Regression: serial: imx: overrun errors on debug UART

[Francesco Dolcini]

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? This is very likely a difference compared to the RS232
console port.

I am also not really convinced this is a proper regression, while 7a637784d517
clearly is making the situation _worst_, we had some issues even before -
unfortunately I have no much more details available.

Francesco