernel - Re: [PATCH] tty: serial: imx: fix rs485 rx after tx

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Date:   Mon, 07 Aug 2023 11:48:08 +0200
From:   Sebastien Laveze <slaveze@...rtandconnective.com>
To:     Martin Fuzzey <martin.fuzzey@...wbird.group>,
        linux-serial@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Fabio Estevam <festevam@...il.com>,
        Marek Vasut <marex@...x.de>, linux-kernel@...r.kernel.org,
        Thibaud Canale <tcanale@...rtandconnective.com>
Subject: Re: [PATCH] tty: serial: imx: fix rs485 rx after tx

> Since commit 79d0224f6bf2 ("tty: serial: imx: Handle RS485 DE signal
> active high") RS485 reception no longer works after a transmission.

I can confirm on a Modbus/RS485 setup.

> Fix this by moving the existing loopback disable code to a helper
> function
> and calling it from imx_uart_start_rx() too.
> 
> Fixes: 79d0224f6bf2 ("tty: serial: imx: Handle RS485 DE signal active
> high")

Unfortunately this doesn't fix the regression on my setup and I had to
fully revert 79d0224f6bf2. 

Since there's a Modbus layer on top, it's always TX to remote then RX.

Note that RS485 communication has never been perfect on my setup. After
TX the DE line is often held active for too long leading to corrupted
RX if too close from last TX. This leads to occasional frame loss in
Modbus but it's not a blocker. Hope to get some time to investigate.

https://bugzilla.kernel.org/show_bug.cgi?id=207751