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

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Message-ID: <a9c6d8e7-a014-4e04-9c6a-617235130c7b@gmx.de>
Date:   Fri, 1 Dec 2023 17:55:18 +0100
From:   Eberhard Stoll <eberhard.stoll@....de>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.se>,
        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
Subject: Re: [PATCH v3] serial: imx: fix tx statemachine deadlock



On 30.11.23 10:09, Rasmus Villemoes wrote:
> On 24/11/2023 14.37, Rasmus Villemoes wrote:
>> 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.
>
> This also seems to fix the problem I had when switching to rs232 and
> back to rs485, and I agree that it seems to be a cleaner fix than mine.
>
> I also tried reproducing what Eberhard reported, and I think I managed
> to do that, and at least my way of reproducing the tx lockup also seems
> to be fixed by this patch. Eberhard, can you test this patch in your setup?
>
> In any case,
>
> Tested-by: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
>
> Rasmus
>

Yes, it also works for my test setup!

Tested-by: Eberhard Stoll <eberhard.stoll@....de>

Best regards
Eberhard