Regression: serial: imx: overrun errors on debug UART

[Sergey Organov]

Hello,

Uwe Kleine-König <u.kleine-koenig at pengutronix.de> writes:
> Hello,
>
> On Sat, Mar 25, 2023 at 12:31:01PM +0100, Stefan Wahren wrote:
[...]
>
> So the current theory that the issue occurs because of a combination of:
>
>  - With a higher watermark value the irq triggers later and so there is
>    less time the until the ISR must run before an overflow happens; and
>
>  - serial console activity disables irqs for a (relative) long time
>
> right?
>
> So on an UP system the problem should occur also on a non-console
> port?

That's exactly what I've experienced, especially when console baud
rate was lower than that of other port(s). I had console at 115200,
and got immediate problems on another port working at 460800 whenever
relatively lengthy printk output has been emitted (in my case it was
info from wlan driver.)

> Local irqs are only disabled if some printk is about to be emitted,
> isn't it?

Yep, and this allows for easy check if it's indeed printk that causes
this by eliminating the output using

# echo 0 > /proc/sys/kernel/printk

> Does this match the error you're seeing?
>
> That makes me wonder if the error doesn't relate to the UART being a
> console port, but the UART being used without DMA?! (So the patch above
> fixes the problem for you because on the console port no DMA is used?)

Indeed DMA is likely to be able to hide the problem if the cause is
printk, though all my results were obtained on DMA-disabled ports, and I
never checked with DMA enabled, so unfortunately I have no tested
confirmation of this idea.

Best regards,
-- Sergey Organov