Regression: serial: imx: overrun errors on debug UART

[Francesco Dolcini]

On Fri, Mar 24, 2023 at 01:49:56PM +0100, Stefan Wahren wrote:
> Am 24.03.23 um 13:35 schrieb Ilpo Järvinen:
> > On Fri, 24 Mar 2023, Francesco Dolcini wrote:
> > > 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 me this is an interesting difference that might be worth
investigating. The console is somehow special since you are going to
echo out the received chars most of the times.

Francesco