[PATCH v3] serial: imx: fix tx statemachine deadlock

[Rasmus Villemoes]

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 at live.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 at prevas.dk>

Rasmus