Regression: serial: imx: overrun errors on debug UART

[Stefan Wahren]

Am 24.03.23 um 13:35 schrieb Ilpo Järvinen:
> On Fri, 24 Mar 2023, Francesco Dolcini wrote:
>
>> Hello
>>
>> On Fri, Mar 24, 2023 at 01:47:59PM +0200, Ilpo Järvinen wrote:
>>> On Fri, 24 Mar 2023, Stefan Wahren wrote:
>>>> after switching to Linux 6.1.21 on our Tarragon board (i.MX6ULL SoC), we
>>>> experience the following issues with the debug UART (115200 baud, 8N1, no
>>>> hardware flow control):
>>>>
>>>> - overrun errors if we paste in multiple text lines while system is idle
>>>> - no reaction to single key strokes while system is on higher load
>>>>
>>>> After reverting 7a637784d517 ("serial: imx: reduce RX interrupt frequency")
>>>> the issue disappear.
>>>>
>>>> Maybe it's worth to mention that the Tarragon board uses two additional
>>>> application UARTs with similiar baud rates (9600 - 115200 baud, no hardware
>>>> flow control) for RS485 communication, but there are no overrun errors (with
>>>> and without the mention change).
>>> This has come up earlier, see e.g.:
>>>
>>> https://lore.kernel.org/linux-serial/20221003110850.GA28338@francesco-nb.int.toradex.com/
>> yep, it looks exactly the same issue.
>>
>> We did not verify if this was affecting other UARTs. However, isn't RS485
>> half-duplex?
> While half-duplex is more likely by far due simplicity, RS485 could also
> be full-duplex. It seems imx driver supports for both modes.

The RS485 on Tarragon is half-duplex, but this is implemented in 
external hardware. So from Linux / driver point of view it's a RS232.

To us the current behavior (overrun errors and no reaction under load) 
is not acceptable. I agree that increasing the rx threshold isn't the 
real issue. But i needed a starting point for a discussion.

So any ideas how to investigate this further are welcome.

>