linux-kernel - Re: [PATCH v3] serial: imx: fix tx statemachine deadlock

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Date:   Fri, 24 Nov 2023 14:37:11 +0100
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Paul Geurts <paul_geurts@...e.nl>, gregkh@...uxfoundation.org,
        jirislaby@...nel.org, shawnguo@...nel.org, s.hauer@...gutronix.de,
        kernel@...gutronix.de, festevam@...il.com, linux-imx@....com,
        u.kleine-koenig@...gutronix.de, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc:     Eberhard Stoll <eberhard.stoll@....de>
Subject: Re: [PATCH v3] serial: imx: fix tx statemachine deadlock

On 24/11/2023 14.11, Paul Geurts wrote:
> When using the serial port as RS485 port, the tx statemachine is used to
> control the RTS pin to drive the RS485 transceiver TX_EN pin. When the
> TTY port is closed in the middle of a transmission (for instance during
> userland application crash), imx_uart_shutdown disables the interface
> and disables the Transmission Complete interrupt. afer that,
> imx_uart_stop_tx bails on an incomplete transmission, to be retriggered
> by the TC interrupt. This interrupt is disabled and therefore the tx
> statemachine never transitions out of SEND. The statemachine is in
> deadlock now, and the TX_EN remains low, making the interface useless.
> 
> imx_uart_stop_tx now checks for incomplete transmission AND whether TC
> interrupts are enabled before bailing to be retriggered. This makes sure
> the state machine handling is reached, and is properly set to
> WAIT_AFTER_SEND.
> 
> Fixes: cb1a60923609 ("serial: imx: implement rts delaying for rs485")
> Signed-off-by: Paul Geurts <paul_geurts@...e.nl>

Hi Paul

Interestingly, both Eberhard (cc'ed) and I have hit similar problems in
this driver recently. See the thread
https://lore.kernel.org/lkml/20231120132256.136625-1-rasmus.villemoes@prevas.dk/
.

It is possible that this also fixes the problems I/we saw, but I can't
get around to testing until sometime next week.

Rasmus