[PATCH v2] serial: imx: ensure RTS signal is not left active after shutdown

[Marek Vasut]

On 6/25/24 10:39 PM, Fabio Estevam wrote:
> [Adding Christoph and Marek]
> 
> On Tue, Jun 25, 2024 at 3:42 PM Rasmus Villemoes
> <linux at rasmusvillemoes.dk> wrote:
>>
>> If a process is killed while writing to a /dev/ttymxc* device in RS485
>> mode, we observe that the RTS signal is left high, thus making it
>> impossible for other devices to transmit anything.
>>
>> Moreover, the ->tx_state variable is left in state SEND, which means
>> that when one next opens the device and configures baud rate etc., the
>> initialization code in imx_uart_set_termios dutifully ensures the RTS
>> pin is pulled down, but since ->tx_state is already SEND, the logic in
>> imx_uart_start_tx() does not in fact pull the pin high before
>> transmitting, so nothing actually gets on the wire on the other side
>> of the transceiver. Only when that transmission is allowed to complete
>> is the state machine then back in a consistent state.
>>
>> This is completely reproducible by doing something as simple as
>>
>>    seq 10000 > /dev/ttymxc0
>>
>> and hitting ctrl-C, and watching with a logic analyzer.
>>
>> Signed-off-by: Rasmus Villemoes <linux at rasmusvillemoes.dk>
>> ---
>> v2: Use dev_warn() instead of dev_WARN_ONCE().
>>
>> v1: https://lore.kernel.org/lkml/20240524121246.1896651-1-linux@rasmusvillemoes.dk/
>>
>> A screen dump from a logic analyzer can be seen at:
>>
>>    https://ibb.co/xCcP7Jy
>>
>> This is on an imx8mp board, with /dev/ttymxc0 and /dev/ttymxc2 both
>> configured for rs485 and connected to each other. I'm writing to
>> /dev/ttymxc2. This demonstrates both bugs; that RTS is left high when
>> a write is interrupted, and that a subsequent write actually fails to
>> have RTS high while TX'ing.
>>
>> I'm not sure what commit to name as a Fixes:. This certainly happens
>> on 6.6 and onwards, but I assume the problem exists since the tx_state
>> machine was introduced in cb1a60923609 (serial: imx: implement rts
>> delaying for rs485), and possibly even before that.

Wow, thank you for the detailed analysis of the issue.

Reviewed-by: Marek Vasut <marex at denx.de>