linux-kernel - Re: [PATCH] serial: imx: also enable Transmit Complete interrupt in rs232 mode

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Message-ID: <0dc938c5-0d11-4d21-940f-9be1f771b7df@gmx.de>
Date:   Tue, 21 Nov 2023 21:59:19 +0100
From:   Eberhard Stoll <eberhard.stoll@....de>
To:     rasmus.villemoes@...vas.dk
Cc:     festevam@...il.com, gregkh@...uxfoundation.org,
        jirislaby@...nel.org, kernel@...gutronix.de,
        linux-arm-kernel@...ts.infradead.org, linux-imx@....com,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        s.hauer@...gutronix.de, shawnguo@...nel.org,
        eberhard.stoll@...tron.de
Subject: Re: [PATCH] serial: imx: also enable Transmit Complete interrupt in
 rs232 mode

> Currently, if one switches to rs232 mode, writes something to the
> device, and then switches to rs485 mode, the imx_port's ->tx_state is
> left as SEND. This then prevents a subsequent write in rs485 mode from
> properly asserting the rts pin (i.e. enabling the transceiver),
> because imx_uart_start_rx() does not enter the "if (sport->tx_state ==
> OFF)" branch. Hence nothing is actually transmitted.
>
> The problem is that in rs232 mode, ->tx_state never gets set to OFF,
> due to
>
> 	usr2 = imx_uart_readl(sport, USR2);
> 	if (!(usr2 & USR2_TXDC)) {
> 		/* The shifter is still busy, so retry once TC triggers */
> 		return;
> 	}
>
> in imx_uart_stop_tx(), and TC never triggers because the Transmit
> Complete interrupt is not enabled for rs232.
>
> Signed-off-by: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
> ---
> I'm not sure this is the best fix.
>
> At first I considered doing something much more targeted, but
> definitely also more hacky: In imx_uart_rs485_config(), if switching
> on rs485 mode, simply add "sport->tx_state = OFF;".
>
> If someone has a better suggestion, I'm all ears.

Hello Rasmus,

i can observe a very similar situation, but with a litte different
configuration. This is how i can trigger the situation very quickly:

   1) open the port
   2) send 1 byte out
   3) close the port

Do it in a loop. As faster, the lockup may occur earlier (but not
mandatory, 100ms is sufficient in my setup at 115200 Baud on an
i.mx8mm board).
With this configuration i get the lockup in around 1 minute.

For my setup it's clear what happens:

   - when the tty is closed imx_uart_shutdown() is called. This calls
     imx_uart_stop_tx()
   - for a lockup, the shifter is still busy and imx_uart_stop_tx()
     returns early (as you explained) without modifying ->tx_state.
   - imx_uart_shutdown() proceeds and finally closes the port. Due to
     imx_uart_stop_tx() is not executed completely tx_state is left in
     state ->tx_state == SEND.
   - When the port is opened again, tx_state is SEND and nothing can
     be transmitted any more. The tx path has locked up!

Setting ->tx_state = SEND in imx_uart_shutdown() helps for my issue
(and should be ok IMHO).

But IMHO there is one next issue with this situation: When the port
operates with WAIT_AFTER_RTS and WAIT_AFTER_SEND then some timers
for callback functions might be active. I did not discover where they
are stopped for the case when the serial port is closed. Maybe stopping
is not required ...

I'd appreciate someone with more experience could review or revise it

Best Regards
Eberhard